v5, 7 partsFast git status via a file system watcher

dir.c, dir.h

diff --git a/dir.c b/dir.c
index 9efcf1eab6..5f531d9eed 100644
--- a/dir.c
+++ b/dir.c
@@ -666,7 +666,7 @@ static void trim_trailing_spaces(char *buf)
  *
  * If "name" has the trailing slash, it'll be excluded in the search.
  */
-static struct untracked_cache_dir *lookup_untracked(struct untracked_cache *uc,
+struct untracked_cache_dir *lookup_untracked(struct untracked_cache *uc,
 						    struct untracked_cache_dir *dir,
 						    const char *name, int len)
 {
diff --git a/dir.h b/dir.h
index a89c13e27a..75655ebe35 100644
--- a/dir.h
+++ b/dir.h
@@ -354,4 +354,7 @@ extern void connect_work_tree_and_git_dir(const char *work_tree, const char *git
 extern void relocate_gitdir(const char *path,
 			    const char *old_git_dir,
 			    const char *new_git_dir);
+struct untracked_cache_dir *lookup_untracked(struct untracked_cache *uc,
+					     struct untracked_cache_dir *dir,
+					     const char *name, int len);
 #endif

diff --git a/compat/bswap.h b/compat/bswap.h
index d47c003544..f89fe7f4b5 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -158,6 +158,7 @@ static inline uint64_t git_bswap64(uint64_t x)
 
 #define get_be16(p)	ntohs(*(unsigned short *)(p))
 #define get_be32(p)	ntohl(*(unsigned int *)(p))
+#define get_be64(p)	ntohll(*(uint64_t *)(p))
 #define put_be32(p, v)	do { *(unsigned int *)(p) = htonl(v); } while (0)
 
 #else
@@ -170,6 +171,9 @@ static inline uint64_t git_bswap64(uint64_t x)
 	(*((unsigned char *)(p) + 1) << 16) | \
 	(*((unsigned char *)(p) + 2) <<  8) | \
 	(*((unsigned char *)(p) + 3) <<  0) )
+#define get_be64(p)	( \
+	((uint64_t)get_be32((unsigned char *)(p) + 0) << 32) | \
+	((uint64_t)get_be32((unsigned char *)(p) + 4) <<  0)
 #define put_be32(p, v)	do { \
 	unsigned int __v = (v); \
 	*((unsigned char *)(p) + 0) = __v >> 24; \

diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
new file mode 100755
index 0000000000..458eabe6dc
--- /dev/null
+++ b/t/t7519-status-fsmonitor.sh
@@ -0,0 +1,173 @@
+#!/bin/sh
+
+test_description='git status with file system watcher'
+
+. ./test-lib.sh
+
+clean_repo () {
+	git reset --hard HEAD &&
+	git clean -fd &&
+	rm -f marker
+}
+
+dirty_repo () {
+	: >untracked &&
+	: >dir1/untracked &&
+	: >dir2/untracked &&
+	echo 1 >modified &&
+	echo 2 >dir1/modified &&
+	echo 3 >dir2/modified &&
+	echo 4 >new &&
+	echo 5 >dir1/new &&
+	echo 6 >dir2/new &&
+	git add new &&
+	git add dir1/new &&
+	git add dir2/new
+}
+
+# The test query-fsmonitor hook proc will output a marker file we can use to
+# ensure the hook was actually used to generate the correct results.
+
+# fsmonitor works correctly with or without the untracked cache
+# but if it is available, we'll turn it on to ensure we test that
+# codepath as well.
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+if test_have_prereq UNTRACKED_CACHE; then
+	git config core.untrackedcache true
+else
+	git config core.untrackedcache false
+fi
+
+test_expect_success 'setup' '
+	mkdir -p .git/hooks &&
+	: >tracked &&
+	: >modified &&
+	mkdir dir1 &&
+	: >dir1/tracked &&
+	: >dir1/modified &&
+	mkdir dir2 &&
+	: >dir2/tracked &&
+	: >dir2/modified &&
+	git add . &&
+	test_tick &&
+	git commit -m initial &&
+	git config core.fsmonitor true &&
+	cat >.gitignore <<-\EOF
+	.gitignore
+	expect*
+	output*
+	marker*
+	EOF
+'
+
+# Ensure commands that call refresh_index() to move the index back in time
+# properly invalidate the fsmonitor cache
+
+test_expect_success 'refresh_index() invalidates fsmonitor cache' '
+	git status &&
+	test_path_is_missing marker &&
+	dirty_repo &&
+	write_script .git/hooks/query-fsmonitor<<-\EOF &&
+	:>marker
+	EOF
+	git add . &&
+	git commit -m "to reset" &&
+	git status &&
+	test_path_is_file marker &&
+	git reset HEAD~1 &&
+	rm -f marker &&
+	git status >output &&
+	test_path_is_file marker &&
+	git -c core.fsmonitor=false status >expect &&
+	test_i18ncmp expect output
+'
+
+# Now make sure it's actually skipping the check for modified and untracked
+# files unless it is told about them.  Note, after "git reset --hard HEAD" no
+# extensions exist other than 'TREE' so do a "git status" to get the extension
+# written before testing the results.
+
+test_expect_success "status doesn't detect unreported modifications" '
+	write_script .git/hooks/query-fsmonitor<<-\EOF &&
+	:>marker
+	EOF
+	clean_repo &&
+	git status &&
+	test_path_is_missing marker &&
+	: >untracked &&
+	echo 2 >dir1/modified &&
+	git status >output &&
+	test_path_is_file marker &&
+	test_i18ngrep ! "Changes not staged for commit:" output &&
+	test_i18ngrep ! "Untracked files:" output &&
+	write_script .git/hooks/query-fsmonitor<<-\EOF &&
+	:>marker
+	printf "untracked\0"
+	printf "dir1/modified\0"
+	EOF
+	rm -f marker &&
+	git status >output &&
+	test_path_is_file marker &&
+	test_i18ngrep "Changes not staged for commit:" output &&
+	test_i18ngrep "Untracked files:" output
+'
+
+# Status is well tested elsewhere so we'll just ensure that the results are
+# the same when using core.fsmonitor. First call after turning on the option
+# does a complete scan so we need to do two calls to ensure we test the new
+# codepath.
+
+test_expect_success 'status with core.untrackedcache false' '
+	git config core.untrackedcache false &&
+	write_script .git/hooks/query-fsmonitor<<-\EOF &&
+	if [ $1 -ne 1 ]
+	then
+		echo -e "Unsupported query-fsmonitor hook version.\n" >&2
+		exit 1;
+	fi
+	: >marker
+	printf "untracked\0"
+	printf "dir1/untracked\0"
+	printf "dir2/untracked\0"
+	printf "modified\0"
+	printf "dir1/modified\0"
+	printf "dir2/modified\0"
+	printf "new\0""
+	printf "dir1/new\0"
+	printf "dir2/new\0"
+	EOF
+	clean_repo &&
+	dirty_repo &&
+	git -c core.fsmonitor=false status >expect &&
+	clean_repo &&
+	git status &&
+	test_path_is_missing marker &&
+	dirty_repo &&
+	git status >output &&
+	test_path_is_file marker &&
+	test_i18ncmp expect output
+'
+
+if ! test_have_prereq UNTRACKED_CACHE; then
+	skip_all='This system does not support untracked cache'
+	test_done
+fi
+
+test_expect_success 'status with core.untrackedcache true' '
+	git config core.untrackedcache true &&
+	git -c core.fsmonitor=false status >expect &&
+	clean_repo &&
+	git status &&
+	test_path_is_missing marker &&
+	dirty_repo &&
+	git status >output &&
+	test_path_is_file marker &&
+	test_i18ncmp expect output
+'
+
+test_done

Makefile, builtin/update-index.c, cache.h, config.c, dir.c, dir.h, entry.c, environment.c, fsmonitor.c, fsmonitor.h, read-cache.c, unpack-trees.c

diff --git a/Makefile b/Makefile
index 7c621f7f76..992dd58801 100644
--- a/Makefile
+++ b/Makefile
@@ -763,6 +763,7 @@ LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec_cmd.o
 LIB_OBJS += fetch-pack.o
 LIB_OBJS += fsck.o
+LIB_OBJS += fsmonitor.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o
diff --git a/builtin/update-index.c b/builtin/update-index.c
index ebfc09faa0..32fd977b43 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -232,6 +232,7 @@ static int mark_ce_flags(const char *path, int flag, int mark)
 		else
 			active_cache[pos]->ce_flags &= ~flag;
 		active_cache[pos]->ce_flags |= CE_UPDATE_IN_BASE;
+		active_cache[pos]->ce_flags |= CE_FSMONITOR_DIRTY;
 		cache_tree_invalidate_path(&the_index, path);
 		active_cache_changed |= CE_ENTRY_CHANGED;
 		return 0;
diff --git a/cache.h b/cache.h
index 4d92aae0e8..220efbfde7 100644
--- a/cache.h
+++ b/cache.h
@@ -201,6 +201,7 @@ struct cache_entry {
 #define CE_ADDED             (1 << 19)
 
 #define CE_HASHED            (1 << 20)
+#define CE_FSMONITOR_DIRTY   (1 << 21)
 #define CE_WT_REMOVE         (1 << 22) /* remove in work directory */
 #define CE_CONFLICTED        (1 << 23)
 
@@ -324,6 +325,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define CACHE_TREE_CHANGED	(1 << 5)
 #define SPLIT_INDEX_ORDERED	(1 << 6)
 #define UNTRACKED_CHANGED	(1 << 7)
+#define FSMONITOR_CHANGED	(1 << 8)
 
 struct split_index;
 struct untracked_cache;
@@ -342,6 +344,8 @@ struct index_state {
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
 	struct untracked_cache *untracked;
+	uint64_t fsmonitor_last_update;
+	struct ewah_bitmap *fsmonitor_dirty;
 };
 
 extern struct index_state the_index;
@@ -767,6 +771,7 @@ extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
 extern int git_db_env, git_index_env, git_graft_env, git_common_dir_env;
+extern int core_fsmonitor;
 
 /*
  * Include broken refs in all ref iterations, which will
diff --git a/config.c b/config.c
index 146cb3452a..c70f667640 100644
--- a/config.c
+++ b/config.c
@@ -1245,6 +1245,10 @@ static int git_default_core_config(const char *var, const char *value)
 			hide_dotfiles = HIDE_DOTFILES_DOTGITONLY;
 		else
 			hide_dotfiles = git_config_bool(var, value);
+	}
+
+	if (!strcmp(var, "core.fsmonitor")) {
+		core_fsmonitor = git_config_bool(var, value);
 		return 0;
 	}
 
diff --git a/dir.c b/dir.c
index 5f531d9eed..1f4a43dc75 100644
--- a/dir.c
+++ b/dir.c
@@ -17,6 +17,7 @@
 #include "utf8.h"
 #include "varint.h"
 #include "ewah/ewok.h"
+#include "fsmonitor.h"
 
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
@@ -1680,17 +1681,23 @@ static int valid_cached_dir(struct dir_struct *dir,
 	if (!untracked)
 		return 0;
 
-	if (stat(path->len ? path->buf : ".", &st)) {
-		invalidate_directory(dir->untracked, untracked);
-		memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
-		return 0;
-	}
-	if (!untracked->valid ||
-	    match_stat_data_racy(istate, &untracked->stat_data, &st)) {
-		if (untracked->valid)
+	/*
+	 * With fsmonitor, we can trust the untracked cache's valid field.
+	 */
+	refresh_by_fsmonitor(istate);
+	if (!(dir->untracked->use_fsmonitor && untracked->valid)) {
+		if (stat(path->len ? path->buf : ".", &st)) {
 			invalidate_directory(dir->untracked, untracked);
-		fill_stat_data(&untracked->stat_data, &st);
-		return 0;
+			memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
+			return 0;
+		}
+		if (!untracked->valid ||
+		    match_stat_data_racy(istate, &untracked->stat_data, &st)) {
+			if (untracked->valid)
+				invalidate_directory(dir->untracked, untracked);
+			fill_stat_data(&untracked->stat_data, &st);
+			return 0;
+		}
 	}
 
 	if (untracked->check_only != !!check_only) {
diff --git a/dir.h b/dir.h
index 75655ebe35..1a67cb4324 100644
--- a/dir.h
+++ b/dir.h
@@ -139,6 +139,8 @@ struct untracked_cache {
 	int gitignore_invalidated;
 	int dir_invalidated;
 	int dir_opened;
+	/* fsmonitor invalidation data */
+	unsigned int use_fsmonitor : 1;
 };
 
 struct dir_struct {
diff --git a/entry.c b/entry.c
index d6b263f78e..667ca5734b 100644
--- a/entry.c
+++ b/entry.c
@@ -222,6 +222,7 @@ static int write_entry(struct cache_entry *ce,
 			lstat(ce->name, &st);
 		fill_stat_cache_info(ce, &st);
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		ce->ce_flags |= CE_FSMONITOR_DIRTY;
 		state->istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 	return 0;
diff --git a/environment.c b/environment.c
index aa478e71de..ea63ee4604 100644
--- a/environment.c
+++ b/environment.c
@@ -64,6 +64,7 @@ int precomposed_unicode = -1; /* see probe_utf8_pathname_composition() */
 unsigned long pack_size_limit_cfg;
 enum hide_dotfiles_type hide_dotfiles = HIDE_DOTFILES_DOTGITONLY;
 enum log_refs_config log_all_ref_updates = LOG_REFS_UNSET;
+int core_fsmonitor;
 
 #ifndef PROTECT_HFS_DEFAULT
 #define PROTECT_HFS_DEFAULT 0
diff --git a/fsmonitor.c b/fsmonitor.c
new file mode 100644
index 0000000000..c1f89abccd
--- /dev/null
+++ b/fsmonitor.c
@@ -0,0 +1,261 @@
+#include "cache.h"
+#include "dir.h"
+#include "ewah/ewok.h"
+#include "run-command.h"
+#include "strbuf.h"
+#include "fsmonitor.h"
+
+#define INDEX_EXTENSION_VERSION	1
+#define HOOK_INTERFACE_VERSION		1
+
+int read_fsmonitor_extension(struct index_state *istate, const void *data,
+	unsigned long sz)
+{
+	const char *index = data;
+	uint32_t hdr_version;
+	uint32_t ewah_size;
+	int ret;
+
+	if (sz < sizeof(uint32_t) + sizeof(uint64_t) + sizeof(uint32_t))
+		return error("corrupt fsmonitor extension (too short)");
+
+	hdr_version = get_be32(index);
+	index += sizeof(uint32_t);
+	if (hdr_version != INDEX_EXTENSION_VERSION)
+		return error("bad fsmonitor version %d", hdr_version);
+
+	istate->fsmonitor_last_update = get_be64(index);
+	index += sizeof(uint64_t);
+
+	ewah_size = get_be32(index);
+	index += sizeof(uint32_t);
+
+	istate->fsmonitor_dirty = ewah_new();
+	ret = ewah_read_mmap(istate->fsmonitor_dirty, index, ewah_size);
+	if (ret != ewah_size) {
+		ewah_free(istate->fsmonitor_dirty);
+		istate->fsmonitor_dirty = NULL;
+		return error("failed to parse ewah bitmap reading fsmonitor index extension");
+	}
+
+	return 0;
+}
+
+void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
+{
+	uint32_t hdr_version;
+	uint64_t tm;
+	struct ewah_bitmap *bitmap;
+	int i;
+	uint32_t ewah_start;
+	uint32_t ewah_size = 0;
+	int fixup = 0;
+
+	hdr_version = htonl(INDEX_EXTENSION_VERSION);
+	strbuf_add(sb, &hdr_version, sizeof(uint32_t));
+
+	tm = htonll((uint64_t)istate->fsmonitor_last_update);
+	strbuf_add(sb, &tm, sizeof(uint64_t));
+	fixup = sb->len;
+	strbuf_add(sb, &ewah_size, sizeof(uint32_t)); /* we'll fix this up later */
+
+	ewah_start = sb->len;
+	bitmap = ewah_new();
+	for (i = 0; i < istate->cache_nr; i++)
+		if (istate->cache[i]->ce_flags & CE_FSMONITOR_DIRTY)
+			ewah_set(bitmap, i);
+	ewah_serialize_strbuf(bitmap, sb);
+	ewah_free(bitmap);
+
+	/* fix up size field */
+	ewah_size = htonl(sb->len - ewah_start);
+	memcpy(sb->buf + fixup, &ewah_size, sizeof(uint32_t));
+}
+
+static struct untracked_cache_dir *find_untracked_cache_dir(
+	struct untracked_cache *uc, struct untracked_cache_dir *ucd,
+	const char *name)
+{
+	const char *end;
+	struct untracked_cache_dir *dir = ucd;
+
+	if (!*name)
+		return dir;
+
+	end = strchr(name, '/');
+	if (end) {
+		dir = lookup_untracked(uc, ucd, name, end - name);
+		if (dir)
+			return find_untracked_cache_dir(uc, dir, end + 1);
+	}
+
+	return dir;
+}
+
+/* This function will be passed to ewah_each_bit() */
+static void mark_fsmonitor_dirty(size_t pos, void *is)
+{
+	struct index_state *istate = is;
+	struct untracked_cache_dir *dir;
+	struct cache_entry *ce = istate->cache[pos];
+
+	assert(pos < istate->cache_nr);
+	ce->ce_flags |= CE_FSMONITOR_DIRTY;
+
+	if (!istate->untracked || !istate->untracked->root)
+		return;
+
+	dir = find_untracked_cache_dir(istate->untracked, istate->untracked->root, ce->name);
+	if (dir)
+		dir->valid = 0;
+}
+
+void tweak_fsmonitor_extension(struct index_state *istate)
+{
+	int val, fsmonitor = 0;
+
+	if (!git_config_get_maybe_bool("core.fsmonitor", &val))
+		fsmonitor = val;
+
+	if (fsmonitor) {
+		if (!istate->fsmonitor_last_update)
+			istate->cache_changed |= FSMONITOR_CHANGED;
+		if (istate->fsmonitor_dirty)
+			ewah_each_bit(istate->fsmonitor_dirty, mark_fsmonitor_dirty, istate);
+	} else {
+		if (istate->fsmonitor_last_update)
+			istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = 0;
+	}
+
+	if (istate->fsmonitor_dirty) {
+		ewah_free(istate->fsmonitor_dirty);
+		istate->fsmonitor_dirty = NULL;
+	}
+}
+
+/*
+ * Call the query-fsmonitor hook passing the time of the last saved results.
+ */
+static int query_fsmonitor(int version, uint64_t last_update, struct strbuf *query_result)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	char ver[64];
+	char date[64];
+	const char *argv[4];
+
+	if (!(argv[0] = find_hook("query-fsmonitor")))
+		return -1;
+
+	snprintf(ver, sizeof(version), "%d", version);
+	snprintf(date, sizeof(date), "%" PRIuMAX, (uintmax_t)last_update);
+	argv[1] = ver;
+	argv[2] = date;
+	argv[3] = NULL;
+	cp.argv = argv;
+	cp.out = -1;
+
+	return capture_command(&cp, query_result, 1024);
+}
+
+static void mark_file_dirty(struct index_state *istate, const char *name)
+{
+	struct untracked_cache_dir *dir;
+	int pos;
+
+	/* find it in the index and mark that entry as dirty */
+	pos = index_name_pos(istate, name, strlen(name));
+	if (pos >= 0) {
+		if (!(istate->cache[pos]->ce_flags & CE_FSMONITOR_DIRTY)) {
+			istate->cache[pos]->ce_flags |= CE_FSMONITOR_DIRTY;
+			istate->cache_changed |= FSMONITOR_CHANGED;
+		}
+	}
+
+	/*
+	 * Find the corresponding directory in the untracked cache
+	 * and mark it as invalid
+	 */
+	if (!istate->untracked || !istate->untracked->root)
+		return;
+
+	dir = find_untracked_cache_dir(istate->untracked, istate->untracked->root, name);
+	if (dir) {
+		if (dir->valid) {
+			dir->valid = 0;
+			istate->cache_changed |= FSMONITOR_CHANGED;
+		}
+	}
+}
+
+void refresh_by_fsmonitor(struct index_state *istate)
+{
+	static int has_run_once = 0;
+	struct strbuf query_result = STRBUF_INIT;
+	int query_success = 0;
+	size_t bol = 0; /* beginning of line */
+	uint64_t last_update;
+	char *buf, *entry;
+	int i;
+
+	if (!core_fsmonitor || has_run_once)
+		return;
+	has_run_once = 1;
+
+	/*
+	 * This could be racy so save the date/time now and the hook
+	 * should be inclusive to ensure we don't miss potential changes.
+	 */
+	last_update = getnanotime();
+
+	/*
+	 * If we have a last update time, call query-monitor for the set of
+	 * changes since that time.
+	 */
+	if (istate->fsmonitor_last_update) {
+		query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION,
+			istate->fsmonitor_last_update, &query_result);
+		trace_performance_since(last_update, "query-fsmonitor");
+	}
+
+	if (query_success) {
+		/* Mark all entries returned by the monitor as dirty */
+		buf = entry = query_result.buf;
+		for (i = 0; i < query_result.len; i++) {
+			if (buf[i] != '\0')
+				continue;
+			mark_file_dirty(istate, buf + bol);
+			bol = i + 1;
+		}
+		if (bol < query_result.len)
+			mark_file_dirty(istate, buf + bol);
+
+		/* Mark all clean entries up-to-date */
+		for (i = 0; i < istate->cache_nr; i++) {
+			struct cache_entry *ce = istate->cache[i];
+			if (ce_stage(ce) || (ce->ce_flags & CE_FSMONITOR_DIRTY))
+				continue;
+			ce_mark_uptodate(ce);
+		}
+
+		/*
+		 * Now that we've marked the invalid entries in the
+		 * untracked-cache itself, we can mark the untracked cache for
+		 * fsmonitor usage.
+		 */
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 1;
+	} else {
+		/* if we can't update the cache, fall back to checking them all */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags |= CE_FSMONITOR_DIRTY;
+
+		/* mark the untracked cache as unusable for fsmonitor */
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 0;
+	}
+	strbuf_release(&query_result);
+
+	/* Now that we've updated istate, save the last_update time */
+	istate->fsmonitor_last_update = last_update;
+}
diff --git a/fsmonitor.h b/fsmonitor.h
new file mode 100644
index 0000000000..9c1e2b480f
--- /dev/null
+++ b/fsmonitor.h
@@ -0,0 +1,9 @@
+#ifndef FSMONITOR_H
+#define FSMONITOR_H
+
+int read_fsmonitor_extension(struct index_state *istate, const void *data, unsigned long sz);
+void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate);
+void tweak_fsmonitor_extension(struct index_state *istate);
+void refresh_by_fsmonitor(struct index_state *istate);
+
+#endif
diff --git a/read-cache.c b/read-cache.c
index bc156a133e..68a1580c96 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -18,6 +18,7 @@
 #include "varint.h"
 #include "split-index.h"
 #include "utf8.h"
+#include "fsmonitor.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -37,11 +38,12 @@
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
 #define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
+#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -61,6 +63,7 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 	free(old);
 	set_index_entry(istate, nr, ce);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	ce->ce_flags |= CE_FSMONITOR_DIRTY;
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 }
 
@@ -777,6 +780,7 @@ int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
 	}
 	cache_tree_invalidate_path(istate, ce->name);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	ce->ce_flags |= CE_FSMONITOR_DIRTY;
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 
 	return 0;
@@ -1344,6 +1348,8 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 	const char *added_fmt;
 	const char *unmerged_fmt;
 
+	refresh_by_fsmonitor(istate);
+
 	modified_fmt = (in_porcelain ? "M\t%s\n" : "%s: needs update\n");
 	deleted_fmt = (in_porcelain ? "D\t%s\n" : "%s: needs update\n");
 	typechange_fmt = (in_porcelain ? "T\t%s\n" : "%s needs update\n");
@@ -1380,8 +1386,11 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 			continue;
 
 		new = refresh_cache_ent(istate, ce, options, &cache_errno, &changed);
-		if (new == ce)
+		if (new == ce) {
+			ce->ce_flags &= ~CE_FSMONITOR_DIRTY;
 			continue;
+		}
+
 		if (!new) {
 			const char *fmt;
 
@@ -1391,6 +1400,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 				 */
 				ce->ce_flags &= ~CE_VALID;
 				ce->ce_flags |= CE_UPDATE_IN_BASE;
+				ce->ce_flags |= CE_FSMONITOR_DIRTY;
 				istate->cache_changed |= CE_ENTRY_CHANGED;
 			}
 			if (quiet)
@@ -1549,6 +1559,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_UNTRACKED:
 		istate->untracked = read_untracked_extension(data, sz);
 		break;
+	case CACHE_EXT_FSMONITOR:
+		read_fsmonitor_extension(istate, data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1721,6 +1734,7 @@ static void post_read_index_from(struct index_state *istate)
 	check_ce_order(istate);
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
+	tweak_fsmonitor_extension(istate);
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2295,6 +2309,16 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->fsmonitor_last_update) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_fsmonitor_extension(&sb, istate);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1))
 		return -1;
diff --git a/unpack-trees.c b/unpack-trees.c
index d38c37e38c..0ebc505b6c 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -417,6 +417,7 @@ static int apply_sparse_checkout(struct index_state *istate,
 		ce->ce_flags &= ~CE_SKIP_WORKTREE;
 	if (was_skip_worktree != ce_skip_worktree(ce)) {
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		ce->ce_flags |= CE_FSMONITOR_DIRTY;
 		istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 

diff --git a/templates/hooks--query-fsmonitor.sample b/templates/hooks--query-fsmonitor.sample
new file mode 100755
index 0000000000..8d05b87a90
--- /dev/null
+++ b/templates/hooks--query-fsmonitor.sample
@@ -0,0 +1,76 @@
+#!/bin/sh
+#
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to provide fast
+# git status.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+# To enable this hook, rename this file to "query-fsmonitor"
+
+# check the hook interface version
+if [ "$1" -eq 1 ]
+then
+	# convert nanoseconds to seconds
+	time_t=$(($2/1000000000))
+else
+	echo -e "Unsupported query-fsmonitor hook version.\nFalling back to scanning...\n" >&2
+	exit 1;
+fi
+
+# Convert unix style paths to what Watchman expects
+case "$(uname -s)" in
+MINGW*|MSYS_NT*)
+  GIT_WORK_TREE="$(cygpath -aw "$PWD" | sed 's,\\,/,g')"
+  ;;
+*)
+  GIT_WORK_TREE="$PWD"
+  ;;
+esac
+
+# In the query expression below we're asking for names of files that
+# changed since $time_t but were not transient (ie created after
+# $time_t but no longer exist).
+#
+# To accomplish this, we're using the "since" generator to use the
+# recency index to select candidate nodes and "fields" to limit the
+# output to file names only. Then we're using the "expression" term to
+# further constrain the results.
+#
+# The category of transient files that we want to ignore will have a
+# creation clock (cclock) newer than $time_t value and will also not
+# currently exist.
+
+echo  "[\"query\", \"$GIT_WORK_TREE\", { \
+	\"since\": $time_t, \
+	\"fields\": [\"name\"], \
+	\"expression\": [\"not\", [\"allof\", [\"since\", $time_t, \"cclock\"], [\"not\", \"exists\"]]] \
+	}]" | \
+	watchman -j |
+	perl -0666 -e '
+		use strict;
+		use warnings;
+
+		my $stdin = <>;
+		die "Watchman: command returned no output.\nFalling back to scanning...\n" if $stdin eq "";
+		die "Watchman: command returned invalid output: $stdin\nFalling back to scanning...\n" unless $stdin =~ /^\{/;
+
+		my $json_pkg;
+		eval {
+			require JSON::XS;
+			$json_pkg = "JSON::XS";
+			1;
+		} or do {
+			require JSON::PP;
+			$json_pkg = "JSON::PP";
+		};
+
+		my $o = $json_pkg->new->utf8->decode($stdin);
+		die "Watchman: $o->{error}.\nFalling back to scanning...\n" if $o->{error};
+
+		local $, = "\0";
+		print @{$o->{files}};
+	'

Makefile, t/helper/test-drop-caches.c, t/perf/p7519-fsmonitor.sh

diff --git a/Makefile b/Makefile
index 992dd58801..893947839f 100644
--- a/Makefile
+++ b/Makefile
@@ -648,6 +648,7 @@ TEST_PROGRAMS_NEED_X += test-subprocess
 TEST_PROGRAMS_NEED_X += test-svn-fe
 TEST_PROGRAMS_NEED_X += test-urlmatch-normalization
 TEST_PROGRAMS_NEED_X += test-wildmatch
+TEST_PROGRAMS_NEED_X += test-drop-caches
 
 TEST_PROGRAMS = $(patsubst %,t/helper/%$X,$(TEST_PROGRAMS_NEED_X))
 
diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
new file mode 100644
index 0000000000..80830d920b
--- /dev/null
+++ b/t/helper/test-drop-caches.c
@@ -0,0 +1,107 @@
+#include "git-compat-util.h"
+#include <stdio.h>
+
+typedef DWORD NTSTATUS;
+
+#ifdef GIT_WINDOWS_NATIVE
+#include <tchar.h>
+
+#define STATUS_SUCCESS			(0x00000000L)
+#define STATUS_PRIVILEGE_NOT_HELD	(0xC0000061L)
+
+typedef enum _SYSTEM_INFORMATION_CLASS {
+	SystemMemoryListInformation = 80, // 80, q: SYSTEM_MEMORY_LIST_INFORMATION; s: SYSTEM_MEMORY_LIST_COMMAND (requires SeProfileSingleProcessPrivilege)
+} SYSTEM_INFORMATION_CLASS;
+
+// private
+typedef enum _SYSTEM_MEMORY_LIST_COMMAND
+{
+	MemoryCaptureAccessedBits,
+	MemoryCaptureAndResetAccessedBits,
+	MemoryEmptyWorkingSets,
+	MemoryFlushModifiedList,
+	MemoryPurgeStandbyList,
+	MemoryPurgeLowPriorityStandbyList,
+	MemoryCommandMax
+} SYSTEM_MEMORY_LIST_COMMAND;
+
+BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
+{
+	BOOL bResult;
+	DWORD dwBufferLength;
+	LUID luid;
+	TOKEN_PRIVILEGES tpPreviousState;
+	TOKEN_PRIVILEGES tpNewState;
+
+	dwBufferLength = 16;
+	bResult = LookupPrivilegeValueA(0, lpName, &luid);
+	if (bResult)
+	{
+		tpNewState.PrivilegeCount = 1;
+		tpNewState.Privileges[0].Luid = luid;
+		tpNewState.Privileges[0].Attributes = 0;
+		bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpNewState, (DWORD)((LPBYTE)&(tpNewState.Privileges[1]) - (LPBYTE)&tpNewState), &tpPreviousState, &dwBufferLength);
+		if (bResult)
+		{
+			tpPreviousState.PrivilegeCount = 1;
+			tpPreviousState.Privileges[0].Luid = luid;
+			tpPreviousState.Privileges[0].Attributes = flags != 0 ? 2 : 0;
+			bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpPreviousState, dwBufferLength, 0, 0);
+		}
+	}
+	return bResult;
+}
+#endif
+
+int cmd_main(int argc, const char **argv)
+{
+	NTSTATUS status = 1;
+#ifdef GIT_WINDOWS_NATIVE
+	HANDLE hProcess = GetCurrentProcess();
+	HANDLE hToken;
+	if (!OpenProcessToken(hProcess, TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &hToken))
+	{
+		_ftprintf(stderr, _T("Can't open current process token\n"));
+		return 1;
+	}
+
+	if (!GetPrivilege(hToken, "SeProfileSingleProcessPrivilege", 1))
+	{
+		_ftprintf(stderr, _T("Can't get SeProfileSingleProcessPrivilege\n"));
+		return 1;
+	}
+
+	CloseHandle(hToken);
+
+	HMODULE ntdll = LoadLibrary(_T("ntdll.dll"));
+	if (!ntdll)
+	{
+		_ftprintf(stderr, _T("Can't load ntdll.dll, wrong Windows version?\n"));
+		return 1;
+	}
+
+	NTSTATUS(WINAPI *NtSetSystemInformation)(INT, PVOID, ULONG) = (NTSTATUS(WINAPI *)(INT, PVOID, ULONG))GetProcAddress(ntdll, "NtSetSystemInformation");
+	if (!NtSetSystemInformation)
+	{
+		_ftprintf(stderr, _T("Can't get function addresses, wrong Windows version?\n"));
+		return 1;
+	}
+
+	SYSTEM_MEMORY_LIST_COMMAND command = MemoryPurgeStandbyList;
+	status = NtSetSystemInformation(
+		SystemMemoryListInformation,
+		&command,
+		sizeof(SYSTEM_MEMORY_LIST_COMMAND)
+	);
+	if (status == STATUS_PRIVILEGE_NOT_HELD)
+	{
+		_ftprintf(stderr, _T("Insufficient privileges to execute the memory list command"));
+	}
+	else if (status != STATUS_SUCCESS)
+	{
+		_ftprintf(stderr, _T("Unable to execute the memory list command %lX"), status);
+	}
+#endif
+
+	return status;
+}
diff --git a/t/perf/p7519-fsmonitor.sh b/t/perf/p7519-fsmonitor.sh
new file mode 100755
index 0000000000..e41905cb9b
--- /dev/null
+++ b/t/perf/p7519-fsmonitor.sh
@@ -0,0 +1,161 @@
+#!/bin/sh
+
+test_description="Test core.fsmonitor"
+
+. ./perf-lib.sh
+
+# This has to be run with GIT_PERF_REPEAT_COUNT=1 to generate valid results.
+# Otherwise the caching that happens for the nth run will negate the validity
+# of the comparisons.
+if [ "$GIT_PERF_REPEAT_COUNT" -ne 1 ]
+then
+	echo "warning: This test must be run with GIT_PERF_REPEAT_COUNT=1 to generate valid results." >&2
+	echo "warning: Setting GIT_PERF_REPEAT_COUNT=1" >&2
+	GIT_PERF_REPEAT_COUNT=1
+fi
+
+test_perf_large_repo
+test_checkout_worktree
+
+# Convert unix style paths to what Watchman expects
+case "$(uname -s)" in
+MINGW*|MSYS_NT*)
+  GIT_WORK_TREE="$(cygpath -aw "$PWD" | sed 's,\\,/,g')"
+  ;;
+*)
+  GIT_WORK_TREE="$PWD"
+  ;;
+esac
+
+# The big win for using fsmonitor is the elimination of the need to scan
+# the working directory looking for changed files and untracked files. If
+# the file information is all cached in RAM, the benefits are reduced.
+
+flush_disk_cache () {
+	case "$(uname -s)" in
+	MINGW*|MSYS_NT*)
+	  sync && test-drop-caches
+	  ;;
+	*)
+	  sudo sync && echo 3 | sudo tee /proc/sys/vm/drop_caches
+	  ;;
+	esac
+
+}
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+test_expect_success "setup" '
+	# Maybe set untrackedCache & splitIndex depending on the environment
+	if test -n "$GIT_PERF_7519_UNTRACKED_CACHE"
+	then
+		git config core.untrackedCache "$GIT_PERF_7519_UNTRACKED_CACHE"
+	else
+		if test_have_prereq UNTRACKED_CACHE
+		then
+			git config core.untrackedCache true
+		else
+			git config core.untrackedCache false
+		fi
+	fi &&
+
+	if test -n "$GIT_PERF_7519_SPLIT_INDEX"
+	then
+		git config core.splitIndex "$GIT_PERF_7519_SPLIT_INDEX"
+	fi &&
+
+	# Hook scaffolding
+	mkdir .git/hooks &&
+	cp ../../../templates/hooks--query-fsmonitor.sample .git/hooks/query-fsmonitor &&
+
+	# have Watchman monitor the test folder
+	watchman watch "$GIT_WORK_TREE" &&
+	watchman watch-list | grep -q -F "$GIT_WORK_TREE"
+'
+
+# Worst case without fsmonitor
+test_expect_success "clear fs cache" '
+	git config core.fsmonitor false &&
+	flush_disk_cache
+'
+test_perf "status (fsmonitor=false, cold fs cache)" '
+	git status
+'
+
+# Best case without fsmonitor
+test_perf "status (fsmonitor=false, warm fs cache)" '
+	git status
+'
+
+# Let's see if -uno & -uall make any difference
+test_expect_success "clear fs cache" '
+	flush_disk_cache
+'
+test_perf "status -uno (fsmonitor=false, cold fs cache)" '
+	git status -uno
+'
+
+test_expect_success "clear fs cache" '
+	flush_disk_cache
+'
+test_perf "status -uall (fsmonitor=false, cold fs cache)" '
+	git status -uall
+'
+
+# The first run with core.fsmonitor=true has to do a normal scan and write
+# out the index extension.
+test_expect_success "populate extension" '
+	# core.preloadIndex defeats the benefits of core.fsMonitor as it
+	# calls lstat for the index entries. Turn it off as _not_ doing
+	# the work is faster than doing the work across multiple threads.
+	git config core.fsmonitor true &&
+	git config core.preloadIndex false &&
+	git status
+'
+
+# Worst case with fsmonitor
+test_expect_success "shutdown fsmonitor, clear fs cache" '
+	watchman shutdown-server &&
+	flush_disk_cache
+'
+test_perf "status (fsmonitor=true, cold fs cache, cold fsmonitor)" '
+	git status
+'
+
+# Best case with fsmonitor
+test_perf "status (fsmonitor=true, warm fs cache, warm fsmonitor)" '
+	git status
+'
+
+# Best improved with fsmonitor (compare to worst case without fsmonitor)
+test_expect_success "clear fs cache" '
+	flush_disk_cache
+'
+test_perf "status (fsmonitor=true, cold fs cache, warm fsmonitor)" '
+	git status
+'
+
+# Let's see if -uno & -uall make any difference
+test_expect_success "clear fs cache" '
+	flush_disk_cache
+'
+test_perf "status -uno (fsmonitor=true, cold fs cache)" '
+	git status -uno
+'
+
+test_expect_success "clear fs cache" '
+	flush_disk_cache
+'
+test_perf "status -uall (fsmonitor=true, cold fs cache)" '
+	git status -uall
+'
+
+test_expect_success "cleanup" '
+	watchman watch-del "$GIT_WORK_TREE" &&
+	watchman shutdown-server
+'
+
+test_done

Documentation/config.txt, Documentation/githooks.txt, Documentation/technical/index-format.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dd4beec39d..d883e3318c 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -410,6 +410,13 @@ core.protectNTFS::
 	8.3 "short" names.
 	Defaults to `true` on Windows, and `false` elsewhere.
 
+core.fsmonitor::
+	If set to true, call the query-fsmonitor hook proc which will
+	identify all files that may have had changes since the last
+	request. This information is used to speed up operations like
+	'git commit' and 'git status' by limiting what git must scan to
+	detect changes.
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index b2514f4d44..219786b2da 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -456,6 +456,29 @@ non-zero status causes 'git send-email' to abort before sending any
 e-mails.
 
 
+[[query-fsmonitor]]
+query-fsmonitor
+~~~~~~~~~~~~~~~
+
+This hook is invoked when the configuration option core.fsmonitor is
+set and git needs to identify changed or untracked files.  It takes
+two arguments, a version (currently 1) and the time in elapsed
+nanoseconds since midnight, January 1, 1970.
+
+The hook should output to stdout the list of all files in the working
+directory that may have changed since the requested time.  The logic
+should be inclusive so that it does not miss any potential changes.
+The paths should be relative to the root of the working directory
+and be separated by a single NUL.
+
+Git will limit what files it checks for changes as well as which
+directories are checked for untracked files based on the path names
+given.
+
+The exit status determines whether git will use the data from the
+hook to limit its search.  On error, it will fall back to verifying
+all files and folders.
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index ade0b0c445..7aeeea6f94 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type:
     in the previous ewah bitmap.
 
   - One NUL.
+
+== File System Monitor cache
+
+  The file system monitor cache tracks files for which the query-fsmonitor
+  hook has told us about changes.  The signature for this extension is
+  { 'F', 'S', 'M', 'N' }.
+
+  The extension starts with
+
+  - 32-bit version number: the current supported version is 1.
+
+  - 64-bit time: the extension data reflects all changes through the given
+	time which is stored as the nanoseconds elapsed since midnight,
+	January 1, 1970.
+
+  - 32-bit bitmap size: the size of the CE_FSMONITOR_DIRTY bitmap.
+
+  - An ewah bitmap, the n-th bit indicates whether the n-th index entry
+    is CE_FSMONITOR_DIRTY.

diff --git a/compat/bswap.h b/compat/bswap.h
index 7d063e9e40..6b22c46214 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -158,7 +158,9 @@ static inline uint64_t git_bswap64(uint64_t x)
 
 #define get_be16(p)	ntohs(*(unsigned short *)(p))
 #define get_be32(p)	ntohl(*(unsigned int *)(p))
+#define get_be64(p)	ntohll(*(uint64_t *)(p))
 #define put_be32(p, v)	do { *(unsigned int *)(p) = htonl(v); } while (0)
+#define put_be64(p, v)	do { *(uint64_t *)(p) = htonll(v); } while (0)
 
 #else
 
@@ -178,6 +180,13 @@ static inline uint32_t get_be32(const void *ptr)
 		(uint32_t)p[3] <<  0;
 }
 
+static inline uint64_t get_be64(const void *ptr)
+{
+	const unsigned char *p = ptr;
+	return	(uint64_t)get_be32(p[0]) << 32 |
+		(uint64_t)get_be32(p[4]) <<  0;
+}
+
 static inline void put_be32(void *ptr, uint32_t value)
 {
 	unsigned char *p = ptr;
@@ -187,4 +196,17 @@ static inline void put_be32(void *ptr, uint32_t value)
 	p[3] = value >>  0;
 }
 
+static inline void put_be64(void *ptr, uint64_t value)
+{
+	unsigned char *p = ptr;
+	p[0] = value >> 56;
+	p[1] = value >> 48;
+	p[2] = value >> 40;
+	p[3] = value >> 32;
+	p[4] = value >> 24;
+	p[5] = value >> 16;
+	p[6] = value >>  8;
+	p[7] = value >>  0;
+}
+
 #endif

Makefile, apply.c, builtin/update-index.c, cache.h, config.c, config.h, diff-lib.c, dir.c, dir.h, entry.c, environment.c, fsmonitor.c, fsmonitor.h, preload-index.c, read-cache.c, submodule.c, unpack-trees.c

diff --git a/Makefile b/Makefile
index f2bb7f2f63..9d6ec9c1e9 100644
--- a/Makefile
+++ b/Makefile
@@ -786,6 +786,7 @@ LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec_cmd.o
 LIB_OBJS += fetch-pack.o
 LIB_OBJS += fsck.o
+LIB_OBJS += fsmonitor.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o
diff --git a/apply.c b/apply.c
index 71cbbd141c..9061cc5f15 100644
--- a/apply.c
+++ b/apply.c
@@ -3399,7 +3399,7 @@ static int verify_index_match(const struct cache_entry *ce, struct stat *st)
 			return -1;
 		return 0;
 	}
-	return ce_match_stat(ce, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+	return ce_match_stat(ce, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 }
 
 #define SUBMODULE_PATCH_WITHOUT_INDEX 1
diff --git a/builtin/update-index.c b/builtin/update-index.c
index e1ca0759d5..6f39ee9274 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -16,6 +16,7 @@
 #include "pathspec.h"
 #include "dir.h"
 #include "split-index.h"
+#include "fsmonitor.h"
 
 /*
  * Default to not allowing changes to the list of files. The
@@ -233,6 +234,7 @@ static int mark_ce_flags(const char *path, int flag, int mark)
 		else
 			active_cache[pos]->ce_flags &= ~flag;
 		active_cache[pos]->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(&the_index, active_cache[pos]);
 		cache_tree_invalidate_path(&the_index, path);
 		active_cache_changed |= CE_ENTRY_CHANGED;
 		return 0;
diff --git a/cache.h b/cache.h
index a916bc79e3..eccab968bd 100644
--- a/cache.h
+++ b/cache.h
@@ -203,6 +203,7 @@ struct cache_entry {
 #define CE_ADDED             (1 << 19)
 
 #define CE_HASHED            (1 << 20)
+#define CE_FSMONITOR_VALID   (1 << 21)
 #define CE_WT_REMOVE         (1 << 22) /* remove in work directory */
 #define CE_CONFLICTED        (1 << 23)
 
@@ -326,6 +327,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define CACHE_TREE_CHANGED	(1 << 5)
 #define SPLIT_INDEX_ORDERED	(1 << 6)
 #define UNTRACKED_CHANGED	(1 << 7)
+#define FSMONITOR_CHANGED	(1 << 8)
 
 struct split_index;
 struct untracked_cache;
@@ -344,6 +346,7 @@ struct index_state {
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
 	struct untracked_cache *untracked;
+	uint64_t fsmonitor_last_update;
 };
 
 extern struct index_state the_index;
@@ -679,8 +682,10 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_IGNORE_MISSING		0x08
 /* enable stat refresh */
 #define CE_MATCH_REFRESH		0x10
-extern int ie_match_stat(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
-extern int ie_modified(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+/* do stat comparison even if CE_FSMONITOR_VALID is true */
+#define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
 #define HASH_WRITE_OBJECT 1
 #define HASH_FORMAT_CHECK 2
@@ -773,6 +778,7 @@ extern int core_apply_sparse_checkout;
 extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
+extern const char *core_fsmonitor;
 
 /*
  * Include broken refs in all ref iterations, which will
diff --git a/config.c b/config.c
index d0d8ce823a..ddda96e584 100644
--- a/config.c
+++ b/config.c
@@ -2165,6 +2165,20 @@ int git_config_get_max_percent_split_change(void)
 	return -1; /* default value */
 }
 
+int git_config_get_fsmonitor(void)
+{
+	if (git_config_get_pathname("core.fsmonitor", &core_fsmonitor))
+		core_fsmonitor = getenv("GIT_FSMONITOR_TEST");
+
+	if (core_fsmonitor && !*core_fsmonitor)
+		core_fsmonitor = NULL;
+
+	if (core_fsmonitor)
+		return 1;
+
+	return 0;
+}
+
 NORETURN
 void git_die_config_linenr(const char *key, const char *filename, int linenr)
 {
diff --git a/config.h b/config.h
index 97471b8873..c9fcf691ba 100644
--- a/config.h
+++ b/config.h
@@ -211,6 +211,7 @@ extern int git_config_get_pathname(const char *key, const char **dest);
 extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
+extern int git_config_get_fsmonitor(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);
diff --git a/diff-lib.c b/diff-lib.c
index 2a52b07954..23c6d03ca9 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -12,6 +12,7 @@
 #include "refs.h"
 #include "submodule.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 /*
  * diff-files
@@ -228,6 +229,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 
 		if (!changed && !dirty_submodule) {
 			ce_mark_uptodate(ce);
+			mark_fsmonitor_valid(ce);
 			if (!DIFF_OPT_TST(&revs->diffopt, FIND_COPIES_HARDER))
 				continue;
 		}
diff --git a/dir.c b/dir.c
index 1c55dc3e36..ac9833daec 100644
--- a/dir.c
+++ b/dir.c
@@ -18,6 +18,7 @@
 #include "utf8.h"
 #include "varint.h"
 #include "ewah/ewok.h"
+#include "fsmonitor.h"
 
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
@@ -1688,17 +1689,23 @@ static int valid_cached_dir(struct dir_struct *dir,
 	if (!untracked)
 		return 0;
 
-	if (stat(path->len ? path->buf : ".", &st)) {
-		invalidate_directory(dir->untracked, untracked);
-		memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
-		return 0;
-	}
-	if (!untracked->valid ||
-	    match_stat_data_racy(istate, &untracked->stat_data, &st)) {
-		if (untracked->valid)
+	/*
+	 * With fsmonitor, we can trust the untracked cache's valid field.
+	 */
+	refresh_fsmonitor(istate);
+	if (!(dir->untracked->use_fsmonitor && untracked->valid)) {
+		if (stat(path->len ? path->buf : ".", &st)) {
 			invalidate_directory(dir->untracked, untracked);
-		fill_stat_data(&untracked->stat_data, &st);
-		return 0;
+			memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
+			return 0;
+		}
+		if (!untracked->valid ||
+			match_stat_data_racy(istate, &untracked->stat_data, &st)) {
+			if (untracked->valid)
+				invalidate_directory(dir->untracked, untracked);
+			fill_stat_data(&untracked->stat_data, &st);
+			return 0;
+		}
 	}
 
 	if (untracked->check_only != !!check_only) {
diff --git a/dir.h b/dir.h
index e3717055d1..fab8fc1561 100644
--- a/dir.h
+++ b/dir.h
@@ -139,6 +139,8 @@ struct untracked_cache {
 	int gitignore_invalidated;
 	int dir_invalidated;
 	int dir_opened;
+	/* fsmonitor invalidation data */
+	unsigned int use_fsmonitor : 1;
 };
 
 struct dir_struct {
diff --git a/entry.c b/entry.c
index cb291aa88b..5e6794f9fc 100644
--- a/entry.c
+++ b/entry.c
@@ -4,6 +4,7 @@
 #include "streaming.h"
 #include "submodule.h"
 #include "progress.h"
+#include "fsmonitor.h"
 
 static void create_directories(const char *path, int path_len,
 			       const struct checkout *state)
@@ -357,6 +358,7 @@ static int write_entry(struct cache_entry *ce,
 			lstat(ce->name, &st);
 		fill_stat_cache_info(ce, &st);
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(state->istate, ce);
 		state->istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 	return 0;
@@ -402,7 +404,7 @@ int checkout_entry(struct cache_entry *ce,
 
 	if (!check_path(path.buf, path.len, &st, state->base_dir_len)) {
 		const struct submodule *sub;
-		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 		/*
 		 * Needs to be checked before !changed returns early,
 		 * as the possibly empty directory was not changed
diff --git a/environment.c b/environment.c
index 3fd4b10845..d0b9fc64d4 100644
--- a/environment.c
+++ b/environment.c
@@ -76,6 +76,7 @@ int protect_hfs = PROTECT_HFS_DEFAULT;
 #define PROTECT_NTFS_DEFAULT 0
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
+const char *core_fsmonitor;
 
 /*
  * The character that begins a commented line in user-editable file
diff --git a/fsmonitor.c b/fsmonitor.c
new file mode 100644
index 0000000000..144294b8df
--- /dev/null
+++ b/fsmonitor.c
@@ -0,0 +1,253 @@
+#include "cache.h"
+#include "config.h"
+#include "dir.h"
+#include "ewah/ewok.h"
+#include "fsmonitor.h"
+#include "run-command.h"
+#include "strbuf.h"
+
+#define INDEX_EXTENSION_VERSION	(1)
+#define HOOK_INTERFACE_VERSION	(1)
+
+struct trace_key trace_fsmonitor = TRACE_KEY_INIT(FSMONITOR);
+
+static void fsmonitor_ewah_callback(size_t pos, void *is)
+{
+	struct index_state *istate = (struct index_state *)is;
+	struct cache_entry *ce = istate->cache[pos];
+
+	ce->ce_flags &= ~CE_FSMONITOR_VALID;
+}
+
+int read_fsmonitor_extension(struct index_state *istate, const void *data,
+	unsigned long sz)
+{
+	const char *index = data;
+	uint32_t hdr_version;
+	uint32_t ewah_size;
+	struct ewah_bitmap *fsmonitor_dirty;
+	int i;
+	int ret;
+
+	if (sz < sizeof(uint32_t) + sizeof(uint64_t) + sizeof(uint32_t))
+		return error("corrupt fsmonitor extension (too short)");
+
+	hdr_version = get_be32(index);
+	index += sizeof(uint32_t);
+	if (hdr_version != INDEX_EXTENSION_VERSION)
+		return error("bad fsmonitor version %d", hdr_version);
+
+	istate->fsmonitor_last_update = get_be64(index);
+	index += sizeof(uint64_t);
+
+	ewah_size = get_be32(index);
+	index += sizeof(uint32_t);
+
+	fsmonitor_dirty = ewah_new();
+	ret = ewah_read_mmap(fsmonitor_dirty, index, ewah_size);
+	if (ret != ewah_size) {
+		ewah_free(fsmonitor_dirty);
+		return error("failed to parse ewah bitmap reading fsmonitor index extension");
+	}
+
+	if (git_config_get_fsmonitor()) {
+		/* Mark all entries valid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags |= CE_FSMONITOR_VALID;
+
+		/* Mark all previously saved entries as dirty */
+		ewah_each_bit(fsmonitor_dirty, fsmonitor_ewah_callback, istate);
+		ewah_free(fsmonitor_dirty);
+
+		/* Now mark the untracked cache for fsmonitor usage */
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 1;
+	}
+
+	trace_printf_key(&trace_fsmonitor, "read fsmonitor extension successful");
+	return 0;
+}
+
+void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
+{
+	uint32_t hdr_version;
+	uint64_t tm;
+	struct ewah_bitmap *bitmap;
+	int i;
+	uint32_t ewah_start;
+	uint32_t ewah_size = 0;
+	int fixup = 0;
+
+	put_be32(&hdr_version, INDEX_EXTENSION_VERSION);
+	strbuf_add(sb, &hdr_version, sizeof(uint32_t));
+
+	put_be64(&tm, istate->fsmonitor_last_update);
+	strbuf_add(sb, &tm, sizeof(uint64_t));
+	fixup = sb->len;
+	strbuf_add(sb, &ewah_size, sizeof(uint32_t)); /* we'll fix this up later */
+
+	ewah_start = sb->len;
+	bitmap = ewah_new();
+	for (i = 0; i < istate->cache_nr; i++)
+		if (!(istate->cache[i]->ce_flags & CE_FSMONITOR_VALID))
+			ewah_set(bitmap, i);
+	ewah_serialize_strbuf(bitmap, sb);
+	ewah_free(bitmap);
+
+	/* fix up size field */
+	put_be32(&ewah_size, sb->len - ewah_start);
+	memcpy(sb->buf + fixup, &ewah_size, sizeof(uint32_t));
+
+	trace_printf_key(&trace_fsmonitor, "write fsmonitor extension successful");
+}
+
+/*
+ * Call the query-fsmonitor hook passing the time of the last saved results.
+ */
+static int query_fsmonitor(int version, uint64_t last_update, struct strbuf *query_result)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	char ver[64];
+	char date[64];
+	const char *argv[4];
+
+	if (!(argv[0] = core_fsmonitor))
+		return -1;
+
+	snprintf(ver, sizeof(version), "%d", version);
+	snprintf(date, sizeof(date), "%" PRIuMAX, (uintmax_t)last_update);
+	argv[1] = ver;
+	argv[2] = date;
+	argv[3] = NULL;
+	cp.argv = argv;
+	cp.use_shell = 1;
+
+	return capture_command(&cp, query_result, 1024);
+}
+
+static void fsmonitor_refresh_callback(struct index_state *istate, const char *name)
+{
+	int pos = index_name_pos(istate, name, strlen(name));
+
+	if (pos >= 0) {
+		struct cache_entry *ce = istate->cache[pos];
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+	}
+
+	/*
+	 * Mark the untracked cache dirty even if it wasn't found in the index
+	 * as it could be a new untracked file.
+	 */
+	trace_printf_key(&trace_fsmonitor, "fsmonitor_refresh_callback '%s'", name);
+	untracked_cache_invalidate_path(istate, name);
+}
+
+void refresh_fsmonitor(struct index_state *istate)
+{
+	static int has_run_once = 0;
+	struct strbuf query_result = STRBUF_INIT;
+	int query_success = 0;
+	size_t bol; /* beginning of line */
+	uint64_t last_update;
+	char *buf;
+	int i;
+
+	if (!core_fsmonitor || has_run_once)
+		return;
+	has_run_once = 1;
+
+	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
+	/*
+	 * This could be racy so save the date/time now and query_fsmonitor
+	 * should be inclusive to ensure we don't miss potential changes.
+	 */
+	last_update = getnanotime();
+
+	/*
+	 * If we have a last update time, call query_fsmonitor for the set of
+	 * changes since that time, else assume everything is possibly dirty
+	 * and check it all.
+	 */
+	if (istate->fsmonitor_last_update) {
+		query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION,
+			istate->fsmonitor_last_update, &query_result);
+		trace_performance_since(last_update, "fsmonitor process '%s'", core_fsmonitor);
+		trace_printf_key(&trace_fsmonitor, "fsmonitor process '%s' returned %s",
+			core_fsmonitor, query_success ? "success" : "failure");
+	}
+
+	/* a fsmonitor process can return '*' to indicate all entries are invalid */
+	if (query_success && query_result.buf[0] != '*') {
+		/* Mark all entries returned by the monitor as dirty */
+		buf = query_result.buf;
+		bol = 0;
+		for (i = 0; i < query_result.len; i++) {
+			if (buf[i] != '\0')
+				continue;
+			fsmonitor_refresh_callback(istate, buf + bol);
+			bol = i + 1;
+		}
+		if (bol < query_result.len)
+			fsmonitor_refresh_callback(istate, buf + bol);
+	} else {
+		/* Mark all entries invalid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 0;
+	}
+	strbuf_release(&query_result);
+
+	/* Now that we've updated istate, save the last_update time */
+	istate->fsmonitor_last_update = last_update;
+}
+
+void add_fsmonitor(struct index_state *istate)
+{
+	int i;
+
+	if (!istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "add fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = getnanotime();
+
+		/* reset the fsmonitor state */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		/* reset the untracked cache */
+		if (istate->untracked) {
+			add_untracked_cache(istate);
+			istate->untracked->use_fsmonitor = 1;
+		}
+
+		/* Update the fsmonitor state */
+		refresh_fsmonitor(istate);
+	}
+}
+
+void remove_fsmonitor(struct index_state *istate)
+{
+	if (istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "remove fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = 0;
+	}
+}
+
+void tweak_fsmonitor(struct index_state *istate)
+{
+	switch (git_config_get_fsmonitor()) {
+	case -1: /* keep: do nothing */
+		break;
+	case 0: /* false */
+		remove_fsmonitor(istate);
+		break;
+	case 1: /* true */
+		add_fsmonitor(istate);
+		break;
+	default: /* unknown value: do nothing */
+		break;
+	}
+}
diff --git a/fsmonitor.h b/fsmonitor.h
new file mode 100644
index 0000000000..dadbe90283
--- /dev/null
+++ b/fsmonitor.h
@@ -0,0 +1,61 @@
+#ifndef FSMONITOR_H
+#define FSMONITOR_H
+
+extern struct trace_key trace_fsmonitor;
+
+/*
+ * Read the the fsmonitor index extension and (if configured) restore the
+ * CE_FSMONITOR_VALID state.
+ */
+extern int read_fsmonitor_extension(struct index_state *istate, const void *data, unsigned long sz);
+
+/*
+ * Write the CE_FSMONITOR_VALID state into the fsmonitor index extension.
+ */
+extern void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension
+ */
+extern void add_fsmonitor(struct index_state *istate);
+extern void remove_fsmonitor(struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension as necessary based on the current
+ * core.fsmonitor setting.
+ */
+extern void tweak_fsmonitor(struct index_state *istate);
+
+/*
+ * Run the configured fsmonitor integration script and clear the
+ * CE_FSMONITOR_VALID bit for any files returned as dirty.  Also invalidate
+ * any corresponding untracked cache directory structures. Optimized to only
+ * run the first time it is called.
+ */
+extern void refresh_fsmonitor(struct index_state *istate);
+
+/*
+ * Set the given cache entries CE_FSMONITOR_VALID bit.
+ */
+static inline void mark_fsmonitor_valid(struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags |= CE_FSMONITOR_VALID;
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_clean '%s'", ce->name);
+	}
+}
+
+/*
+ * Clear the given cache entries CE_FSMONITOR_VALID bit and invalidate any
+ * corresponding untracked cache directory structures.
+ */
+static inline void mark_fsmonitor_invalid(struct index_state *istate, struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+		untracked_cache_invalidate_path(istate, ce->name);
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_invalid '%s'", ce->name);
+	}
+}
+
+#endif
diff --git a/preload-index.c b/preload-index.c
index 75564c497a..2a83255e4e 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -4,6 +4,7 @@
 #include "cache.h"
 #include "pathspec.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 #ifdef NO_PTHREADS
 static void preload_index(struct index_state *index,
@@ -55,15 +56,18 @@ static void *preload_thread(void *_data)
 			continue;
 		if (ce_skip_worktree(ce))
 			continue;
+		if (ce->ce_flags & CE_FSMONITOR_VALID)
+			continue;
 		if (!ce_path_match(ce, &p->pathspec, NULL))
 			continue;
 		if (threaded_has_symlink_leading_path(&cache, ce->name, ce_namelen(ce)))
 			continue;
 		if (lstat(ce->name, &st))
 			continue;
-		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY))
+		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR))
 			continue;
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
 	} while (--nr > 0);
 	cache_def_clear(&cache);
 	return NULL;
diff --git a/read-cache.c b/read-cache.c
index 40da87ea71..53093dbebf 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -19,6 +19,7 @@
 #include "varint.h"
 #include "split-index.h"
 #include "utf8.h"
+#include "fsmonitor.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -38,11 +39,12 @@
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
 #define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
+#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -62,6 +64,7 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 	free(old);
 	set_index_entry(istate, nr, ce);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 }
 
@@ -150,8 +153,10 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 	if (assume_unchanged)
 		ce->ce_flags |= CE_VALID;
 
-	if (S_ISREG(st->st_mode))
+	if (S_ISREG(st->st_mode)) {
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
+	}
 }
 
 static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
@@ -300,7 +305,7 @@ int match_stat_data_racy(const struct index_state *istate,
 	return match_stat_data(sd, st);
 }
 
-int ie_match_stat(const struct index_state *istate,
+int ie_match_stat(struct index_state *istate,
 		  const struct cache_entry *ce, struct stat *st,
 		  unsigned int options)
 {
@@ -308,7 +313,10 @@ int ie_match_stat(const struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * If it's marked as always valid in the index, it's
 	 * valid whatever the checked-out copy says.
@@ -319,6 +327,8 @@ int ie_match_stat(const struct index_state *istate,
 		return 0;
 	if (!ignore_valid && (ce->ce_flags & CE_VALID))
 		return 0;
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
+		return 0;
 
 	/*
 	 * Intent-to-add entries have not been added, so the index entry
@@ -356,7 +366,7 @@ int ie_match_stat(const struct index_state *istate,
 	return changed;
 }
 
-int ie_modified(const struct index_state *istate,
+int ie_modified(struct index_state *istate,
 		const struct cache_entry *ce,
 		struct stat *st, unsigned int options)
 {
@@ -631,7 +641,7 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
 	int size, namelen, was_same;
 	mode_t st_mode = st->st_mode;
 	struct cache_entry *ce, *alias;
-	unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY;
+	unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR;
 	int verbose = flags & (ADD_CACHE_VERBOSE | ADD_CACHE_PRETEND);
 	int pretend = flags & ADD_CACHE_PRETEND;
 	int intent_only = flags & ADD_CACHE_INTENT;
@@ -777,6 +787,7 @@ int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
 	}
 	cache_tree_invalidate_path(istate, ce->name);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 
 	return 0;
@@ -1228,10 +1239,13 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
 	if (!refresh || ce_uptodate(ce))
 		return ce;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * CE_VALID or CE_SKIP_WORKTREE means the user promised us
 	 * that the change to the work tree does not matter and told
@@ -1245,6 +1259,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 		ce_mark_uptodate(ce);
 		return ce;
 	}
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
+		ce_mark_uptodate(ce);
+		return ce;
+	}
 
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
 		if (ignore_missing)
@@ -1282,8 +1300,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 			 * because CE_UPTODATE flag is in-core only;
 			 * we are not going to write this change out.
 			 */
-			if (!S_ISGITLINK(ce->ce_mode))
+			if (!S_ISGITLINK(ce->ce_mode)) {
 				ce_mark_uptodate(ce);
+				mark_fsmonitor_valid(ce);
+			}
 			return ce;
 		}
 	}
@@ -1336,7 +1356,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 	int first = 1;
 	int in_porcelain = (flags & REFRESH_IN_PORCELAIN);
 	unsigned int options = (CE_MATCH_REFRESH |
-				(really ? CE_MATCH_IGNORE_VALID : 0) |
+				(really ? CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_FSMONITOR : 0) |
 				(not_new ? CE_MATCH_IGNORE_MISSING : 0));
 	const char *modified_fmt;
 	const char *deleted_fmt;
@@ -1391,6 +1411,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 				 */
 				ce->ce_flags &= ~CE_VALID;
 				ce->ce_flags |= CE_UPDATE_IN_BASE;
+				mark_fsmonitor_invalid(istate, ce);
 				istate->cache_changed |= CE_ENTRY_CHANGED;
 			}
 			if (quiet)
@@ -1550,6 +1571,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_UNTRACKED:
 		istate->untracked = read_untracked_extension(data, sz);
 		break;
+	case CACHE_EXT_FSMONITOR:
+		read_fsmonitor_extension(istate, data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1722,6 +1746,7 @@ static void post_read_index_from(struct index_state *istate)
 	check_ce_order(istate);
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
+	tweak_fsmonitor(istate);
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2306,6 +2331,16 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->fsmonitor_last_update) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_fsmonitor_extension(&sb, istate);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1))
 		return -1;
diff --git a/submodule.c b/submodule.c
index 3cea8221e0..8a931a1aaa 100644
--- a/submodule.c
+++ b/submodule.c
@@ -62,7 +62,7 @@ int is_staging_gitmodules_ok(const struct index_state *istate)
 	if ((pos >= 0) && (pos < istate->cache_nr)) {
 		struct stat st;
 		if (lstat(GITMODULES_FILE, &st) == 0 &&
-		    ce_match_stat(istate->cache[pos], &st, 0) & DATA_CHANGED)
+		    ce_match_stat(istate->cache[pos], &st, CE_MATCH_IGNORE_FSMONITOR) & DATA_CHANGED)
 			return 0;
 	}
 
diff --git a/unpack-trees.c b/unpack-trees.c
index 71b70ccb12..f724a61ac0 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -14,6 +14,7 @@
 #include "dir.h"
 #include "submodule.h"
 #include "submodule-config.h"
+#include "fsmonitor.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -408,6 +409,7 @@ static int apply_sparse_checkout(struct index_state *istate,
 		ce->ce_flags &= ~CE_SKIP_WORKTREE;
 	if (was_skip_worktree != ce_skip_worktree(ce)) {
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(istate, ce);
 		istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 
@@ -1454,7 +1456,7 @@ static int verify_uptodate_1(const struct cache_entry *ce,
 		return 0;
 
 	if (!lstat(ce->name, &st)) {
-		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
+		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR;
 		unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);
 
 		if (submodule_from_ce(ce)) {
@@ -1610,7 +1612,7 @@ static int icase_exists(struct unpack_trees_options *o, const char *name, int le
 	const struct cache_entry *src;
 
 	src = index_file_exists(o->src_index, name, len, 1);
-	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 }
 
 static int check_ok_to_remove(const char *name, int len, int dtype,
@@ -2134,7 +2136,7 @@ int oneway_merge(const struct cache_entry * const *src,
 		if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old)) {
 			struct stat st;
 			if (lstat(old->name, &st) ||
-			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
+			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR))
 				update |= CE_UPDATE;
 		}
 		add_entry(o, old, update, 0);

Documentation/config.txt, Documentation/githooks.txt, Documentation/technical/index-format.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dc4e3f58a2..c196007a27 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -413,6 +413,12 @@ core.protectNTFS::
 	8.3 "short" names.
 	Defaults to `true` on Windows, and `false` elsewhere.
 
+core.fsmonitor::
+	If set, the value of this variable is used as a command which
+	will identify all files that may have changed since the
+	requested date/time. This information is used to speed up git by
+	avoiding unnecessary processing of files that have not changed.
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 1bb4f92d4d..da82d64b0b 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -456,6 +456,29 @@ non-zero status causes 'git send-email' to abort before sending any
 e-mails.
 
 
+[[fsmonitor-watchman]]
+fsmonitor-watchman
+~~~~~~~~~~~~~~~
+
+This hook is invoked when the configuration option core.fsmonitor is
+set to .git/hooks/fsmonitor-watchman.  It takes two arguments, a version
+(currently 1) and the time in elapsed nanoseconds since midnight,
+January 1, 1970.
+
+The hook should output to stdout the list of all files in the working
+directory that may have changed since the requested time.  The logic
+should be inclusive so that it does not miss any potential changes.
+The paths should be relative to the root of the working directory
+and be separated by a single NUL.
+
+Git will limit what files it checks for changes as well as which
+directories are checked for untracked files based on the path names
+given.
+
+The exit status determines whether git will use the data from the
+hook to limit its search.  On error, it will fall back to verifying
+all files and folders.
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index ade0b0c445..db3572626b 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type:
     in the previous ewah bitmap.
 
   - One NUL.
+
+== File System Monitor cache
+
+  The file system monitor cache tracks files for which the core.fsmonitor
+  hook has told us about changes.  The signature for this extension is
+  { 'F', 'S', 'M', 'N' }.
+
+  The extension starts with
+
+  - 32-bit version number: the current supported version is 1.
+
+  - 64-bit time: the extension data reflects all changes through the given
+	time which is stored as the nanoseconds elapsed since midnight,
+	January 1, 1970.
+
+  - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
+
+  - An ewah bitmap, the n-th bit indicates whether the n-th index entry
+    is not CE_FSMONITOR_VALID.

diff --git a/builtin/update-index.c b/builtin/update-index.c
index 6f39ee9274..b03afc1f3a 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -918,6 +918,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	int lock_error = 0;
 	int split_index = -1;
 	int force_write = 0;
+	int fsmonitor = -1;
 	struct lock_file *lock_file;
 	struct parse_opt_ctx_t ctx;
 	strbuf_getline_fn getline_fn;
@@ -1011,6 +1012,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			    N_("enable untracked cache without testing the filesystem"), UC_FORCE),
 		OPT_SET_INT(0, "force-write-index", &force_write,
 			N_("write out the index even if is not flagged as changed"), 1),
+		OPT_BOOL(0, "fsmonitor", &fsmonitor,
+			N_("enable or disable file system monitor")),
 		OPT_END()
 	};
 
@@ -1152,6 +1155,22 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		die("BUG: bad untracked_cache value: %d", untracked_cache);
 	}
 
+	if (fsmonitor > 0) {
+		if (git_config_get_fsmonitor() == 0)
+			warning(_("core.fsmonitor is unset; "
+				"set it if you really want to "
+				"enable fsmonitor"));
+		add_fsmonitor(&the_index);
+		report(_("fsmonitor enabled"));
+	} else if (!fsmonitor) {
+		if (git_config_get_fsmonitor() == 1)
+			warning(_("core.fsmonitor is set; "
+				"remove it if you really want to "
+				"disable fsmonitor"));
+		remove_fsmonitor(&the_index);
+		report(_("fsmonitor disabled"));
+	}
+
 	if (active_cache_changed || force_write) {
 		if (newfd < 0) {
 			if (refresh_args.flags & REFRESH_QUIET)

diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index e1339e6d17..313962a0c1 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -31,6 +31,7 @@ static int show_resolve_undo;
 static int show_modified;
 static int show_killed;
 static int show_valid_bit;
+static int show_fsmonitor_bit;
 static int line_terminator = '\n';
 static int debug_mode;
 static int show_eol;
@@ -86,7 +87,8 @@ static const char *get_tag(const struct cache_entry *ce, const char *tag)
 {
 	static char alttag[4];
 
-	if (tag && *tag && show_valid_bit && (ce->ce_flags & CE_VALID)) {
+	if (tag && *tag && ((show_valid_bit && (ce->ce_flags & CE_VALID)) ||
+		(show_fsmonitor_bit && (ce->ce_flags & CE_FSMONITOR_VALID)))) {
 		memcpy(alttag, tag, 3);
 
 		if (isalpha(tag[0])) {
@@ -515,6 +517,8 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 			N_("identify the file status with tags")),
 		OPT_BOOL('v', NULL, &show_valid_bit,
 			N_("use lowercase letters for 'assume unchanged' files")),
+		OPT_BOOL('f', NULL, &show_fsmonitor_bit,
+			N_("use lowercase letters for 'fsmonitor clean' files")),
 		OPT_BOOL('c', "cached", &show_cached,
 			N_("show cached files in the output (default)")),
 		OPT_BOOL('d', "deleted", &show_deleted,
@@ -584,7 +588,7 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 	for (i = 0; i < exclude_list.nr; i++) {
 		add_exclude(exclude_list.items[i].string, "", 0, el, --exclude_args);
 	}
-	if (show_tag || show_valid_bit) {
+	if (show_tag || show_valid_bit || show_fsmonitor_bit) {
 		tag_cached = "H ";
 		tag_unmerged = "M ";
 		tag_removed = "R ";

t/t7519-status-fsmonitor.sh, t/t7519/fsmonitor-all, t/t7519/fsmonitor-none, t/t7519/fsmonitor-watchman

diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
new file mode 100755
index 0000000000..6aa1e4e924
--- /dev/null
+++ b/t/t7519-status-fsmonitor.sh
@@ -0,0 +1,259 @@
+#!/bin/sh
+
+test_description='git status with file system watcher'
+
+. ./test-lib.sh
+
+#
+# To run the entire git test suite using fsmonitor:
+#
+# copy t/t7519/fsmonitor-all to a location in your path and then set
+# GIT_FSMONITOR_TEST=fsmonitor-all and run your tests.
+#
+
+# Note, after "git reset --hard HEAD" no extensions exist other than 'TREE'
+# "git update-index --fsmonitor" can be used to get the extension written
+# before testing the results.
+
+clean_repo () {
+	git reset --hard HEAD &&
+	git clean -fd
+}
+
+dirty_repo () {
+	: >untracked &&
+	: >dir1/untracked &&
+	: >dir2/untracked &&
+	echo 1 >modified &&
+	echo 2 >dir1/modified &&
+	echo 3 >dir2/modified &&
+	echo 4 >new &&
+	echo 5 >dir1/new &&
+	echo 6 >dir2/new
+}
+
+write_integration_script() {
+	write_script .git/hooks/fsmonitor-test<<-\EOF
+	if [ "$#" -ne 2 ]; then
+		echo "$0: exactly 2 arguments expected"
+		exit 2
+	fi
+	if [ "$1" != 1 ]; then
+		echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+		exit 1
+	fi
+	printf "untracked\0"
+	printf "dir1/untracked\0"
+	printf "dir2/untracked\0"
+	printf "modified\0"
+	printf "dir1/modified\0"
+	printf "dir2/modified\0"
+	printf "new\0"
+	printf "dir1/new\0"
+	printf "dir2/new\0"
+	EOF
+}
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+test_expect_success 'setup' '
+	mkdir -p .git/hooks &&
+	: >tracked &&
+	: >modified &&
+	mkdir dir1 &&
+	: >dir1/tracked &&
+	: >dir1/modified &&
+	mkdir dir2 &&
+	: >dir2/tracked &&
+	: >dir2/modified &&
+	git -c core.fsmonitor= add . &&
+	git -c core.fsmonitor= commit -m initial &&
+	git config core.fsmonitor .git/hooks/fsmonitor-test &&
+	cat >.gitignore <<-\EOF
+	.gitignore
+	expect*
+	actual*
+	marker*
+	EOF
+'
+
+# test that the fsmonitor extension is off by default
+test_expect_success 'fsmonitor extension is off by default' '
+	test-dump-fsmonitor >actual &&
+	grep "^no fsmonitor" actual
+'
+
+# test that "update-index --fsmonitor" adds the fsmonitor extension
+test_expect_success 'update-index --fsmonitor" adds the fsmonitor extension' '
+	git update-index --fsmonitor &&
+	test-dump-fsmonitor >actual &&
+	grep "^fsmonitor last update" actual
+'
+
+# test that "update-index --no-fsmonitor" removes the fsmonitor extension
+test_expect_success 'update-index --no-fsmonitor" removes the fsmonitor extension' '
+	git update-index --no-fsmonitor &&
+	test-dump-fsmonitor >actual &&
+	grep "^no fsmonitor" actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+H dir1/tracked
+H dir2/modified
+H dir2/tracked
+H modified
+H tracked
+EOF
+
+# test that all files returned by the script get flagged as invalid
+test_expect_success 'all files returned by integration script get flagged as invalid' '
+	write_integration_script &&
+	dirty_repo &&
+	git update-index --fsmonitor &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/new
+H dir1/tracked
+H dir2/modified
+h dir2/new
+H dir2/tracked
+H modified
+h new
+H tracked
+EOF
+
+# test that newly added files are marked valid
+test_expect_success 'newly added files are marked valid' '
+	git add new &&
+	git add dir1/new &&
+	git add dir2/new &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/new
+h dir1/tracked
+H dir2/modified
+h dir2/new
+h dir2/tracked
+H modified
+h new
+h tracked
+EOF
+
+# test that all unmodified files get marked valid
+test_expect_success 'all unmodified files get marked valid' '
+	# modified files result in update-index returning 1
+	test_must_fail git update-index --refresh --force-write-index &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+h dir1/tracked
+h dir2/modified
+h dir2/tracked
+h modified
+h tracked
+EOF
+
+# test that *only* files returned by the integration script get flagged as invalid
+test_expect_success '*only* files returned by the integration script get flagged as invalid' '
+	write_script .git/hooks/fsmonitor-test<<-\EOF &&
+	printf "dir1/modified\0"
+	EOF
+	clean_repo &&
+	git update-index --refresh --force-write-index &&
+	echo 1 >modified &&
+	echo 2 >dir1/modified &&
+	echo 3 >dir2/modified &&
+	test_must_fail git update-index --refresh --force-write-index &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+# Ensure commands that call refresh_index() to move the index back in time
+# properly invalidate the fsmonitor cache
+test_expect_success 'refresh_index() invalidates fsmonitor cache' '
+	write_script .git/hooks/fsmonitor-test<<-\EOF &&
+	EOF
+	clean_repo &&
+	dirty_repo &&
+	git add . &&
+	git commit -m "to reset" &&
+	git reset HEAD~1 &&
+	git status >actual &&
+	git -c core.fsmonitor= status >expect &&
+	test_i18ncmp expect actual
+'
+
+# test fsmonitor with and without preloadIndex
+preload_values="false true"
+for preload_val in $preload_values
+do
+	test_expect_success "setup preloadIndex to $preload_val" '
+		git config core.preloadIndex $preload_val &&
+		if [ $preload_val -eq true ]
+		then
+			GIT_FORCE_PRELOAD_TEST=$preload_val; export GIT_FORCE_PRELOAD_TEST
+		else
+			unset GIT_FORCE_PRELOAD_TEST
+		fi
+	'
+
+	# test fsmonitor with and without the untracked cache (if available)
+	uc_values="false"
+	test_have_prereq UNTRACKED_CACHE && uc_values="false true"
+	for uc_val in $uc_values
+	do
+		test_expect_success "setup untracked cache to $uc_val" '
+			git config core.untrackedcache $uc_val
+		'
+
+		# Status is well tested elsewhere so we'll just ensure that the results are
+		# the same when using core.fsmonitor.
+		test_expect_success 'compare status with and without fsmonitor' '
+			write_integration_script &&
+			clean_repo &&
+			dirty_repo &&
+			git add new &&
+			git add dir1/new &&
+			git add dir2/new &&
+			git status >actual &&
+			git -c core.fsmonitor= status >expect &&
+			test_i18ncmp expect actual
+		'
+
+		# Make sure it's actually skipping the check for modified and untracked
+		# (if enabled) files unless it is told about them.
+		test_expect_success "status doesn't detect unreported modifications" '
+			write_script .git/hooks/fsmonitor-test<<-\EOF &&
+			:>marker
+			EOF
+			clean_repo &&
+			git status &&
+			test_path_is_file marker &&
+			dirty_repo &&
+			rm -f marker &&
+			git status >actual &&
+			test_path_is_file marker &&
+			test_i18ngrep ! "Changes not staged for commit:" actual &&
+			if [ $uc_val -eq true ]; then test_i18ngrep ! "Untracked files:" actual; fi &&
+			if [ $uc_val -eq false ]; then test_i18ngrep "Untracked files:" actual; fi &&
+			rm -f marker
+		'
+	done
+done
+
+test_done
diff --git a/t/t7519/fsmonitor-all b/t/t7519/fsmonitor-all
new file mode 100755
index 0000000000..a3870e431e
--- /dev/null
+++ b/t/t7519/fsmonitor-all
@@ -0,0 +1,23 @@
+#!/bin/sh
+#
+# An test hook script to integrate with git to test fsmonitor.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+#echo "$0 $*" >&2
+
+if [ "$#" -ne 2 ] ; then
+	echo -e "$0: exactly 2 arguments expected\n" >&2
+	exit 2
+fi
+
+if [ "$1" != 1 ]
+then
+	echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+	exit 1
+fi
+
+echo "*"
\ No newline at end of file
diff --git a/t/t7519/fsmonitor-none b/t/t7519/fsmonitor-none
new file mode 100755
index 0000000000..c500bb0f26
--- /dev/null
+++ b/t/t7519/fsmonitor-none
@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+# An test hook script to integrate with git to test fsmonitor.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+#echo "$0 $*" >&2
+
+if [ "$#" -ne 2 ] ; then
+	echo -e "$0: exactly 2 arguments expected\n" >&2
+	exit 2
+fi
+
+if [ "$1" != 1 ]
+then
+	echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+	exit 1
+fi
diff --git a/t/t7519/fsmonitor-watchman b/t/t7519/fsmonitor-watchman
new file mode 100755
index 0000000000..aaee5d1fe3
--- /dev/null
+++ b/t/t7519/fsmonitor-watchman
@@ -0,0 +1,128 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use IPC::Open2;
+
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to speed up detecting
+# new and modified files.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+# To enable this hook, rename this file to "query-watchman" and set
+# 'git config core.fsmonitor .git/hooks/query-watchman'
+#
+my ($version, $time) = @ARGV;
+print STDERR "$0 $version $time\n";
+
+# Check the hook interface version
+
+if ($version == 1) {
+	# convert nanoseconds to seconds
+	$time = int $time / 1000000000;
+} else {
+	die "Unsupported query-fsmonitor hook version '$version'.\n" .
+	    "Falling back to scanning...\n";
+}
+
+my $git_work_tree = $ENV{'PWD'};
+
+my $retry = 1;
+
+launch_watchman();
+
+sub launch_watchman {
+
+	# Set input record separator
+	local $/ = 0666;
+
+	my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j')
+	    or die "open2() failed: $!\n" .
+	    "Falling back to scanning...\n";
+
+	# In the query expression below we're asking for names of files that
+	# changed since $time but were not transient (ie created after
+	# $time but no longer exist).
+	#
+	# To accomplish this, we're using the "since" generator to use the
+	# recency index to select candidate nodes and "fields" to limit the
+	# output to file names only. Then we're using the "expression" term to
+	# further constrain the results.
+	#
+	# The category of transient files that we want to ignore will have a
+	# creation clock (cclock) newer than $time_t value and will also not
+	# currently exist.
+
+	open (my $fh, ">", ".git/watchman-query.json");
+	print $fh "[\"query\", \"$git_work_tree\", { \
+	\"since\": $time, \
+	\"fields\": [\"name\"], \
+	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
+	}]";
+	close $fh;
+
+	print CHLD_IN "[\"query\", \"$git_work_tree\", { \
+	\"since\": $time, \
+	\"fields\": [\"name\"], \
+	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
+	}]";
+
+	my $response = <CHLD_OUT>;
+
+	open ($fh, ">", ".git/watchman-response.json");
+	print $fh $response;
+	close $fh;
+
+	die "Watchman: command returned no output.\n" .
+	    "Falling back to scanning...\n" if $response eq "";
+	die "Watchman: command returned invalid output: $response\n" .
+	    "Falling back to scanning...\n" unless $response =~ /^\{/;
+
+	my $json_pkg;
+	eval {
+		require JSON::XS;
+		$json_pkg = "JSON::XS";
+		1;
+	} or do {
+		require JSON::PP;
+		$json_pkg = "JSON::PP";
+	};
+
+	my $o = $json_pkg->new->utf8->decode($response);
+
+	if ($retry > 0 and $o->{error} and $o->{error} =~ m/unable to resolve root .* directory (.*) is not watched/) {
+		print STDERR "Adding '$git_work_tree' to watchman's watch list.\n";
+		$retry--;
+		qx/watchman watch "$git_work_tree"/;
+		die "Failed to make watchman watch '$git_work_tree'.\n" .
+		    "Falling back to scanning...\n" if $? != 0;
+		# return fast "everything is dirty" flag"
+		print "*\0";
+		open ($fh, ">", ".git/watchman-output.out");
+		print "*\0";
+		close $fh;
+
+		# Watchman will always return all files on the first query so
+		# return the fast "everything is dirty" flag to git and do the
+		# Watchman query just to get it over with now so we won't pay
+		# the cost in git to look up each individual file.
+		print "*\0";
+		eval { launch_watchman() };
+		exit 0;
+	}
+
+	die "Watchman: $o->{error}.\n" .
+	    "Falling back to scanning...\n" if $o->{error};
+
+	binmode STDOUT, ":utf8";
+	local $, = "\0";
+	print @{$o->{files}};
+
+	open ($fh, ">", ".git/watchman-output.out");
+	print $fh @{$o->{files}};
+	close $fh;
+}
\ No newline at end of file

diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
index c6df85af5e..29ae4e284f 100755
--- a/t/t7519-status-fsmonitor.sh
+++ b/t/t7519-status-fsmonitor.sh
@@ -24,12 +24,14 @@ dirty_repo () {
  	: >untracked &&
  	: >dir1/untracked &&
  	: >dir2/untracked &&
+	: >dir3/untracked &&
  	echo 1 >modified &&
  	echo 2 >dir1/modified &&
  	echo 3 >dir2/modified &&
  	echo 4 >new &&
  	echo 5 >dir1/new &&
  	echo 6 >dir2/new
+	echo 7 >dir3/new
  }

  write_integration_script () {
@@ -47,12 +49,14 @@ write_integration_script () {
  	printf "untracked\0"
  	printf "dir1/untracked\0"
  	printf "dir2/untracked\0"
+	printf "dir3/untracked\0"
  	printf "modified\0"
  	printf "dir1/modified\0"
  	printf "dir2/modified\0"
  	printf "new\0"
  	printf "dir1/new\0"
  	printf "dir2/new\0"
+	printf "dir3/new\0"
  	EOF
  }

@@ -71,6 +75,8 @@ test_expect_success 'setup' '
  	mkdir dir2 &&
  	: >dir2/tracked &&
  	: >dir2/modified &&
+	mkdir dir3 &&
+	: >dir3/tracked &&
  	git -c core.fsmonitor= add . &&
  	git -c core.fsmonitor= commit -m initial &&
  	git config core.fsmonitor .git/hooks/fsmonitor-test &&
@@ -107,6 +113,7 @@ h dir1/modified
  H dir1/tracked
  h dir2/modified
  H dir2/tracked
+H dir3/tracked
  h modified
  H tracked
  EOF
@@ -126,6 +133,7 @@ H dir1/modified
  H dir1/tracked
  H dir2/modified
  H dir2/tracked
+H dir3/tracked
  H modified
  H tracked
  EOF
@@ -144,6 +152,7 @@ H dir1/modified
  H dir1/tracked
  H dir2/modified
  H dir2/tracked
+H dir3/tracked
  H modified
  H tracked
  EOF
@@ -164,6 +173,8 @@ H dir1/tracked
  H dir2/modified
  h dir2/new
  H dir2/tracked
+h dir3/new
+H dir3/tracked
  H modified
  h new
  H tracked
@@ -174,6 +185,7 @@ test_expect_success 'newly added files are marked 
valid' '
  	git add new &&
  	git add dir1/new &&
  	git add dir2/new &&
+	git add dir3/new &&
  	git ls-files -f >actual &&
  	test_cmp expect actual
  '
@@ -185,6 +197,8 @@ h dir1/tracked
  H dir2/modified
  h dir2/new
  h dir2/tracked
+h dir3/new
+h dir3/tracked
  H modified
  h new
  h tracked
@@ -203,6 +217,7 @@ H dir1/modified
  h dir1/tracked
  h dir2/modified
  h dir2/tracked
+h dir3/tracked
  h modified
  h tracked
  EOF
@@ -269,6 +284,7 @@ do
  			git add new &&
  			git add dir1/new &&
  			git add dir2/new &&
+			git add dir3/new &&
  			git status >actual &&
  			git -c core.fsmonitor= status >expect &&
  			test_i18ncmp expect actual



>> -----Original Message-----
>> From: David Turner
>> Sent: Friday, September 15, 2017 6:00 PM
>> To: 'Ben Peart' <benpeart@microsoft.com>
>> Cc: avarab@gmail.com; christian.couder@gmail.com; git@vger.kernel.org;
>> gitster@pobox.com; johannes.schindelin@gmx.de; pclouds@gmail.com;
>> peff@peff.net
>> Subject: RE: [PATCH v6 10/12] fsmonitor: add test cases for fsmonitor
>> extension
>>
>>> -----Original Message-----
>>> +dirty_repo () {
>>> +	: >untracked &&
>>> +	: >dir1/untracked &&
>>> +	: >dir2/untracked &&
>>> +	echo 1 >modified &&
>>> +	echo 2 >dir1/modified &&
>>> +	echo 3 >dir2/modified &&
>>> +	echo 4 >new &&
>>> +	echo 5 >dir1/new &&
>>> +	echo 6 >dir2/new
>>
>> If I add an untracked file named dir3/untracked to dirty_repo  (and
>> write_integration_script), then "status doesn't detect unreported
>> modifications", below, fails.  Did I do something wrong, or does this turn up a
>> bug?
> 
> 

diff --git a/t/check-non-portable-shell.pl b/t/check-non-portable-shell.pl
index b170cbc045..03dc9d2852 100755
--- a/t/check-non-portable-shell.pl
+++ b/t/check-non-portable-shell.pl
@@ -17,7 +17,7 @@ sub err {
 while (<>) {
 	chomp;
 	/\bsed\s+-i/ and err 'sed -i is not portable';
-	/\becho\s+-n/ and err 'echo -n is not portable (please use printf)';
+	/\becho\s+-[neE]/ and err 'echo with option is not portable (please use printf)';
 	/^\s*declare\s+/ and err 'arrays/declare not portable';
 	/^\s*[^#]\s*which\s/ and err 'which is not portable (please use type)';
 	/\btest\s+[^=]*==/ and err '"test a == b" is not portable (please use =)';

Makefile, t/helper/.gitignore, t/helper/test-drop-caches.c, t/perf/p7519-fsmonitor.sh

diff --git a/Makefile b/Makefile
index d970cd00e9..b2653ee64f 100644
--- a/Makefile
+++ b/Makefile
@@ -638,6 +638,7 @@ TEST_PROGRAMS_NEED_X += test-ctype
 TEST_PROGRAMS_NEED_X += test-config
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
+TEST_PROGRAMS_NEED_X += test-drop-caches
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
 TEST_PROGRAMS_NEED_X += test-dump-fsmonitor
 TEST_PROGRAMS_NEED_X += test-dump-split-index
diff --git a/t/helper/.gitignore b/t/helper/.gitignore
index 721650256e..f9328eebdd 100644
--- a/t/helper/.gitignore
+++ b/t/helper/.gitignore
@@ -3,6 +3,7 @@
 /test-config
 /test-date
 /test-delta
+/test-drop-caches
 /test-dump-cache-tree
 /test-dump-split-index
 /test-dump-untracked-cache
diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
new file mode 100644
index 0000000000..717079865c
--- /dev/null
+++ b/t/helper/test-drop-caches.c
@@ -0,0 +1,161 @@
+#include "git-compat-util.h"
+
+#if defined(GIT_WINDOWS_NATIVE)
+
+int cmd_sync(void)
+{
+	char Buffer[MAX_PATH];
+	DWORD dwRet;
+	char szVolumeAccessPath[] = "\\\\.\\X:";
+	HANDLE hVolWrite;
+	int success = 0;
+
+	dwRet = GetCurrentDirectory(MAX_PATH, Buffer);
+	if ((0 == dwRet) || (dwRet > MAX_PATH))
+		return error("Error getting current directory");
+
+	if ((Buffer[0] < 'A') || (Buffer[0] > 'Z'))
+		return error("Invalid drive letter '%c'", Buffer[0]);
+
+	szVolumeAccessPath[4] = Buffer[0];
+	hVolWrite = CreateFile(szVolumeAccessPath, GENERIC_READ | GENERIC_WRITE,
+		FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, NULL);
+	if (INVALID_HANDLE_VALUE == hVolWrite)
+		return error("Unable to open volume for writing, need admin access");
+
+	success = FlushFileBuffers(hVolWrite);
+	if (!success)
+		error("Unable to flush volume");
+
+	CloseHandle(hVolWrite);
+
+	return !success;
+}
+
+#define STATUS_SUCCESS			(0x00000000L)
+#define STATUS_PRIVILEGE_NOT_HELD	(0xC0000061L)
+
+typedef enum _SYSTEM_INFORMATION_CLASS {
+	SystemMemoryListInformation = 80,
+} SYSTEM_INFORMATION_CLASS;
+
+typedef enum _SYSTEM_MEMORY_LIST_COMMAND {
+	MemoryCaptureAccessedBits,
+	MemoryCaptureAndResetAccessedBits,
+	MemoryEmptyWorkingSets,
+	MemoryFlushModifiedList,
+	MemoryPurgeStandbyList,
+	MemoryPurgeLowPriorityStandbyList,
+	MemoryCommandMax
+} SYSTEM_MEMORY_LIST_COMMAND;
+
+BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
+{
+	BOOL bResult;
+	DWORD dwBufferLength;
+	LUID luid;
+	TOKEN_PRIVILEGES tpPreviousState;
+	TOKEN_PRIVILEGES tpNewState;
+
+	dwBufferLength = 16;
+	bResult = LookupPrivilegeValueA(0, lpName, &luid);
+	if (bResult) {
+		tpNewState.PrivilegeCount = 1;
+		tpNewState.Privileges[0].Luid = luid;
+		tpNewState.Privileges[0].Attributes = 0;
+		bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpNewState,
+			(DWORD)((LPBYTE)&(tpNewState.Privileges[1]) - (LPBYTE)&tpNewState),
+			&tpPreviousState, &dwBufferLength);
+		if (bResult) {
+			tpPreviousState.PrivilegeCount = 1;
+			tpPreviousState.Privileges[0].Luid = luid;
+			tpPreviousState.Privileges[0].Attributes = flags != 0 ? 2 : 0;
+			bResult = AdjustTokenPrivileges(TokenHandle, 0, &tpPreviousState,
+				dwBufferLength, 0, 0);
+		}
+	}
+	return bResult;
+}
+
+int cmd_dropcaches(void)
+{
+	HANDLE hProcess = GetCurrentProcess();
+	HANDLE hToken;
+	int status;
+
+	if (!OpenProcessToken(hProcess, TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &hToken))
+		return error("Can't open current process token");
+
+	if (!GetPrivilege(hToken, "SeProfileSingleProcessPrivilege", 1))
+		return error("Can't get SeProfileSingleProcessPrivilege");
+
+	CloseHandle(hToken);
+
+	HMODULE ntdll = LoadLibrary("ntdll.dll");
+	if (!ntdll)
+		return error("Can't load ntdll.dll, wrong Windows version?");
+
+	DWORD(WINAPI *NtSetSystemInformation)(INT, PVOID, ULONG) =
+		(DWORD(WINAPI *)(INT, PVOID, ULONG))GetProcAddress(ntdll, "NtSetSystemInformation");
+	if (!NtSetSystemInformation)
+		return error("Can't get function addresses, wrong Windows version?");
+
+	SYSTEM_MEMORY_LIST_COMMAND command = MemoryPurgeStandbyList;
+	status = NtSetSystemInformation(
+		SystemMemoryListInformation,
+		&command,
+		sizeof(SYSTEM_MEMORY_LIST_COMMAND)
+	);
+	if (status == STATUS_PRIVILEGE_NOT_HELD)
+		error("Insufficient privileges to purge the standby list, need admin access");
+	else if (status != STATUS_SUCCESS)
+		error("Unable to execute the memory list command %d", status);
+
+	FreeLibrary(ntdll);
+
+	return status;
+}
+
+#elif defined(__linux__)
+
+int cmd_sync(void)
+{
+	return system("sync");
+}
+
+int cmd_dropcaches(void)
+{
+	return system("echo 3 | sudo tee /proc/sys/vm/drop_caches");
+}
+
+#elif defined(__APPLE__)
+
+int cmd_sync(void)
+{
+	return system("sync");
+}
+
+int cmd_dropcaches(void)
+{
+	return system("sudo purge");
+}
+
+#else
+
+int cmd_sync(void)
+{
+	return 0;
+}
+
+int cmd_dropcaches(void)
+{
+	return error("drop caches not implemented on this platform");
+}
+
+#endif
+
+int cmd_main(int argc, const char **argv)
+{
+	cmd_sync();
+	return cmd_dropcaches();
+}
diff --git a/t/perf/p7519-fsmonitor.sh b/t/perf/p7519-fsmonitor.sh
new file mode 100755
index 0000000000..1c5978d5c8
--- /dev/null
+++ b/t/perf/p7519-fsmonitor.sh
@@ -0,0 +1,184 @@
+#!/bin/sh
+
+test_description="Test core.fsmonitor"
+
+. ./perf-lib.sh
+
+#
+# Performance test for the fsmonitor feature which enables git to talk to a
+# file system change monitor and avoid having to scan the working directory
+# for new or modified files.
+#
+# By default, the performance test will utilize the Watchman file system
+# monitor if it is installed.  If Watchman is not installed, it will use a
+# dummy integration script that does not report any new or modified files.
+# The dummy script has very little overhead which provides optimistic results.
+#
+# The performance test will also use the untracked cache feature if it is
+# available as fsmonitor uses it to speed up scanning for untracked files.
+#
+# There are 3 environment variables that can be used to alter the default
+# behavior of the performance test:
+#
+# GIT_PERF_7519_UNTRACKED_CACHE: used to configure core.untrackedCache
+# GIT_PERF_7519_SPLIT_INDEX: used to configure core.splitIndex
+# GIT_PERF_7519_FSMONITOR: used to configure core.fsMonitor
+#
+# The big win for using fsmonitor is the elimination of the need to scan the
+# working directory looking for changed and untracked files. If the file
+# information is all cached in RAM, the benefits are reduced.
+#
+# GIT_PERF_7519_DROP_CACHE: if set, the OS caches are dropped between tests
+#
+
+test_perf_large_repo
+test_checkout_worktree
+
+test_lazy_prereq UNTRACKED_CACHE '
+	{ git update-index --test-untracked-cache; ret=$?; } &&
+	test $ret -ne 1
+'
+
+test_lazy_prereq WATCHMAN '
+	{ command -v watchman >/dev/null 2>&1; ret=$?; } &&
+	test $ret -ne 1
+'
+
+if test_have_prereq WATCHMAN
+then
+	# Convert unix style paths to escaped Windows style paths for Watchman
+	case "$(uname -s)" in
+	MSYS_NT*)
+	  GIT_WORK_TREE="$(cygpath -aw "$PWD" | sed 's,\\,/,g')"
+	  ;;
+	*)
+	  GIT_WORK_TREE="$PWD"
+	  ;;
+	esac
+fi
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"
+then
+	# When using GIT_PERF_7519_DROP_CACHE, GIT_PERF_REPEAT_COUNT must be 1 to
+	# generate valid results. Otherwise the caching that happens for the nth
+	# run will negate the validity of the comparisons.
+	if test "$GIT_PERF_REPEAT_COUNT" -ne 1
+	then
+		echo "warning: Setting GIT_PERF_REPEAT_COUNT=1" >&2
+		GIT_PERF_REPEAT_COUNT=1
+	fi
+fi
+
+test_expect_success "setup for fsmonitor" '
+	# set untrackedCache depending on the environment
+	if test -n "$GIT_PERF_7519_UNTRACKED_CACHE"
+	then
+		git config core.untrackedCache "$GIT_PERF_7519_UNTRACKED_CACHE"
+	else
+		if test_have_prereq UNTRACKED_CACHE
+		then
+			git config core.untrackedCache true
+		else
+			git config core.untrackedCache false
+		fi
+	fi &&
+
+	# set core.splitindex depending on the environment
+	if test -n "$GIT_PERF_7519_SPLIT_INDEX"
+	then
+		git config core.splitIndex "$GIT_PERF_7519_SPLIT_INDEX"
+	fi &&
+
+	# set INTEGRATION_SCRIPT depending on the environment
+	if test -n "$GIT_PERF_7519_FSMONITOR"
+	then
+		INTEGRATION_SCRIPT="$GIT_PERF_7519_FSMONITOR"
+	else
+		#
+		# Choose integration script based on existance of Watchman.
+		# If Watchman exists, watch the work tree and attempt a query.
+		# If everything succeeds, use Watchman integration script,
+		# else fall back to an empty integration script.
+		#
+		mkdir .git/hooks &&
+		if test_have_prereq WATCHMAN
+		then
+			INTEGRATION_SCRIPT=".git/hooks/fsmonitor-watchman" &&
+			cp "$TEST_DIRECTORY/../templates/hooks--fsmonitor-watchman.sample" "$INTEGRATION_SCRIPT" &&
+			watchman watch "$GIT_WORK_TREE" &&
+			watchman watch-list | grep -q -F "$GIT_WORK_TREE"
+		else
+			INTEGRATION_SCRIPT=".git/hooks/fsmonitor-empty" &&
+			write_script "$INTEGRATION_SCRIPT"<<-\EOF
+			EOF
+		fi
+	fi &&
+
+	git config core.fsmonitor "$INTEGRATION_SCRIPT" &&
+	git update-index --fsmonitor
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uno (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uno
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uall (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uall
+'
+
+test_expect_success "setup without fsmonitor" '
+	unset INTEGRATION_SCRIPT &&
+	git config --unset core.fsmonitor &&
+	git update-index --no-fsmonitor
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uno (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uno
+'
+
+if test -n "$GIT_PERF_7519_DROP_CACHE"; then
+	test-drop-caches
+fi
+
+test_perf "status -uall (fsmonitor=$INTEGRATION_SCRIPT)" '
+	git status -uall
+'
+
+if test_have_prereq WATCHMAN
+then
+	watchman watch-del "$GIT_WORK_TREE" >/dev/null 2>&1 &&
+
+	# Work around Watchman bug on Windows where it holds on to handles
+	# preventing the removal of the trash directory
+	watchman shutdown-server >/dev/null 2>&1
+fi
+
+test_done

compat/win32/lazyload.h, t/helper/test-drop-caches.c

diff --git a/compat/win32/lazyload.h b/compat/win32/lazyload.h
new file mode 100644
index 00000000000..91c10dad2fb
--- /dev/null
+++ b/compat/win32/lazyload.h
@@ -0,0 +1,44 @@
+#ifndef LAZYLOAD_H
+#define LAZYLOAD_H
+
+/* simplify loading of DLL functions */
+
+struct proc_addr {
+	const char *const dll;
+	const char *const function;
+	FARPROC pfunction;
+	unsigned initialized : 1;
+};
+
+/* Declares a function to be loaded dynamically from a DLL. */
+#define DECLARE_PROC_ADDR(dll, rettype, function, ...) \
+	static struct proc_addr proc_addr_##function = \
+	{ #dll, #function, NULL, 0 }; \
+	static rettype (WINAPI *function)(__VA_ARGS__)
+
+/*
+ * Loads a function from a DLL (once-only).
+ * Returns non-NULL function pointer on success.
+ * Returns NULL + errno == ENOSYS on failure.
+ */
+#define INIT_PROC_ADDR(function) \
+	(function = get_proc_addr(&proc_addr_##function))
+
+static inline void *get_proc_addr(struct proc_addr *proc)
+{
+	/* only do this once */
+	if (!proc->initialized) {
+		HANDLE hnd;
+		proc->initialized = 1;
+		hnd = LoadLibraryExA(proc->dll, NULL,
+				     LOAD_LIBRARY_SEARCH_SYSTEM32);
+		if (hnd)
+			proc->pfunction = GetProcAddress(hnd, proc->function);
+	}
+	/* set ENOSYS if DLL or function was not found */
+	if (!proc->pfunction)
+		errno = ENOSYS;
+	return proc->pfunction;
+}
+
+#endif
-- snap --

With this patch, this fixup to your patch would make things compile (you
can also cherry-pick d05996fb61027512b8ab31a36c4a7a677dea11bb from my
fork):

-- snipsnap --
From d05996fb61027512b8ab31a36c4a7a677dea11bb Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Mon, 18 Sep 2017 14:56:40 +0200
Subject: [PATCH] fixup! fsmonitor: add a performance test

---
 t/helper/test-drop-caches.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
index 717079865cb..b27358528f7 100644
--- a/t/helper/test-drop-caches.c
+++ b/t/helper/test-drop-caches.c
@@ -1,6 +1,7 @@
 #include "git-compat-util.h"
 
 #if defined(GIT_WINDOWS_NATIVE)
+#include "compat/win32/lazyload.h"
 
 int cmd_sync(void)
 {
@@ -82,6 +83,9 @@ int cmd_dropcaches(void)
 	HANDLE hProcess = GetCurrentProcess();
 	HANDLE hToken;
 	int status;
+	SYSTEM_MEMORY_LIST_COMMAND command;
+	DECLARE_PROC_ADDR(ntll,
+			  DWORD, NtSetSystemInformation, INT, PVOID, ULONG);
 
 	if (!OpenProcessToken(hProcess, TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &hToken))
 		return error("Can't open current process token");
@@ -91,16 +95,10 @@ int cmd_dropcaches(void)
 
 	CloseHandle(hToken);
 
-	HMODULE ntdll = LoadLibrary("ntdll.dll");
-	if (!ntdll)
-		return error("Can't load ntdll.dll, wrong Windows
 		version?");
-
-	DWORD(WINAPI *NtSetSystemInformation)(INT, PVOID, ULONG) =
-		(DWORD(WINAPI *)(INT, PVOID, ULONG))GetProcAddress(ntdll,
 		"NtSetSystemInformation");
-	if (!NtSetSystemInformation)
+	if (!INIT_PROC_ADDR(NtSetSystemInformation))
 		return error("Can't get function addresses, wrong Windows version?");
 
-	SYSTEM_MEMORY_LIST_COMMAND command = MemoryPurgeStandbyList;
+	command = MemoryPurgeStandbyList;
 	status = NtSetSystemInformation(
 		SystemMemoryListInformation,
 		&command,
@@ -111,8 +109,6 @@ int cmd_dropcaches(void)
 	else if (status != STATUS_SUCCESS)
 		error("Unable to execute the memory list command %d", status);
 
-	FreeLibrary(ntdll);
-
 	return status;
 }
 

diff --git a/templates/hooks--fsmonitor-watchman.sample b/templates/hooks--fsmonitor-watchman.sample
new file mode 100755
index 0000000000..2779d7edf3
--- /dev/null
+++ b/templates/hooks--fsmonitor-watchman.sample
@@ -0,0 +1,119 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+use IPC::Open2;
+
+# An example hook script to integrate Watchman
+# (https://facebook.github.io/watchman/) with git to speed up detecting
+# new and modified files.
+#
+# The hook is passed a version (currently 1) and a time in nanoseconds
+# formatted as a string and outputs to stdout all files that have been
+# modified since the given time. Paths must be relative to the root of
+# the working tree and separated by a single NUL.
+#
+# To enable this hook, rename this file to "query-watchman" and set
+# 'git config core.fsmonitor .git/hooks/query-watchman'
+#
+my ($version, $time) = @ARGV;
+
+# Check the hook interface version
+
+if ($version == 1) {
+	# convert nanoseconds to seconds
+	$time = int $time / 1000000000;
+} else {
+	die "Unsupported query-fsmonitor hook version '$version'.\n" .
+	    "Falling back to scanning...\n";
+}
+
+# Convert unix style paths to escaped Windows style paths when running
+# in Windows command prompt
+
+my $system = `uname -s`;
+$system =~ s/[\r\n]+//g;
+my $git_work_tree;
+
+if ($system =~ m/^MSYS_NT/) {
+	$git_work_tree = `cygpath -aw "\$PWD"`;
+	$git_work_tree =~ s/[\r\n]+//g;
+	$git_work_tree =~ s,\\,/,g;
+} else {
+	$git_work_tree = $ENV{'PWD'};
+}
+
+my $retry = 1;
+
+launch_watchman();
+
+sub launch_watchman {
+
+	# Set input record separator
+	local $/ = 0666;
+
+	my $pid = open2(\*CHLD_OUT, \*CHLD_IN, 'watchman -j')
+	    or die "open2() failed: $!\n" .
+	    "Falling back to scanning...\n";
+
+	# In the query expression below we're asking for names of files that
+	# changed since $time but were not transient (ie created after
+	# $time but no longer exist).
+	#
+	# To accomplish this, we're using the "since" generator to use the
+	# recency index to select candidate nodes and "fields" to limit the
+	# output to file names only. Then we're using the "expression" term to
+	# further constrain the results.
+	#
+	# The category of transient files that we want to ignore will have a
+	# creation clock (cclock) newer than $time_t value and will also not
+	# currently exist.
+
+	print CHLD_IN "[\"query\", \"$git_work_tree\", { \
+	\"since\": $time, \
+	\"fields\": [\"name\"], \
+	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
+	}]";
+
+	my $response = <CHLD_OUT>;
+
+	die "Watchman: command returned no output.\n" .
+	    "Falling back to scanning...\n" if $response eq "";
+	die "Watchman: command returned invalid output: $response\n" .
+	    "Falling back to scanning...\n" unless $response =~ /^\{/;
+
+	my $json_pkg;
+	eval {
+		require JSON::XS;
+		$json_pkg = "JSON::XS";
+		1;
+	} or do {
+		require JSON::PP;
+		$json_pkg = "JSON::PP";
+	};
+
+	my $o = $json_pkg->new->utf8->decode($response);
+
+	if ($retry > 0 and $o->{error} and $o->{error} =~ m/unable to resolve root .* directory (.*) is not watched/) {
+		print STDERR "Adding '$git_work_tree' to watchman's watch list.\n";
+		$retry--;
+		qx/watchman watch "$git_work_tree"/;
+		die "Failed to make watchman watch '$git_work_tree'.\n" .
+		    "Falling back to scanning...\n" if $? != 0;
+
+		# Watchman will always return all files on the first query so
+		# return the fast "everything is dirty" flag to git and do the
+		# Watchman query just to get it over with now so we won't pay
+		# the cost in git to look up each individual file.
+		print "*\0";
+		eval { launch_watchman() };
+		exit 0;
+	}
+
+	die "Watchman: $o->{error}.\n" .
+	    "Falling back to scanning...\n" if $o->{error};
+
+	binmode STDOUT, ":utf8";
+	local $, = "\0";
+	print @{$o->{files}};
+}

Makefile, t/helper/test-dump-fsmonitor.c

diff --git a/Makefile b/Makefile
index 9d6ec9c1e9..d970cd00e9 100644
--- a/Makefile
+++ b/Makefile
@@ -639,6 +639,7 @@ TEST_PROGRAMS_NEED_X += test-config
 TEST_PROGRAMS_NEED_X += test-date
 TEST_PROGRAMS_NEED_X += test-delta
 TEST_PROGRAMS_NEED_X += test-dump-cache-tree
+TEST_PROGRAMS_NEED_X += test-dump-fsmonitor
 TEST_PROGRAMS_NEED_X += test-dump-split-index
 TEST_PROGRAMS_NEED_X += test-dump-untracked-cache
 TEST_PROGRAMS_NEED_X += test-fake-ssh
diff --git a/t/helper/test-dump-fsmonitor.c b/t/helper/test-dump-fsmonitor.c
new file mode 100644
index 0000000000..482d749bb9
--- /dev/null
+++ b/t/helper/test-dump-fsmonitor.c
@@ -0,0 +1,21 @@
+#include "cache.h"
+
+int cmd_main(int ac, const char **av)
+{
+	struct index_state *istate = &the_index;
+	int i;
+
+	setup_git_directory();
+	if (do_read_index(istate, get_index_file(), 0) < 0)
+		die("unable to read index file");
+	if (!istate->fsmonitor_last_update) {
+		printf("no fsmonitor\n");
+		return 0;
+	}
+	printf("fsmonitor last update %"PRIuMAX"\n", istate->fsmonitor_last_update);
+
+	for (i = 0; i < istate->cache_nr; i++)
+		printf((istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) ? "+" : "-");
+
+	return 0;
+}

diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index 22f69a410b..af9b847761 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -6,6 +6,7 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+sane_unset GIT_FSMONITOR_TEST
 
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&

diff --git a/builtin/update-index.c b/builtin/update-index.c
index d562f2ec69..e1ca0759d5 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -915,6 +915,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	struct refresh_params refresh_args = {0, &has_errors};
 	int lock_error = 0;
 	int split_index = -1;
+	int force_write = 0;
 	struct lock_file *lock_file;
 	struct parse_opt_ctx_t ctx;
 	strbuf_getline_fn getline_fn;
@@ -1006,6 +1007,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			    N_("test if the filesystem supports untracked cache"), UC_TEST),
 		OPT_SET_INT(0, "force-untracked-cache", &untracked_cache,
 			    N_("enable untracked cache without testing the filesystem"), UC_FORCE),
+		OPT_SET_INT(0, "force-write-index", &force_write,
+			N_("write out the index even if is not flagged as changed"), 1),
 		OPT_END()
 	};
 
@@ -1147,7 +1150,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 		die("BUG: bad untracked_cache value: %d", untracked_cache);
 	}
 
-	if (active_cache_changed) {
+	if (active_cache_changed || force_write) {
 		if (newfd < 0) {
 			if (refresh_args.flags & REFRESH_QUIET)
 				exit(128);

diff --git a/preload-index.c b/preload-index.c
index 70a4c80878..75564c497a 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -79,6 +79,8 @@ static void preload_index(struct index_state *index,
 		return;
 
 	threads = index->cache_nr / THREAD_COST;
+	if ((index->cache_nr > 1) && (threads < 2) && getenv("GIT_FORCE_PRELOAD_TEST"))
+		threads = 2;
 	if (threads < 2)
 		return;
 	if (threads > MAX_PARALLEL)

Documentation/config.txt, Documentation/git-ls-files.txt, Documentation/git-update-index.txt, Documentation/githooks.txt, builtin/update-index.c, fsmonitor.c, fsmonitor.h, t/helper/test-drop-caches.c, t/helper/test-dump-fsmonitor.c, t/perf/p7519-fsmonitor.sh, t/t7519-status-fsmonitor.sh, t/t7519/fsmonitor-all, t/t7519/fsmonitor-none, t/t7519/fsmonitor-watchman, templates/hooks--fsmonitor-watchman.sample

diff --git a/Documentation/config.txt b/Documentation/config.txt
index c196007a27..db52645cb4 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -418,6 +418,7 @@ core.fsmonitor::
 	will identify all files that may have changed since the
 	requested date/time. This information is used to speed up git by
 	avoiding unnecessary processing of files that have not changed.
+	See the "fsmonitor-watchman" section of linkgit:githooks[5].
 
 core.trustctime::
 	If false, the ctime differences between the index and the
diff --git a/Documentation/git-ls-files.txt b/Documentation/git-ls-files.txt
index d153c17e06..3ac3e3a77d 100644
--- a/Documentation/git-ls-files.txt
+++ b/Documentation/git-ls-files.txt
@@ -9,7 +9,7 @@ git-ls-files - Show information about files in the index and the working tree
 SYNOPSIS
 --------
 [verse]
-'git ls-files' [-z] [-t] [-v]
+'git ls-files' [-z] [-t] [-v] [-f]
 		(--[cached|deleted|others|ignored|stage|unmerged|killed|modified])*
 		(-[c|d|o|i|s|u|k|m])*
 		[--eol]
@@ -133,6 +133,11 @@ a space) at the start of each line:
 	that are marked as 'assume unchanged' (see
 	linkgit:git-update-index[1]).
 
+-f::
+	Similar to `-t`, but use lowercase letters for files
+	that are marked as 'fsmonitor valid' (see
+	linkgit:git-update-index[1]).
+
 --full-name::
 	When run from a subdirectory, the command usually
 	outputs paths relative to the current directory.  This
diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index e19eba62cd..95231dbfcb 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -16,9 +16,11 @@ SYNOPSIS
 	     [--chmod=(+|-)x]
 	     [--[no-]assume-unchanged]
 	     [--[no-]skip-worktree]
+	     [--[no-]fsmonitor-valid]
 	     [--ignore-submodules]
 	     [--[no-]split-index]
 	     [--[no-|test-|force-]untracked-cache]
+	     [--[no-]fsmonitor]
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin] [--index-version <n>]
@@ -111,6 +113,12 @@ you will need to handle the situation manually.
 	set and unset the "skip-worktree" bit for the paths. See
 	section "Skip-worktree bit" below for more information.
 
+--[no-]fsmonitor-valid::
+	When one of these flags is specified, the object name recorded
+	for the paths are not updated. Instead, these options
+	set and unset the "fsmonitor valid" bit for the paths. See
+	section "File System Monitor" below for more information.
+
 -g::
 --again::
 	Runs 'git update-index' itself on the paths whose index
@@ -201,6 +209,15 @@ will remove the intended effect of the option.
 	`--untracked-cache` used to imply `--test-untracked-cache` but
 	this option would enable the extension unconditionally.
 
+--fsmonitor::
+--no-fsmonitor::
+	Enable or disable files system monitor feature. These options
+	take effect whatever the value of the `core.fsmonitor`
+	configuration variable (see linkgit:git-config[1]). But a warning
+	is emitted when the change goes against the configured value, as
+	the configured value will take effect next time the index is
+	read and this will remove the intended effect of the option.
+
 \--::
 	Do not interpret any more arguments as options.
 
@@ -447,6 +464,34 @@ command reads the index; while when `--[no-|force-]untracked-cache`
 are used, the untracked cache is immediately added to or removed from
 the index.
 
+File System Monitor
+-------------------
+
+This feature is intended to speed up git operations for repos that have
+large working directories.
+
+It enables git to work together with a file system monitor (see the
+"fsmonitor-watchman" section of linkgit:githooks[5]) that can
+inform it as to what files have been modified. This enables git to avoid
+having to lstat() every file to find modified files.
+
+When used in conjunction with the untracked cache, it can further improve
+performance by avoiding the cost of scaning the entire working directory
+looking for new files.
+
+If you want to enable (or disable) this feature, it is easier to use
+the `core.fsmonitor` configuration variable (see
+linkgit:git-config[1]) than using the `--fsmonitor` option to
+`git update-index` in each repository, especially if you want to do so
+across all repositories you use, because you can set the configuration
+variable to `true` (or `false`) in your `$HOME/.gitconfig` just once
+and have it affect all repositories you touch.
+
+When the `core.fsmonitor` configuration variable is changed, the
+file system monitor is added to or removed from the index the next time
+a command reads the index. When `--[no-]fsmonitor` are used, the file
+system monitor is immediately added to or removed from the index.
+
 Configuration
 -------------
 
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index da82d64b0b..ae60559cd9 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -455,10 +455,8 @@ the name of the file that holds the e-mail to be sent.  Exiting with a
 non-zero status causes 'git send-email' to abort before sending any
 e-mails.
 
-
-[[fsmonitor-watchman]]
 fsmonitor-watchman
-~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~
 
 This hook is invoked when the configuration option core.fsmonitor is
 set to .git/hooks/fsmonitor-watchman.  It takes two arguments, a version
@@ -471,10 +469,17 @@ should be inclusive so that it does not miss any potential changes.
 The paths should be relative to the root of the working directory
 and be separated by a single NUL.
 
+It is OK to include files which have not actually changed.  All changes
+including newly-created and deleted files should be included. When
+files are renamed, both the old and the new name should be included.
+
 Git will limit what files it checks for changes as well as which
 directories are checked for untracked files based on the path names
 given.
 
+An optimized way to tell git "all files have changed" is to return
+the filename '/'.
+
 The exit status determines whether git will use the data from the
 hook to limit its search.  On error, it will fall back to verifying
 all files and folders.
diff --git a/builtin/update-index.c b/builtin/update-index.c
index b03afc1f3a..41618db098 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -33,6 +33,7 @@ static int force_remove;
 static int verbose;
 static int mark_valid_only;
 static int mark_skip_worktree_only;
+static int mark_fsmonitor_only;
 #define MARK_FLAG 1
 #define UNMARK_FLAG 2
 static struct strbuf mtime_dir = STRBUF_INIT;
@@ -229,12 +230,12 @@ static int mark_ce_flags(const char *path, int flag, int mark)
 	int namelen = strlen(path);
 	int pos = cache_name_pos(path, namelen);
 	if (0 <= pos) {
+		mark_fsmonitor_invalid(&the_index, active_cache[pos]);
 		if (mark)
 			active_cache[pos]->ce_flags |= flag;
 		else
 			active_cache[pos]->ce_flags &= ~flag;
 		active_cache[pos]->ce_flags |= CE_UPDATE_IN_BASE;
-		mark_fsmonitor_invalid(&the_index, active_cache[pos]);
 		cache_tree_invalidate_path(&the_index, path);
 		active_cache_changed |= CE_ENTRY_CHANGED;
 		return 0;
@@ -460,6 +461,11 @@ static void update_one(const char *path)
 			die("Unable to mark file %s", path);
 		return;
 	}
+	if (mark_fsmonitor_only) {
+		if (mark_ce_flags(path, CE_FSMONITOR_VALID, mark_fsmonitor_only == MARK_FLAG))
+			die("Unable to mark file %s", path);
+		return;
+	}
 
 	if (force_remove) {
 		if (remove_file_from_cache(path))
@@ -1014,6 +1020,12 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			N_("write out the index even if is not flagged as changed"), 1),
 		OPT_BOOL(0, "fsmonitor", &fsmonitor,
 			N_("enable or disable file system monitor")),
+		{OPTION_SET_INT, 0, "fsmonitor-valid", &mark_fsmonitor_only, NULL,
+			N_("mark files as fsmonitor valid"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, MARK_FLAG},
+		{OPTION_SET_INT, 0, "no-fsmonitor-valid", &mark_fsmonitor_only, NULL,
+			N_("clear fsmonitor valid bit"),
+			PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, UNMARK_FLAG},
 		OPT_END()
 	};
 
diff --git a/fsmonitor.c b/fsmonitor.c
index 144294b8df..b8b2d88fe1 100644
--- a/fsmonitor.c
+++ b/fsmonitor.c
@@ -57,12 +57,12 @@ int read_fsmonitor_extension(struct index_state *istate, const void *data,
 
 		/* Mark all previously saved entries as dirty */
 		ewah_each_bit(fsmonitor_dirty, fsmonitor_ewah_callback, istate);
-		ewah_free(fsmonitor_dirty);
 
 		/* Now mark the untracked cache for fsmonitor usage */
 		if (istate->untracked)
 			istate->untracked->use_fsmonitor = 1;
 	}
+	ewah_free(fsmonitor_dirty);
 
 	trace_printf_key(&trace_fsmonitor, "read fsmonitor extension successful");
 	return 0;
@@ -177,7 +177,7 @@ void refresh_fsmonitor(struct index_state *istate)
 	}
 
 	/* a fsmonitor process can return '*' to indicate all entries are invalid */
-	if (query_success && query_result.buf[0] != '*') {
+	if (query_success && query_result.buf[0] != '/') {
 		/* Mark all entries returned by the monitor as dirty */
 		buf = query_result.buf;
 		bol = 0;
diff --git a/fsmonitor.h b/fsmonitor.h
index dadbe90283..c2240b811a 100644
--- a/fsmonitor.h
+++ b/fsmonitor.h
@@ -46,7 +46,7 @@ static inline void mark_fsmonitor_valid(struct cache_entry *ce)
 }
 
 /*
- * Clear the given cache entries CE_FSMONITOR_VALID bit and invalidate any
+ * Clear the given cache entry's CE_FSMONITOR_VALID bit and invalidate any
  * corresponding untracked cache directory structures.
  */
 static inline void mark_fsmonitor_invalid(struct index_state *istate, struct cache_entry *ce)
diff --git a/t/helper/test-drop-caches.c b/t/helper/test-drop-caches.c
index 717079865c..4e5ca8f397 100644
--- a/t/helper/test-drop-caches.c
+++ b/t/helper/test-drop-caches.c
@@ -2,7 +2,7 @@
 
 #if defined(GIT_WINDOWS_NATIVE)
 
-int cmd_sync(void)
+static int cmd_sync(void)
 {
 	char Buffer[MAX_PATH];
 	DWORD dwRet;
@@ -49,7 +49,7 @@ typedef enum _SYSTEM_MEMORY_LIST_COMMAND {
 	MemoryCommandMax
 } SYSTEM_MEMORY_LIST_COMMAND;
 
-BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
+static BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
 {
 	BOOL bResult;
 	DWORD dwBufferLength;
@@ -77,10 +77,11 @@ BOOL GetPrivilege(HANDLE TokenHandle, LPCSTR lpName, int flags)
 	return bResult;
 }
 
-int cmd_dropcaches(void)
+static int cmd_dropcaches(void)
 {
 	HANDLE hProcess = GetCurrentProcess();
 	HANDLE hToken;
+	HMODULE ntdll;
 	int status;
 
 	if (!OpenProcessToken(hProcess, TOKEN_QUERY | TOKEN_ADJUST_PRIVILEGES, &hToken))
@@ -91,7 +92,7 @@ int cmd_dropcaches(void)
 
 	CloseHandle(hToken);
 
-	HMODULE ntdll = LoadLibrary("ntdll.dll");
+	ntdll = LoadLibrary("ntdll.dll");
 	if (!ntdll)
 		return error("Can't load ntdll.dll, wrong Windows version?");
 
@@ -118,36 +119,36 @@ int cmd_dropcaches(void)
 
 #elif defined(__linux__)
 
-int cmd_sync(void)
+static int cmd_sync(void)
 {
 	return system("sync");
 }
 
-int cmd_dropcaches(void)
+static int cmd_dropcaches(void)
 {
 	return system("echo 3 | sudo tee /proc/sys/vm/drop_caches");
 }
 
 #elif defined(__APPLE__)
 
-int cmd_sync(void)
+static int cmd_sync(void)
 {
 	return system("sync");
 }
 
-int cmd_dropcaches(void)
+static int cmd_dropcaches(void)
 {
 	return system("sudo purge");
 }
 
 #else
 
-int cmd_sync(void)
+static int cmd_sync(void)
 {
 	return 0;
 }
 
-int cmd_dropcaches(void)
+static int cmd_dropcaches(void)
 {
 	return error("drop caches not implemented on this platform");
 }
diff --git a/t/helper/test-dump-fsmonitor.c b/t/helper/test-dump-fsmonitor.c
index 482d749bb9..ad452707e8 100644
--- a/t/helper/test-dump-fsmonitor.c
+++ b/t/helper/test-dump-fsmonitor.c
@@ -12,7 +12,7 @@ int cmd_main(int ac, const char **av)
 		printf("no fsmonitor\n");
 		return 0;
 	}
-	printf("fsmonitor last update %"PRIuMAX"\n", istate->fsmonitor_last_update);
+	printf("fsmonitor last update %"PRIuMAX"\n", (uintmax_t)istate->fsmonitor_last_update);
 
 	for (i = 0; i < istate->cache_nr; i++)
 		printf((istate->cache[i]->ce_flags & CE_FSMONITOR_VALID) ? "+" : "-");
diff --git a/t/perf/p7519-fsmonitor.sh b/t/perf/p7519-fsmonitor.sh
index 1c5978d5c8..16d1bf72e5 100755
--- a/t/perf/p7519-fsmonitor.sh
+++ b/t/perf/p7519-fsmonitor.sh
@@ -95,7 +95,7 @@ test_expect_success "setup for fsmonitor" '
 		INTEGRATION_SCRIPT="$GIT_PERF_7519_FSMONITOR"
 	else
 		#
-		# Choose integration script based on existance of Watchman.
+		# Choose integration script based on existence of Watchman.
 		# If Watchman exists, watch the work tree and attempt a query.
 		# If everything succeeds, use Watchman integration script,
 		# else fall back to an empty integration script.
diff --git a/t/t7519-status-fsmonitor.sh b/t/t7519-status-fsmonitor.sh
index 6aa1e4e924..c6df85af5e 100755
--- a/t/t7519-status-fsmonitor.sh
+++ b/t/t7519-status-fsmonitor.sh
@@ -32,14 +32,16 @@ dirty_repo () {
 	echo 6 >dir2/new
 }
 
-write_integration_script() {
+write_integration_script () {
 	write_script .git/hooks/fsmonitor-test<<-\EOF
-	if [ "$#" -ne 2 ]; then
+	if test "$#" -ne 2
+	then
 		echo "$0: exactly 2 arguments expected"
 		exit 2
 	fi
-	if [ "$1" != 1 ]; then
-		echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+	if test "$1" != 1
+	then
+		echo "Unsupported core.fsmonitor hook version." >&2
 		exit 1
 	fi
 	printf "untracked\0"
@@ -100,6 +102,43 @@ test_expect_success 'update-index --no-fsmonitor" removes the fsmonitor extensio
 	grep "^no fsmonitor" actual
 '
 
+cat >expect <<EOF &&
+h dir1/modified
+H dir1/tracked
+h dir2/modified
+H dir2/tracked
+h modified
+H tracked
+EOF
+
+# test that "update-index --fsmonitor-valid" sets the fsmonitor valid bit
+test_expect_success 'update-index --fsmonitor-valid" sets the fsmonitor valid bit' '
+	git update-index --fsmonitor &&
+	git update-index --fsmonitor-valid dir1/modified &&
+	git update-index --fsmonitor-valid dir2/modified &&
+	git update-index --fsmonitor-valid modified &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
+cat >expect <<EOF &&
+H dir1/modified
+H dir1/tracked
+H dir2/modified
+H dir2/tracked
+H modified
+H tracked
+EOF
+
+# test that "update-index --no-fsmonitor-valid" clears the fsmonitor valid bit
+test_expect_success 'update-index --no-fsmonitor-valid" clears the fsmonitor valid bit' '
+	git update-index --no-fsmonitor-valid dir1/modified &&
+	git update-index --no-fsmonitor-valid dir2/modified &&
+	git update-index --no-fsmonitor-valid modified &&
+	git ls-files -f >actual &&
+	test_cmp expect actual
+'
+
 cat >expect <<EOF &&
 H dir1/modified
 H dir1/tracked
@@ -204,7 +243,7 @@ for preload_val in $preload_values
 do
 	test_expect_success "setup preloadIndex to $preload_val" '
 		git config core.preloadIndex $preload_val &&
-		if [ $preload_val -eq true ]
+		if test $preload_val = true
 		then
 			GIT_FORCE_PRELOAD_TEST=$preload_val; export GIT_FORCE_PRELOAD_TEST
 		else
@@ -249,8 +288,14 @@ do
 			git status >actual &&
 			test_path_is_file marker &&
 			test_i18ngrep ! "Changes not staged for commit:" actual &&
-			if [ $uc_val -eq true ]; then test_i18ngrep ! "Untracked files:" actual; fi &&
-			if [ $uc_val -eq false ]; then test_i18ngrep "Untracked files:" actual; fi &&
+			if test $uc_val = true
+			then
+				test_i18ngrep ! "Untracked files:" actual
+			fi &&
+			if test $uc_val = false
+			then
+				test_i18ngrep "Untracked files:" actual
+			fi &&
 			rm -f marker
 		'
 	done
diff --git a/t/t7519/fsmonitor-all b/t/t7519/fsmonitor-all
index a3870e431e..691bc94dc2 100755
--- a/t/t7519/fsmonitor-all
+++ b/t/t7519/fsmonitor-all
@@ -9,15 +9,16 @@
 #
 #echo "$0 $*" >&2
 
-if [ "$#" -ne 2 ] ; then
-	echo -e "$0: exactly 2 arguments expected\n" >&2
+if test "$#" -ne 2
+then
+	echo "$0: exactly 2 arguments expected" >&2
 	exit 2
 fi
 
-if [ "$1" != 1 ]
+if test "$1" != 1
 then
-	echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+	echo "Unsupported core.fsmonitor hook version." >&2
 	exit 1
 fi
 
-echo "*"
\ No newline at end of file
+echo "/"
diff --git a/t/t7519/fsmonitor-none b/t/t7519/fsmonitor-none
index c500bb0f26..ed9cf5a6a9 100755
--- a/t/t7519/fsmonitor-none
+++ b/t/t7519/fsmonitor-none
@@ -9,13 +9,14 @@
 #
 #echo "$0 $*" >&2
 
-if [ "$#" -ne 2 ] ; then
-	echo -e "$0: exactly 2 arguments expected\n" >&2
+if test "$#" -ne 2
+then
+	echo "$0: exactly 2 arguments expected" >&2
 	exit 2
 fi
 
-if [ "$1" != 1 ]
+if test "$1" != 1
 then
-	echo -e "Unsupported core.fsmonitor hook version.\n" >&2
+	echo "Unsupported core.fsmonitor hook version." >&2
 	exit 1
 fi
diff --git a/t/t7519/fsmonitor-watchman b/t/t7519/fsmonitor-watchman
index aaee5d1fe3..7ceb32dc18 100755
--- a/t/t7519/fsmonitor-watchman
+++ b/t/t7519/fsmonitor-watchman
@@ -17,7 +17,7 @@ use IPC::Open2;
 # 'git config core.fsmonitor .git/hooks/query-watchman'
 #
 my ($version, $time) = @ARGV;
-print STDERR "$0 $version $time\n";
+#print STDERR "$0 $version $time\n";
 
 # Check the hook interface version
 
@@ -29,7 +29,20 @@ if ($version == 1) {
 	    "Falling back to scanning...\n";
 }
 
-my $git_work_tree = $ENV{'PWD'};
+# Convert unix style paths to escaped Windows style paths when running
+# in Windows command prompt
+
+my $system = `uname -s`;
+$system =~ s/[\r\n]+//g;
+my $git_work_tree;
+
+if ($system =~ m/^MSYS_NT/) {
+	$git_work_tree = `cygpath -aw "\$PWD"`;
+	$git_work_tree =~ s/[\r\n]+//g;
+	$git_work_tree =~ s,\\,/,g;
+} else {
+	$git_work_tree = $ENV{'PWD'};
+}
 
 my $retry = 1;
 
@@ -57,20 +70,19 @@ sub launch_watchman {
 	# creation clock (cclock) newer than $time_t value and will also not
 	# currently exist.
 
+	my $query = <<"	END";
+		["query", "$git_work_tree", {
+			"since": $time,
+			"fields": ["name"],
+			"expression": ["not", ["allof", ["since", $time, "cclock"], ["not", "exists"]]]
+		}]
+	END
+	
 	open (my $fh, ">", ".git/watchman-query.json");
-	print $fh "[\"query\", \"$git_work_tree\", { \
-	\"since\": $time, \
-	\"fields\": [\"name\"], \
-	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
-	}]";
+	print $fh $query;
 	close $fh;
 
-	print CHLD_IN "[\"query\", \"$git_work_tree\", { \
-	\"since\": $time, \
-	\"fields\": [\"name\"], \
-	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
-	}]";
-
+	print CHLD_IN $query;
 	my $response = <CHLD_OUT>;
 
 	open ($fh, ">", ".git/watchman-response.json");
@@ -100,17 +112,17 @@ sub launch_watchman {
 		qx/watchman watch "$git_work_tree"/;
 		die "Failed to make watchman watch '$git_work_tree'.\n" .
 		    "Falling back to scanning...\n" if $? != 0;
-		# return fast "everything is dirty" flag"
-		print "*\0";
-		open ($fh, ">", ".git/watchman-output.out");
-		print "*\0";
-		close $fh;
 
 		# Watchman will always return all files on the first query so
 		# return the fast "everything is dirty" flag to git and do the
 		# Watchman query just to get it over with now so we won't pay
 		# the cost in git to look up each individual file.
-		print "*\0";
+
+		open ($fh, ">", ".git/watchman-output.out");
+		print "/\0";
+		close $fh;
+
+		print "/\0";
 		eval { launch_watchman() };
 		exit 0;
 	}
@@ -118,11 +130,11 @@ sub launch_watchman {
 	die "Watchman: $o->{error}.\n" .
 	    "Falling back to scanning...\n" if $o->{error};
 
-	binmode STDOUT, ":utf8";
-	local $, = "\0";
-	print @{$o->{files}};
-
 	open ($fh, ">", ".git/watchman-output.out");
 	print $fh @{$o->{files}};
 	close $fh;
-}
\ No newline at end of file
+
+	binmode STDOUT, ":utf8";
+	local $, = "\0";
+	print @{$o->{files}};
+}
diff --git a/templates/hooks--fsmonitor-watchman.sample b/templates/hooks--fsmonitor-watchman.sample
index 2779d7edf3..870a59d237 100755
--- a/templates/hooks--fsmonitor-watchman.sample
+++ b/templates/hooks--fsmonitor-watchman.sample
@@ -69,12 +69,15 @@ sub launch_watchman {
 	# creation clock (cclock) newer than $time_t value and will also not
 	# currently exist.
 
-	print CHLD_IN "[\"query\", \"$git_work_tree\", { \
-	\"since\": $time, \
-	\"fields\": [\"name\"], \
-	\"expression\": [\"not\", [\"allof\", [\"since\", $time, \"cclock\"], [\"not\", \"exists\"]]] \
-	}]";
-
+	my $query = <<"	END";
+		["query", "$git_work_tree", {
+			"since": $time,
+			"fields": ["name"],
+			"expression": ["not", ["allof", ["since", $time, "cclock"], ["not", "exists"]]]
+		}]
+	END
+
+	print CHLD_IN $query;
 	my $response = <CHLD_OUT>;
 
 	die "Watchman: command returned no output.\n" .
@@ -105,7 +108,7 @@ sub launch_watchman {
 		# return the fast "everything is dirty" flag to git and do the
 		# Watchman query just to get it over with now so we won't pay
 		# the cost in git to look up each individual file.
-		print "*\0";
+		print "/\0";
 		eval { launch_watchman() };
 		exit 0;
 	}

Ben Peart (12):
  bswap: add 64 bit endianness helper get_be64
  preload-index: add override to enable testing preload-index
  update-index: add a new --force-write-index option
  fsmonitor: teach git to optionally utilize a file system monitor to
    speed up detecting new or changed files.
  fsmonitor: add documentation for the fsmonitor extension.
  ls-files: Add support in ls-files to display the fsmonitor valid bit
  update-index: add fsmonitor support to update-index
  fsmonitor: add a test tool to dump the index extension
  split-index: disable the fsmonitor extension when running the split
    index test
  fsmonitor: add test cases for fsmonitor extension
  fsmonitor: add a sample integration script for Watchman
  fsmonitor: add a performance test

 Documentation/config.txt                   |   7 +
 Documentation/git-ls-files.txt             |   7 +-
 Documentation/git-update-index.txt         |  45 +++++
 Documentation/githooks.txt                 |  28 +++
 Documentation/technical/index-format.txt   |  19 ++
 Makefile                                   |   3 +
 apply.c                                    |   2 +-
 builtin/ls-files.c                         |   8 +-
 builtin/update-index.c                     |  38 +++-
 cache.h                                    |  10 +-
 compat/bswap.h                             |  22 +++
 config.c                                   |  14 ++
 config.h                                   |   1 +
 diff-lib.c                                 |   2 +
 dir.c                                      |  27 ++-
 dir.h                                      |   2 +
 entry.c                                    |   4 +-
 environment.c                              |   1 +
 fsmonitor.c                                | 253 ++++++++++++++++++++++++
 fsmonitor.h                                |  61 ++++++
 preload-index.c                            |   8 +-
 read-cache.c                               |  49 ++++-
 submodule.c                                |   2 +-
 t/helper/.gitignore                        |   1 +
 t/helper/test-drop-caches.c                | 162 +++++++++++++++
 t/helper/test-dump-fsmonitor.c             |  21 ++
 t/perf/p7519-fsmonitor.sh                  | 184 +++++++++++++++++
 t/t1700-split-index.sh                     |   1 +
 t/t7519-status-fsmonitor.sh                | 304 +++++++++++++++++++++++++++++
 t/t7519/fsmonitor-all                      |  24 +++
 t/t7519/fsmonitor-none                     |  22 +++
 t/t7519/fsmonitor-watchman                 | 140 +++++++++++++
 templates/hooks--fsmonitor-watchman.sample | 122 ++++++++++++
 unpack-trees.c                             |   8 +-
 34 files changed, 1572 insertions(+), 30 deletions(-)
 create mode 100644 fsmonitor.c
 create mode 100644 fsmonitor.h
 create mode 100644 t/helper/test-drop-caches.c
 create mode 100644 t/helper/test-dump-fsmonitor.c
 create mode 100755 t/perf/p7519-fsmonitor.sh
 create mode 100755 t/t7519-status-fsmonitor.sh
 create mode 100755 t/t7519/fsmonitor-all
 create mode 100755 t/t7519/fsmonitor-none
 create mode 100755 t/t7519/fsmonitor-watchman
 create mode 100755 templates/hooks--fsmonitor-watchman.sample

diff --git a/compat/bswap.h b/compat/bswap.h
index 7d063e9e40..6b22c46214 100644
--- a/compat/bswap.h
+++ b/compat/bswap.h
@@ -158,7 +158,9 @@ static inline uint64_t git_bswap64(uint64_t x)
 
 #define get_be16(p)	ntohs(*(unsigned short *)(p))
 #define get_be32(p)	ntohl(*(unsigned int *)(p))
+#define get_be64(p)	ntohll(*(uint64_t *)(p))
 #define put_be32(p, v)	do { *(unsigned int *)(p) = htonl(v); } while (0)
+#define put_be64(p, v)	do { *(uint64_t *)(p) = htonll(v); } while (0)
 
 #else
 
@@ -178,6 +180,13 @@ static inline uint32_t get_be32(const void *ptr)
 		(uint32_t)p[3] <<  0;
 }
 
+static inline uint64_t get_be64(const void *ptr)
+{
+	const unsigned char *p = ptr;
+	return	(uint64_t)get_be32(p[0]) << 32 |
+		(uint64_t)get_be32(p[4]) <<  0;
+}
+
 static inline void put_be32(void *ptr, uint32_t value)
 {
 	unsigned char *p = ptr;
@@ -187,4 +196,17 @@ static inline void put_be32(void *ptr, uint32_t value)
 	p[3] = value >>  0;
 }
 
+static inline void put_be64(void *ptr, uint64_t value)
+{
+	unsigned char *p = ptr;
+	p[0] = value >> 56;
+	p[1] = value >> 48;
+	p[2] = value >> 40;
+	p[3] = value >> 32;
+	p[4] = value >> 24;
+	p[5] = value >> 16;
+	p[6] = value >>  8;
+	p[7] = value >>  0;
+}
+
 #endif

diff --git a/preload-index.c b/preload-index.c
index 70a4c80878..75564c497a 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -79,6 +79,8 @@ static void preload_index(struct index_state *index,
 		return;
 
 	threads = index->cache_nr / THREAD_COST;
+	if ((index->cache_nr > 1) && (threads < 2) && getenv("GIT_FORCE_PRELOAD_TEST"))
+		threads = 2;
 	if (threads < 2)
 		return;
 	if (threads > MAX_PARALLEL)

Documentation/config.txt, Documentation/git-ls-files.txt, Documentation/git-update-index.txt, Documentation/githooks.txt, Documentation/technical/index-format.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dc4e3f58a2..db52645cb4 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -413,6 +413,13 @@ core.protectNTFS::
 	8.3 "short" names.
 	Defaults to `true` on Windows, and `false` elsewhere.
 
+core.fsmonitor::
+	If set, the value of this variable is used as a command which
+	will identify all files that may have changed since the
+	requested date/time. This information is used to speed up git by
+	avoiding unnecessary processing of files that have not changed.
+	See the "fsmonitor-watchman" section of linkgit:githooks[5].
+
 core.trustctime::
 	If false, the ctime differences between the index and the
 	working tree are ignored; useful when the inode change time
diff --git a/Documentation/git-ls-files.txt b/Documentation/git-ls-files.txt
index d153c17e06..3ac3e3a77d 100644
--- a/Documentation/git-ls-files.txt
+++ b/Documentation/git-ls-files.txt
@@ -9,7 +9,7 @@ git-ls-files - Show information about files in the index and the working tree
 SYNOPSIS
 --------
 [verse]
-'git ls-files' [-z] [-t] [-v]
+'git ls-files' [-z] [-t] [-v] [-f]
 		(--[cached|deleted|others|ignored|stage|unmerged|killed|modified])*
 		(-[c|d|o|i|s|u|k|m])*
 		[--eol]
@@ -133,6 +133,11 @@ a space) at the start of each line:
 	that are marked as 'assume unchanged' (see
 	linkgit:git-update-index[1]).
 
+-f::
+	Similar to `-t`, but use lowercase letters for files
+	that are marked as 'fsmonitor valid' (see
+	linkgit:git-update-index[1]).
+
 --full-name::
 	When run from a subdirectory, the command usually
 	outputs paths relative to the current directory.  This
diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index e19eba62cd..95231dbfcb 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -16,9 +16,11 @@ SYNOPSIS
 	     [--chmod=(+|-)x]
 	     [--[no-]assume-unchanged]
 	     [--[no-]skip-worktree]
+	     [--[no-]fsmonitor-valid]
 	     [--ignore-submodules]
 	     [--[no-]split-index]
 	     [--[no-|test-|force-]untracked-cache]
+	     [--[no-]fsmonitor]
 	     [--really-refresh] [--unresolve] [--again | -g]
 	     [--info-only] [--index-info]
 	     [-z] [--stdin] [--index-version <n>]
@@ -111,6 +113,12 @@ you will need to handle the situation manually.
 	set and unset the "skip-worktree" bit for the paths. See
 	section "Skip-worktree bit" below for more information.
 
+--[no-]fsmonitor-valid::
+	When one of these flags is specified, the object name recorded
+	for the paths are not updated. Instead, these options
+	set and unset the "fsmonitor valid" bit for the paths. See
+	section "File System Monitor" below for more information.
+
 -g::
 --again::
 	Runs 'git update-index' itself on the paths whose index
@@ -201,6 +209,15 @@ will remove the intended effect of the option.
 	`--untracked-cache` used to imply `--test-untracked-cache` but
 	this option would enable the extension unconditionally.
 
+--fsmonitor::
+--no-fsmonitor::
+	Enable or disable files system monitor feature. These options
+	take effect whatever the value of the `core.fsmonitor`
+	configuration variable (see linkgit:git-config[1]). But a warning
+	is emitted when the change goes against the configured value, as
+	the configured value will take effect next time the index is
+	read and this will remove the intended effect of the option.
+
 \--::
 	Do not interpret any more arguments as options.
 
@@ -447,6 +464,34 @@ command reads the index; while when `--[no-|force-]untracked-cache`
 are used, the untracked cache is immediately added to or removed from
 the index.
 
+File System Monitor
+-------------------
+
+This feature is intended to speed up git operations for repos that have
+large working directories.
+
+It enables git to work together with a file system monitor (see the
+"fsmonitor-watchman" section of linkgit:githooks[5]) that can
+inform it as to what files have been modified. This enables git to avoid
+having to lstat() every file to find modified files.
+
+When used in conjunction with the untracked cache, it can further improve
+performance by avoiding the cost of scaning the entire working directory
+looking for new files.
+
+If you want to enable (or disable) this feature, it is easier to use
+the `core.fsmonitor` configuration variable (see
+linkgit:git-config[1]) than using the `--fsmonitor` option to
+`git update-index` in each repository, especially if you want to do so
+across all repositories you use, because you can set the configuration
+variable to `true` (or `false`) in your `$HOME/.gitconfig` just once
+and have it affect all repositories you touch.
+
+When the `core.fsmonitor` configuration variable is changed, the
+file system monitor is added to or removed from the index the next time
+a command reads the index. When `--[no-]fsmonitor` are used, the file
+system monitor is immediately added to or removed from the index.
+
 Configuration
 -------------
 
diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 1bb4f92d4d..ae60559cd9 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -455,6 +455,34 @@ the name of the file that holds the e-mail to be sent.  Exiting with a
 non-zero status causes 'git send-email' to abort before sending any
 e-mails.
 
+fsmonitor-watchman
+~~~~~~~~~~~~~~~~~~
+
+This hook is invoked when the configuration option core.fsmonitor is
+set to .git/hooks/fsmonitor-watchman.  It takes two arguments, a version
+(currently 1) and the time in elapsed nanoseconds since midnight,
+January 1, 1970.
+
+The hook should output to stdout the list of all files in the working
+directory that may have changed since the requested time.  The logic
+should be inclusive so that it does not miss any potential changes.
+The paths should be relative to the root of the working directory
+and be separated by a single NUL.
+
+It is OK to include files which have not actually changed.  All changes
+including newly-created and deleted files should be included. When
+files are renamed, both the old and the new name should be included.
+
+Git will limit what files it checks for changes as well as which
+directories are checked for untracked files based on the path names
+given.
+
+An optimized way to tell git "all files have changed" is to return
+the filename '/'.
+
+The exit status determines whether git will use the data from the
+hook to limit its search.  On error, it will fall back to verifying
+all files and folders.
 
 GIT
 ---
diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index ade0b0c445..db3572626b 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -295,3 +295,22 @@ The remaining data of each directory block is grouped by type:
     in the previous ewah bitmap.
 
   - One NUL.
+
+== File System Monitor cache
+
+  The file system monitor cache tracks files for which the core.fsmonitor
+  hook has told us about changes.  The signature for this extension is
+  { 'F', 'S', 'M', 'N' }.
+
+  The extension starts with
+
+  - 32-bit version number: the current supported version is 1.
+
+  - 64-bit time: the extension data reflects all changes through the given
+	time which is stored as the nanoseconds elapsed since midnight,
+	January 1, 1970.
+
+  - 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap.
+
+  - An ewah bitmap, the n-th bit indicates whether the n-th index entry
+    is not CE_FSMONITOR_VALID.

diff --git a/Documentation/git-update-index.txt 
b/Documentation/git-update-index.txt
index 95231dbfcb..7c2f880a22 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -476,7 +476,7 @@ inform it as to what files have been modified. This 
enables git to avoid
  having to lstat() every file to find modified files.

  When used in conjunction with the untracked cache, it can further improve
-performance by avoiding the cost of scaning the entire working directory
+performance by avoiding the cost of scanning the entire working directory
  looking for new files.

  If you want to enable (or disable) this feature, it is easier to use


>> +If you want to enable (or disable) this feature, it is easier to use
>> +the `core.fsmonitor` configuration variable (see
>> +linkgit:git-config[1]) than using the `--fsmonitor` option to
>> +`git update-index` in each repository, especially if you want to do so
>> +across all repositories you use, because you can set the configuration
>> +variable to `true` (or `false`) in your `$HOME/.gitconfig` just once
>> +and have it affect all repositories you touch.
> 
> This is a mouthful. Maybe you could split it a little, perhaps like so:
> 
>    If you want to enable (or disable) this feature, you will probably
>    want to use the `core.fsmonitor` configuration variable (see
>    linkgit:git-config[1]). By setting it to `true` (or `false`) in your
>    `$HOME/.gitconfig`, it will affect all repositories you touch. For a
>    more fine-grained control, you can set it per repository, or use the
>    `--fsmonitor` option with `git update-index` in each repository.
> 

I'm going to sound like a broken record here. :) The description favored 
consistency with the untracked cache feature immediate above this entry. 
  It is literally a copy/paste/edit.

This is based on the assumption that the text had already been reviewed 
and found to be acceptable.  This also means if you have figured it out 
for one option, when you read the next, you're understanding can carry 
forward speeding up your comprehension.

> The part about $HOME/.gitconfig vs per-repo config is perhaps generic
> enough that it doesn't belong here. So it'd only be about config vs.
> option. Where to place the config item and what implications that has is
> arguably orthogonal to knowing that the option exists and what it does.
> 
> Martin
> 

Makefile, apply.c, builtin/update-index.c, cache.h, config.c, config.h, diff-lib.c, dir.c, dir.h, entry.c, environment.c, fsmonitor.c, fsmonitor.h, preload-index.c, read-cache.c, submodule.c, unpack-trees.c

diff --git a/Makefile b/Makefile
index f2bb7f2f63..9d6ec9c1e9 100644
--- a/Makefile
+++ b/Makefile
@@ -786,6 +786,7 @@ LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec_cmd.o
 LIB_OBJS += fetch-pack.o
 LIB_OBJS += fsck.o
+LIB_OBJS += fsmonitor.o
 LIB_OBJS += gettext.o
 LIB_OBJS += gpg-interface.o
 LIB_OBJS += graph.o
diff --git a/apply.c b/apply.c
index 71cbbd141c..9061cc5f15 100644
--- a/apply.c
+++ b/apply.c
@@ -3399,7 +3399,7 @@ static int verify_index_match(const struct cache_entry *ce, struct stat *st)
 			return -1;
 		return 0;
 	}
-	return ce_match_stat(ce, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+	return ce_match_stat(ce, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 }
 
 #define SUBMODULE_PATCH_WITHOUT_INDEX 1
diff --git a/builtin/update-index.c b/builtin/update-index.c
index e1ca0759d5..6f39ee9274 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -16,6 +16,7 @@
 #include "pathspec.h"
 #include "dir.h"
 #include "split-index.h"
+#include "fsmonitor.h"
 
 /*
  * Default to not allowing changes to the list of files. The
@@ -233,6 +234,7 @@ static int mark_ce_flags(const char *path, int flag, int mark)
 		else
 			active_cache[pos]->ce_flags &= ~flag;
 		active_cache[pos]->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(&the_index, active_cache[pos]);
 		cache_tree_invalidate_path(&the_index, path);
 		active_cache_changed |= CE_ENTRY_CHANGED;
 		return 0;
diff --git a/cache.h b/cache.h
index a916bc79e3..eccab968bd 100644
--- a/cache.h
+++ b/cache.h
@@ -203,6 +203,7 @@ struct cache_entry {
 #define CE_ADDED             (1 << 19)
 
 #define CE_HASHED            (1 << 20)
+#define CE_FSMONITOR_VALID   (1 << 21)
 #define CE_WT_REMOVE         (1 << 22) /* remove in work directory */
 #define CE_CONFLICTED        (1 << 23)
 
@@ -326,6 +327,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 #define CACHE_TREE_CHANGED	(1 << 5)
 #define SPLIT_INDEX_ORDERED	(1 << 6)
 #define UNTRACKED_CHANGED	(1 << 7)
+#define FSMONITOR_CHANGED	(1 << 8)
 
 struct split_index;
 struct untracked_cache;
@@ -344,6 +346,7 @@ struct index_state {
 	struct hashmap dir_hash;
 	unsigned char sha1[20];
 	struct untracked_cache *untracked;
+	uint64_t fsmonitor_last_update;
 };
 
 extern struct index_state the_index;
@@ -679,8 +682,10 @@ extern void *read_blob_data_from_index(const struct index_state *, const char *,
 #define CE_MATCH_IGNORE_MISSING		0x08
 /* enable stat refresh */
 #define CE_MATCH_REFRESH		0x10
-extern int ie_match_stat(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
-extern int ie_modified(const struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+/* do stat comparison even if CE_FSMONITOR_VALID is true */
+#define CE_MATCH_IGNORE_FSMONITOR 0X20
+extern int ie_match_stat(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
+extern int ie_modified(struct index_state *, const struct cache_entry *, struct stat *, unsigned int);
 
 #define HASH_WRITE_OBJECT 1
 #define HASH_FORMAT_CHECK 2
@@ -773,6 +778,7 @@ extern int core_apply_sparse_checkout;
 extern int precomposed_unicode;
 extern int protect_hfs;
 extern int protect_ntfs;
+extern const char *core_fsmonitor;
 
 /*
  * Include broken refs in all ref iterations, which will
diff --git a/config.c b/config.c
index d0d8ce823a..ddda96e584 100644
--- a/config.c
+++ b/config.c
@@ -2165,6 +2165,20 @@ int git_config_get_max_percent_split_change(void)
 	return -1; /* default value */
 }
 
+int git_config_get_fsmonitor(void)
+{
+	if (git_config_get_pathname("core.fsmonitor", &core_fsmonitor))
+		core_fsmonitor = getenv("GIT_FSMONITOR_TEST");
+
+	if (core_fsmonitor && !*core_fsmonitor)
+		core_fsmonitor = NULL;
+
+	if (core_fsmonitor)
+		return 1;
+
+	return 0;
+}
+
 NORETURN
 void git_die_config_linenr(const char *key, const char *filename, int linenr)
 {
diff --git a/config.h b/config.h
index 97471b8873..c9fcf691ba 100644
--- a/config.h
+++ b/config.h
@@ -211,6 +211,7 @@ extern int git_config_get_pathname(const char *key, const char **dest);
 extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
+extern int git_config_get_fsmonitor(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);
diff --git a/diff-lib.c b/diff-lib.c
index 2a52b07954..23c6d03ca9 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -12,6 +12,7 @@
 #include "refs.h"
 #include "submodule.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 /*
  * diff-files
@@ -228,6 +229,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 
 		if (!changed && !dirty_submodule) {
 			ce_mark_uptodate(ce);
+			mark_fsmonitor_valid(ce);
 			if (!DIFF_OPT_TST(&revs->diffopt, FIND_COPIES_HARDER))
 				continue;
 		}
diff --git a/dir.c b/dir.c
index 1c55dc3e36..ac9833daec 100644
--- a/dir.c
+++ b/dir.c
@@ -18,6 +18,7 @@
 #include "utf8.h"
 #include "varint.h"
 #include "ewah/ewok.h"
+#include "fsmonitor.h"
 
 /*
  * Tells read_directory_recursive how a file or directory should be treated.
@@ -1688,17 +1689,23 @@ static int valid_cached_dir(struct dir_struct *dir,
 	if (!untracked)
 		return 0;
 
-	if (stat(path->len ? path->buf : ".", &st)) {
-		invalidate_directory(dir->untracked, untracked);
-		memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
-		return 0;
-	}
-	if (!untracked->valid ||
-	    match_stat_data_racy(istate, &untracked->stat_data, &st)) {
-		if (untracked->valid)
+	/*
+	 * With fsmonitor, we can trust the untracked cache's valid field.
+	 */
+	refresh_fsmonitor(istate);
+	if (!(dir->untracked->use_fsmonitor && untracked->valid)) {
+		if (stat(path->len ? path->buf : ".", &st)) {
 			invalidate_directory(dir->untracked, untracked);
-		fill_stat_data(&untracked->stat_data, &st);
-		return 0;
+			memset(&untracked->stat_data, 0, sizeof(untracked->stat_data));
+			return 0;
+		}
+		if (!untracked->valid ||
+			match_stat_data_racy(istate, &untracked->stat_data, &st)) {
+			if (untracked->valid)
+				invalidate_directory(dir->untracked, untracked);
+			fill_stat_data(&untracked->stat_data, &st);
+			return 0;
+		}
 	}
 
 	if (untracked->check_only != !!check_only) {
diff --git a/dir.h b/dir.h
index e3717055d1..fab8fc1561 100644
--- a/dir.h
+++ b/dir.h
@@ -139,6 +139,8 @@ struct untracked_cache {
 	int gitignore_invalidated;
 	int dir_invalidated;
 	int dir_opened;
+	/* fsmonitor invalidation data */
+	unsigned int use_fsmonitor : 1;
 };
 
 struct dir_struct {
diff --git a/entry.c b/entry.c
index cb291aa88b..5e6794f9fc 100644
--- a/entry.c
+++ b/entry.c
@@ -4,6 +4,7 @@
 #include "streaming.h"
 #include "submodule.h"
 #include "progress.h"
+#include "fsmonitor.h"
 
 static void create_directories(const char *path, int path_len,
 			       const struct checkout *state)
@@ -357,6 +358,7 @@ static int write_entry(struct cache_entry *ce,
 			lstat(ce->name, &st);
 		fill_stat_cache_info(ce, &st);
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(state->istate, ce);
 		state->istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 	return 0;
@@ -402,7 +404,7 @@ int checkout_entry(struct cache_entry *ce,
 
 	if (!check_path(path.buf, path.len, &st, state->base_dir_len)) {
 		const struct submodule *sub;
-		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+		unsigned changed = ce_match_stat(ce, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 		/*
 		 * Needs to be checked before !changed returns early,
 		 * as the possibly empty directory was not changed
diff --git a/environment.c b/environment.c
index 3fd4b10845..d0b9fc64d4 100644
--- a/environment.c
+++ b/environment.c
@@ -76,6 +76,7 @@ int protect_hfs = PROTECT_HFS_DEFAULT;
 #define PROTECT_NTFS_DEFAULT 0
 #endif
 int protect_ntfs = PROTECT_NTFS_DEFAULT;
+const char *core_fsmonitor;
 
 /*
  * The character that begins a commented line in user-editable file
diff --git a/fsmonitor.c b/fsmonitor.c
new file mode 100644
index 0000000000..b8b2d88fe1
--- /dev/null
+++ b/fsmonitor.c
@@ -0,0 +1,253 @@
+#include "cache.h"
+#include "config.h"
+#include "dir.h"
+#include "ewah/ewok.h"
+#include "fsmonitor.h"
+#include "run-command.h"
+#include "strbuf.h"
+
+#define INDEX_EXTENSION_VERSION	(1)
+#define HOOK_INTERFACE_VERSION	(1)
+
+struct trace_key trace_fsmonitor = TRACE_KEY_INIT(FSMONITOR);
+
+static void fsmonitor_ewah_callback(size_t pos, void *is)
+{
+	struct index_state *istate = (struct index_state *)is;
+	struct cache_entry *ce = istate->cache[pos];
+
+	ce->ce_flags &= ~CE_FSMONITOR_VALID;
+}
+
+int read_fsmonitor_extension(struct index_state *istate, const void *data,
+	unsigned long sz)
+{
+	const char *index = data;
+	uint32_t hdr_version;
+	uint32_t ewah_size;
+	struct ewah_bitmap *fsmonitor_dirty;
+	int i;
+	int ret;
+
+	if (sz < sizeof(uint32_t) + sizeof(uint64_t) + sizeof(uint32_t))
+		return error("corrupt fsmonitor extension (too short)");
+
+	hdr_version = get_be32(index);
+	index += sizeof(uint32_t);
+	if (hdr_version != INDEX_EXTENSION_VERSION)
+		return error("bad fsmonitor version %d", hdr_version);
+
+	istate->fsmonitor_last_update = get_be64(index);
+	index += sizeof(uint64_t);
+
+	ewah_size = get_be32(index);
+	index += sizeof(uint32_t);
+
+	fsmonitor_dirty = ewah_new();
+	ret = ewah_read_mmap(fsmonitor_dirty, index, ewah_size);
+	if (ret != ewah_size) {
+		ewah_free(fsmonitor_dirty);
+		return error("failed to parse ewah bitmap reading fsmonitor index extension");
+	}
+
+	if (git_config_get_fsmonitor()) {
+		/* Mark all entries valid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags |= CE_FSMONITOR_VALID;
+
+		/* Mark all previously saved entries as dirty */
+		ewah_each_bit(fsmonitor_dirty, fsmonitor_ewah_callback, istate);
+
+		/* Now mark the untracked cache for fsmonitor usage */
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 1;
+	}
+	ewah_free(fsmonitor_dirty);
+
+	trace_printf_key(&trace_fsmonitor, "read fsmonitor extension successful");
+	return 0;
+}
+
+void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate)
+{
+	uint32_t hdr_version;
+	uint64_t tm;
+	struct ewah_bitmap *bitmap;
+	int i;
+	uint32_t ewah_start;
+	uint32_t ewah_size = 0;
+	int fixup = 0;
+
+	put_be32(&hdr_version, INDEX_EXTENSION_VERSION);
+	strbuf_add(sb, &hdr_version, sizeof(uint32_t));
+
+	put_be64(&tm, istate->fsmonitor_last_update);
+	strbuf_add(sb, &tm, sizeof(uint64_t));
+	fixup = sb->len;
+	strbuf_add(sb, &ewah_size, sizeof(uint32_t)); /* we'll fix this up later */
+
+	ewah_start = sb->len;
+	bitmap = ewah_new();
+	for (i = 0; i < istate->cache_nr; i++)
+		if (!(istate->cache[i]->ce_flags & CE_FSMONITOR_VALID))
+			ewah_set(bitmap, i);
+	ewah_serialize_strbuf(bitmap, sb);
+	ewah_free(bitmap);
+
+	/* fix up size field */
+	put_be32(&ewah_size, sb->len - ewah_start);
+	memcpy(sb->buf + fixup, &ewah_size, sizeof(uint32_t));
+
+	trace_printf_key(&trace_fsmonitor, "write fsmonitor extension successful");
+}
+
+/*
+ * Call the query-fsmonitor hook passing the time of the last saved results.
+ */
+static int query_fsmonitor(int version, uint64_t last_update, struct strbuf *query_result)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	char ver[64];
+	char date[64];
+	const char *argv[4];
+
+	if (!(argv[0] = core_fsmonitor))
+		return -1;
+
+	snprintf(ver, sizeof(version), "%d", version);
+	snprintf(date, sizeof(date), "%" PRIuMAX, (uintmax_t)last_update);
+	argv[1] = ver;
+	argv[2] = date;
+	argv[3] = NULL;
+	cp.argv = argv;
+	cp.use_shell = 1;
+
+	return capture_command(&cp, query_result, 1024);
+}
+
+static void fsmonitor_refresh_callback(struct index_state *istate, const char *name)
+{
+	int pos = index_name_pos(istate, name, strlen(name));
+
+	if (pos >= 0) {
+		struct cache_entry *ce = istate->cache[pos];
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+	}
+
+	/*
+	 * Mark the untracked cache dirty even if it wasn't found in the index
+	 * as it could be a new untracked file.
+	 */
+	trace_printf_key(&trace_fsmonitor, "fsmonitor_refresh_callback '%s'", name);
+	untracked_cache_invalidate_path(istate, name);
+}
+
+void refresh_fsmonitor(struct index_state *istate)
+{
+	static int has_run_once = 0;
+	struct strbuf query_result = STRBUF_INIT;
+	int query_success = 0;
+	size_t bol; /* beginning of line */
+	uint64_t last_update;
+	char *buf;
+	int i;
+
+	if (!core_fsmonitor || has_run_once)
+		return;
+	has_run_once = 1;
+
+	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
+	/*
+	 * This could be racy so save the date/time now and query_fsmonitor
+	 * should be inclusive to ensure we don't miss potential changes.
+	 */
+	last_update = getnanotime();
+
+	/*
+	 * If we have a last update time, call query_fsmonitor for the set of
+	 * changes since that time, else assume everything is possibly dirty
+	 * and check it all.
+	 */
+	if (istate->fsmonitor_last_update) {
+		query_success = !query_fsmonitor(HOOK_INTERFACE_VERSION,
+			istate->fsmonitor_last_update, &query_result);
+		trace_performance_since(last_update, "fsmonitor process '%s'", core_fsmonitor);
+		trace_printf_key(&trace_fsmonitor, "fsmonitor process '%s' returned %s",
+			core_fsmonitor, query_success ? "success" : "failure");
+	}
+
+	/* a fsmonitor process can return '*' to indicate all entries are invalid */
+	if (query_success && query_result.buf[0] != '/') {
+		/* Mark all entries returned by the monitor as dirty */
+		buf = query_result.buf;
+		bol = 0;
+		for (i = 0; i < query_result.len; i++) {
+			if (buf[i] != '\0')
+				continue;
+			fsmonitor_refresh_callback(istate, buf + bol);
+			bol = i + 1;
+		}
+		if (bol < query_result.len)
+			fsmonitor_refresh_callback(istate, buf + bol);
+	} else {
+		/* Mark all entries invalid */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		if (istate->untracked)
+			istate->untracked->use_fsmonitor = 0;
+	}
+	strbuf_release(&query_result);
+
+	/* Now that we've updated istate, save the last_update time */
+	istate->fsmonitor_last_update = last_update;
+}
+
+void add_fsmonitor(struct index_state *istate)
+{
+	int i;
+
+	if (!istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "add fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = getnanotime();
+
+		/* reset the fsmonitor state */
+		for (i = 0; i < istate->cache_nr; i++)
+			istate->cache[i]->ce_flags &= ~CE_FSMONITOR_VALID;
+
+		/* reset the untracked cache */
+		if (istate->untracked) {
+			add_untracked_cache(istate);
+			istate->untracked->use_fsmonitor = 1;
+		}
+
+		/* Update the fsmonitor state */
+		refresh_fsmonitor(istate);
+	}
+}
+
+void remove_fsmonitor(struct index_state *istate)
+{
+	if (istate->fsmonitor_last_update) {
+		trace_printf_key(&trace_fsmonitor, "remove fsmonitor");
+		istate->cache_changed |= FSMONITOR_CHANGED;
+		istate->fsmonitor_last_update = 0;
+	}
+}
+
+void tweak_fsmonitor(struct index_state *istate)
+{
+	switch (git_config_get_fsmonitor()) {
+	case -1: /* keep: do nothing */
+		break;
+	case 0: /* false */
+		remove_fsmonitor(istate);
+		break;
+	case 1: /* true */
+		add_fsmonitor(istate);
+		break;
+	default: /* unknown value: do nothing */
+		break;
+	}
+}
diff --git a/fsmonitor.h b/fsmonitor.h
new file mode 100644
index 0000000000..c2240b811a
--- /dev/null
+++ b/fsmonitor.h
@@ -0,0 +1,61 @@
+#ifndef FSMONITOR_H
+#define FSMONITOR_H
+
+extern struct trace_key trace_fsmonitor;
+
+/*
+ * Read the the fsmonitor index extension and (if configured) restore the
+ * CE_FSMONITOR_VALID state.
+ */
+extern int read_fsmonitor_extension(struct index_state *istate, const void *data, unsigned long sz);
+
+/*
+ * Write the CE_FSMONITOR_VALID state into the fsmonitor index extension.
+ */
+extern void write_fsmonitor_extension(struct strbuf *sb, struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension
+ */
+extern void add_fsmonitor(struct index_state *istate);
+extern void remove_fsmonitor(struct index_state *istate);
+
+/*
+ * Add/remove the fsmonitor index extension as necessary based on the current
+ * core.fsmonitor setting.
+ */
+extern void tweak_fsmonitor(struct index_state *istate);
+
+/*
+ * Run the configured fsmonitor integration script and clear the
+ * CE_FSMONITOR_VALID bit for any files returned as dirty.  Also invalidate
+ * any corresponding untracked cache directory structures. Optimized to only
+ * run the first time it is called.
+ */
+extern void refresh_fsmonitor(struct index_state *istate);
+
+/*
+ * Set the given cache entries CE_FSMONITOR_VALID bit.
+ */
+static inline void mark_fsmonitor_valid(struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags |= CE_FSMONITOR_VALID;
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_clean '%s'", ce->name);
+	}
+}
+
+/*
+ * Clear the given cache entry's CE_FSMONITOR_VALID bit and invalidate any
+ * corresponding untracked cache directory structures.
+ */
+static inline void mark_fsmonitor_invalid(struct index_state *istate, struct cache_entry *ce)
+{
+	if (core_fsmonitor) {
+		ce->ce_flags &= ~CE_FSMONITOR_VALID;
+		untracked_cache_invalidate_path(istate, ce->name);
+		trace_printf_key(&trace_fsmonitor, "mark_fsmonitor_invalid '%s'", ce->name);
+	}
+}
+
+#endif
diff --git a/preload-index.c b/preload-index.c
index 75564c497a..2a83255e4e 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -4,6 +4,7 @@
 #include "cache.h"
 #include "pathspec.h"
 #include "dir.h"
+#include "fsmonitor.h"
 
 #ifdef NO_PTHREADS
 static void preload_index(struct index_state *index,
@@ -55,15 +56,18 @@ static void *preload_thread(void *_data)
 			continue;
 		if (ce_skip_worktree(ce))
 			continue;
+		if (ce->ce_flags & CE_FSMONITOR_VALID)
+			continue;
 		if (!ce_path_match(ce, &p->pathspec, NULL))
 			continue;
 		if (threaded_has_symlink_leading_path(&cache, ce->name, ce_namelen(ce)))
 			continue;
 		if (lstat(ce->name, &st))
 			continue;
-		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY))
+		if (ie_match_stat(index, ce, &st, CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR))
 			continue;
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
 	} while (--nr > 0);
 	cache_def_clear(&cache);
 	return NULL;
diff --git a/read-cache.c b/read-cache.c
index 40da87ea71..53093dbebf 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -19,6 +19,7 @@
 #include "varint.h"
 #include "split-index.h"
 #include "utf8.h"
+#include "fsmonitor.h"
 
 /* Mask for the name length in ce_flags in the on-disk index */
 
@@ -38,11 +39,12 @@
 #define CACHE_EXT_RESOLVE_UNDO 0x52455543 /* "REUC" */
 #define CACHE_EXT_LINK 0x6c696e6b	  /* "link" */
 #define CACHE_EXT_UNTRACKED 0x554E5452	  /* "UNTR" */
+#define CACHE_EXT_FSMONITOR 0x46534D4E	  /* "FSMN" */
 
 /* changes that can be kept in $GIT_DIR/index (basically all extensions) */
 #define EXTMASK (RESOLVE_UNDO_CHANGED | CACHE_TREE_CHANGED | \
 		 CE_ENTRY_ADDED | CE_ENTRY_REMOVED | CE_ENTRY_CHANGED | \
-		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED)
+		 SPLIT_INDEX_ORDERED | UNTRACKED_CHANGED | FSMONITOR_CHANGED)
 
 struct index_state the_index;
 static const char *alternate_index_output;
@@ -62,6 +64,7 @@ static void replace_index_entry(struct index_state *istate, int nr, struct cache
 	free(old);
 	set_index_entry(istate, nr, ce);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 }
 
@@ -150,8 +153,10 @@ void fill_stat_cache_info(struct cache_entry *ce, struct stat *st)
 	if (assume_unchanged)
 		ce->ce_flags |= CE_VALID;
 
-	if (S_ISREG(st->st_mode))
+	if (S_ISREG(st->st_mode)) {
 		ce_mark_uptodate(ce);
+		mark_fsmonitor_valid(ce);
+	}
 }
 
 static int ce_compare_data(const struct cache_entry *ce, struct stat *st)
@@ -300,7 +305,7 @@ int match_stat_data_racy(const struct index_state *istate,
 	return match_stat_data(sd, st);
 }
 
-int ie_match_stat(const struct index_state *istate,
+int ie_match_stat(struct index_state *istate,
 		  const struct cache_entry *ce, struct stat *st,
 		  unsigned int options)
 {
@@ -308,7 +313,10 @@ int ie_match_stat(const struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int assume_racy_is_modified = options & CE_MATCH_RACY_IS_DIRTY;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * If it's marked as always valid in the index, it's
 	 * valid whatever the checked-out copy says.
@@ -319,6 +327,8 @@ int ie_match_stat(const struct index_state *istate,
 		return 0;
 	if (!ignore_valid && (ce->ce_flags & CE_VALID))
 		return 0;
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID))
+		return 0;
 
 	/*
 	 * Intent-to-add entries have not been added, so the index entry
@@ -356,7 +366,7 @@ int ie_match_stat(const struct index_state *istate,
 	return changed;
 }
 
-int ie_modified(const struct index_state *istate,
+int ie_modified(struct index_state *istate,
 		const struct cache_entry *ce,
 		struct stat *st, unsigned int options)
 {
@@ -631,7 +641,7 @@ int add_to_index(struct index_state *istate, const char *path, struct stat *st,
 	int size, namelen, was_same;
 	mode_t st_mode = st->st_mode;
 	struct cache_entry *ce, *alias;
-	unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY;
+	unsigned ce_option = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR;
 	int verbose = flags & (ADD_CACHE_VERBOSE | ADD_CACHE_PRETEND);
 	int pretend = flags & ADD_CACHE_PRETEND;
 	int intent_only = flags & ADD_CACHE_INTENT;
@@ -777,6 +787,7 @@ int chmod_index_entry(struct index_state *istate, struct cache_entry *ce,
 	}
 	cache_tree_invalidate_path(istate, ce->name);
 	ce->ce_flags |= CE_UPDATE_IN_BASE;
+	mark_fsmonitor_invalid(istate, ce);
 	istate->cache_changed |= CE_ENTRY_CHANGED;
 
 	return 0;
@@ -1228,10 +1239,13 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 	int ignore_valid = options & CE_MATCH_IGNORE_VALID;
 	int ignore_skip_worktree = options & CE_MATCH_IGNORE_SKIP_WORKTREE;
 	int ignore_missing = options & CE_MATCH_IGNORE_MISSING;
+	int ignore_fsmonitor = options & CE_MATCH_IGNORE_FSMONITOR;
 
 	if (!refresh || ce_uptodate(ce))
 		return ce;
 
+	if (!ignore_fsmonitor)
+		refresh_fsmonitor(istate);
 	/*
 	 * CE_VALID or CE_SKIP_WORKTREE means the user promised us
 	 * that the change to the work tree does not matter and told
@@ -1245,6 +1259,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 		ce_mark_uptodate(ce);
 		return ce;
 	}
+	if (!ignore_fsmonitor && (ce->ce_flags & CE_FSMONITOR_VALID)) {
+		ce_mark_uptodate(ce);
+		return ce;
+	}
 
 	if (has_symlink_leading_path(ce->name, ce_namelen(ce))) {
 		if (ignore_missing)
@@ -1282,8 +1300,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 			 * because CE_UPTODATE flag is in-core only;
 			 * we are not going to write this change out.
 			 */
-			if (!S_ISGITLINK(ce->ce_mode))
+			if (!S_ISGITLINK(ce->ce_mode)) {
 				ce_mark_uptodate(ce);
+				mark_fsmonitor_valid(ce);
+			}
 			return ce;
 		}
 	}
@@ -1336,7 +1356,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 	int first = 1;
 	int in_porcelain = (flags & REFRESH_IN_PORCELAIN);
 	unsigned int options = (CE_MATCH_REFRESH |
-				(really ? CE_MATCH_IGNORE_VALID : 0) |
+				(really ? CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_FSMONITOR : 0) |
 				(not_new ? CE_MATCH_IGNORE_MISSING : 0));
 	const char *modified_fmt;
 	const char *deleted_fmt;
@@ -1391,6 +1411,7 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 				 */
 				ce->ce_flags &= ~CE_VALID;
 				ce->ce_flags |= CE_UPDATE_IN_BASE;
+				mark_fsmonitor_invalid(istate, ce);
 				istate->cache_changed |= CE_ENTRY_CHANGED;
 			}
 			if (quiet)
@@ -1550,6 +1571,9 @@ static int read_index_extension(struct index_state *istate,
 	case CACHE_EXT_UNTRACKED:
 		istate->untracked = read_untracked_extension(data, sz);
 		break;
+	case CACHE_EXT_FSMONITOR:
+		read_fsmonitor_extension(istate, data, sz);
+		break;
 	default:
 		if (*ext < 'A' || 'Z' < *ext)
 			return error("index uses %.4s extension, which we do not understand",
@@ -1722,6 +1746,7 @@ static void post_read_index_from(struct index_state *istate)
 	check_ce_order(istate);
 	tweak_untracked_cache(istate);
 	tweak_split_index(istate);
+	tweak_fsmonitor(istate);
 }
 
 /* remember to discard_cache() before reading a different cache! */
@@ -2306,6 +2331,16 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 		if (err)
 			return -1;
 	}
+	if (!strip_extensions && istate->fsmonitor_last_update) {
+		struct strbuf sb = STRBUF_INIT;
+
+		write_fsmonitor_extension(&sb, istate);
+		err = write_index_ext_header(&c, newfd, CACHE_EXT_FSMONITOR, sb.len) < 0
+			|| ce_write(&c, newfd, sb.buf, sb.len) < 0;
+		strbuf_release(&sb);
+		if (err)
+			return -1;
+	}
 
 	if (ce_flush(&c, newfd, istate->sha1))
 		return -1;
diff --git a/submodule.c b/submodule.c
index 3cea8221e0..8a931a1aaa 100644
--- a/submodule.c
+++ b/submodule.c
@@ -62,7 +62,7 @@ int is_staging_gitmodules_ok(const struct index_state *istate)
 	if ((pos >= 0) && (pos < istate->cache_nr)) {
 		struct stat st;
 		if (lstat(GITMODULES_FILE, &st) == 0 &&
-		    ce_match_stat(istate->cache[pos], &st, 0) & DATA_CHANGED)
+		    ce_match_stat(istate->cache[pos], &st, CE_MATCH_IGNORE_FSMONITOR) & DATA_CHANGED)
 			return 0;
 	}
 
diff --git a/unpack-trees.c b/unpack-trees.c
index 71b70ccb12..f724a61ac0 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -14,6 +14,7 @@
 #include "dir.h"
 #include "submodule.h"
 #include "submodule-config.h"
+#include "fsmonitor.h"
 
 /*
  * Error messages expected by scripts out of plumbing commands such as
@@ -408,6 +409,7 @@ static int apply_sparse_checkout(struct index_state *istate,
 		ce->ce_flags &= ~CE_SKIP_WORKTREE;
 	if (was_skip_worktree != ce_skip_worktree(ce)) {
 		ce->ce_flags |= CE_UPDATE_IN_BASE;
+		mark_fsmonitor_invalid(istate, ce);
 		istate->cache_changed |= CE_ENTRY_CHANGED;
 	}
 
@@ -1454,7 +1456,7 @@ static int verify_uptodate_1(const struct cache_entry *ce,
 		return 0;
 
 	if (!lstat(ce->name, &st)) {
-		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
+		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR;
 		unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);
 
 		if (submodule_from_ce(ce)) {
@@ -1610,7 +1612,7 @@ static int icase_exists(struct unpack_trees_options *o, const char *name, int le
 	const struct cache_entry *src;
 
 	src = index_file_exists(o->src_index, name, len, 1);
-	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
+	return src && !ie_match_stat(o->src_index, src, st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
 }
 
 static int check_ok_to_remove(const char *name, int len, int dtype,
@@ -2134,7 +2136,7 @@ int oneway_merge(const struct cache_entry * const *src,
 		if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old)) {
 			struct stat st;
 			if (lstat(old->name, &st) ||
-			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
+			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR))
 				update |= CE_UPDATE;
 		}
 		add_entry(o, old, update, 0);

fsmonitor.h, fsmonitor.c

diff --git a/fsmonitor.h b/fsmonitor.h
index c2240b811a..03bf3efe61 100644
--- a/fsmonitor.h
+++ b/fsmonitor.h
@@ -34,9 +34,11 @@ extern void tweak_fsmonitor(struct index_state *istate);
   */
  extern void refresh_fsmonitor(struct index_state *istate);

-/*
- * Set the given cache entries CE_FSMONITOR_VALID bit.
- */
+/*
+ * Set the given cache entries CE_FSMONITOR_VALID bit. This should be
+ * called any time the cache entry has been updated to reflect the
+ * current state of the file on disk.
+ */
  static inline void mark_fsmonitor_valid(struct cache_entry *ce)
  {
         if (core_fsmonitor) {
@@ -46,8 +48,11 @@ static inline void mark_fsmonitor_valid(struct 
cache_entry *ce)
  }

  /*
- * Clear the given cache entry's CE_FSMONITOR_VALID bit and invalidate any
- * corresponding untracked cache directory structures.
+ * Clear the given cache entry's CE_FSMONITOR_VALID bit and invalidate
+ * any corresponding untracked cache directory structures. This should
+ * be called any time git creates or modifies a file that should
+ * trigger an lstat() or invalidate the untracked cache for the
+ * corresponding directory
   */
  static inline void mark_fsmonitor_invalid(struct index_state *istate, 
struct cache_entry *ce)
  {


>> diff --git a/dir.h b/dir.h
>> index e3717055d1..fab8fc1561 100644
>> --- a/dir.h
>> +++ b/dir.h
>> @@ -139,6 +139,8 @@ struct untracked_cache {
>>   	int gitignore_invalidated;
>>   	int dir_invalidated;
>>   	int dir_opened;
>> +	/* fsmonitor invalidation data */
>> +	unsigned int use_fsmonitor : 1;
> 
> This makes it look like we will add a bit more fields in later
> patches that are about "invalidation" around fsmonitor, but it
> appears that such an addition never happens til the end of the
> series.  And use_fsmonitor boolean does not seem to be what the
> comment says---it just tells us if fsmonitor is in use in the
> operation of the untracked cache, no?

I don't have any more planned bit fields.  I only needed a single bit so 
used a bit field just in case someone else comes by later and needs 
another bit.  If you aren't worried about that, we can just make this an 
int.

Correct.  The bit just indicates whether fsmonitor has been used to 
ensure the cache is current or if it needs to be checked.  I have 
comments to this effect where the flag is used in the code but could 
move/duplicate them into the header if wished. For example:

/* With fsmonitor, we can trust the untracked cache's valid field. */

and

/* Now mark the untracked cache for fsmonitor usage */

> 
>> diff --git a/entry.c b/entry.c
>> index cb291aa88b..5e6794f9fc 100644
>> --- a/entry.c
>> +++ b/entry.c
>> @@ -4,6 +4,7 @@
>>   #include "streaming.h"
>>   #include "submodule.h"
>>   #include "progress.h"
>> +#include "fsmonitor.h"
>>   
>>   static void create_directories(const char *path, int path_len,
>>   			       const struct checkout *state)
>> @@ -357,6 +358,7 @@ static int write_entry(struct cache_entry *ce,
>>   			lstat(ce->name, &st);
>>   		fill_stat_cache_info(ce, &st);
>>   		ce->ce_flags |= CE_UPDATE_IN_BASE;
>> +		mark_fsmonitor_invalid(state->istate, ce);
>>   		state->istate->cache_changed |= CE_ENTRY_CHANGED;
> 
> Similar to "how does mark_fsmonitor_valid() relate to
> ce_mark_uptodate()?" question earlier, mark_fsmonitor_invalid()
> seems to often appear in pairs with the update to cache_changed.
> Are there cases where we need CE_ENTRY_CHANGED bit added to the
> index state yet we do not want to call mark_fsmonitor_invalid()?  I
> am wondering if there should be a single helper function that lets
> callers say "I changed this ce in this istate this way.  Please
> update CE_VALID, CE_UPDATE_IN_BASE and CE_FSMONITOR_VALID
> accordingly".
> 
> That "changed _this way_" is deliberately vague in my question
> above, because it is not yet clear to me when mark-valid and
> mark-invalid should and should not be called from the series.
> 

Please let me know if my comment/patch above does not address this 
concern sufficiently.

>> +	/* a fsmonitor process can return '*' to indicate all entries are invalid */
>> +	if (query_success && query_result.buf[0] != '/') {
>> +		/* Mark all entries returned by the monitor as dirty */
> 
> The comment talks about '*' and code checks if it is not '/'?  The
> else clause of this if/else handles the "invalidate all" case, so
> shouldn't we be comparing with '*' instead here?
> 
> Ah, fsmonitor-watchman section of the doc tells us to write the hook
> in a way to return slash, so the comment above the code is stale and
> the comparison with '/' is what we want, I guess.
> 

Correct.  Sorry about missing that.  Here is a patch that can be 
squashed in.

diff --git a/fsmonitor.c b/fsmonitor.c
index b8b2d88fe1..7c1540c054 100644
--- a/fsmonitor.c
+++ b/fsmonitor.c
@@ -176,7 +176,7 @@ void refresh_fsmonitor(struct index_state *istate)
                         core_fsmonitor, query_success ? "success" : 
"failure");
         }

-       /* a fsmonitor process can return '*' to indicate all entries 
are invalid */
+       /* a fsmonitor process can return '/' to indicate all entries 
are invalid */
         if (query_success && query_result.buf[0] != '/') {
                 /* Mark all entries returned by the monitor as dirty */
                 buf = query_result.buf;

apply.c, entry.c, read-cache.c, unpack-trees.c

diff --git a/apply.c b/apply.c
index 9061cc5f15..71cbbd141c 100644
--- a/apply.c
+++ b/apply.c
@@ -3399,7 +3399,7 @@ static int verify_index_match(const struct 
cache_entry *ce, struct stat *st)
  			return -1;
  		return 0;
  	}
-	return ce_match_stat(ce, st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
+	return ce_match_stat(ce, st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
  }

  #define SUBMODULE_PATCH_WITHOUT_INDEX 1
diff --git a/entry.c b/entry.c
index 5e6794f9fc..3a7b667373 100644
--- a/entry.c
+++ b/entry.c
@@ -404,7 +404,7 @@ int checkout_entry(struct cache_entry *ce,

  	if (!check_path(path.buf, path.len, &st, state->base_dir_len)) {
  		const struct submodule *sub;
-		unsigned changed = ce_match_stat(ce, &st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
+		unsigned changed = ce_match_stat(ce, &st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
  		/*
  		 * Needs to be checked before !changed returns early,
  		 * as the possibly empty directory was not changed
diff --git a/read-cache.c b/read-cache.c
index 53093dbebf..05c0a33fdd 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -641,7 +641,7 @@ int add_to_index(struct index_state *istate, const 
char *path, struct stat *st,
  	int size, namelen, was_same;
  	mode_t st_mode = st->st_mode;
  	struct cache_entry *ce, *alias;
-	unsigned ce_option = 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY|CE_MATCH_IGNORE_FSMONITOR;
+	unsigned ce_option = 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_RACY_IS_DIRTY;
  	int verbose = flags & (ADD_CACHE_VERBOSE | ADD_CACHE_PRETEND);
  	int pretend = flags & ADD_CACHE_PRETEND;
  	int intent_only = flags & ADD_CACHE_INTENT;
@@ -1356,7 +1356,7 @@ int refresh_index(struct index_state *istate, 
unsigned int flags,
  	int first = 1;
  	int in_porcelain = (flags & REFRESH_IN_PORCELAIN);
  	unsigned int options = (CE_MATCH_REFRESH |
-				(really ? CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_FSMONITOR : 0) |
+				(really ? CE_MATCH_IGNORE_VALID : 0) |
  				(not_new ? CE_MATCH_IGNORE_MISSING : 0));
  	const char *modified_fmt;
  	const char *deleted_fmt;
diff --git a/unpack-trees.c b/unpack-trees.c
index f724a61ac0..1f5d371636 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1456,7 +1456,7 @@ static int verify_uptodate_1(const struct 
cache_entry *ce,
  		return 0;

  	if (!lstat(ce->name, &st)) {
-		int flags = 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR;
+		int flags = CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE;
  		unsigned changed = ie_match_stat(o->src_index, ce, &st, flags);

  		if (submodule_from_ce(ce)) {
@@ -1612,7 +1612,7 @@ static int icase_exists(struct 
unpack_trees_options *o, const char *name, int le
  	const struct cache_entry *src;

  	src = index_file_exists(o->src_index, name, len, 1);
-	return src && !ie_match_stat(o->src_index, src, st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR);
+	return src && !ie_match_stat(o->src_index, src, st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE);
  }

  static int check_ok_to_remove(const char *name, int len, int dtype,
@@ -2136,7 +2136,7 @@ int oneway_merge(const struct cache_entry * const 
*src,
  		if (o->reset && o->update && !ce_uptodate(old) && 
!ce_skip_worktree(old)) {
  			struct stat st;
  			if (lstat(old->name, &st) ||
-			    ie_match_stat(o->src_index, old, &st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE|CE_MATCH_IGNORE_FSMONITOR))
+			    ie_match_stat(o->src_index, old, &st, 
CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))
  				update |= CE_UPDATE;
  		}
  		add_entry(o, old, update, 0);