# [PATCH 0/2] packfile: small syscall reductions 6 messages from 2019-12-26 to 2019-12-26. Participants: Eric Wong, Junio C Hamano. Thread: https://gitlist.dev/t/52520 ## Eric Wong, 2019-12-26 10:42 Subject: [PATCH 0/2] packfile: small syscall reductions Message-ID: <20191226104220.27325-1-e@80x24.org> URL: https://gitlist.dev/e/20191226104220.27325-1-e%4080x24.org ``` Being an avid user of strace, redundant syscalls clutter up the output and bother me, so I removed some. Folks with CPU mitigations enabled might also benefit from fewer syscalls. Eric Wong (2): packfile: remove redundant fcntl F_GETFD/F_SETFD packfile: replace lseek+read with pread packfile.c | 16 ++-------------- 1 file changed, 2 insertions(+), 14 deletions(-) ``` ## Eric Wong, 2019-12-26 10:42 Subject: [PATCH 1/2] packfile: remove redundant fcntl F_GETFD/F_SETFD Message-ID: <20191226104220.27325-2-e@80x24.org> URL: https://gitlist.dev/e/20191226104220.27325-2-e%4080x24.org In-Reply-To: <20191226104220.27325-1-e@80x24.org> ``` git_open sets close-on-exec since cd66ada06588f797 ("sha1_file: open window into packfiles with O_CLOEXEC"). There's no reason to keep using fcntl to set the close-on-exec flag, anymore. Signed-off-by: Eric Wong --- packfile.c | 11 ----------- 1 file changed, 11 deletions(-) diff --git a/packfile.c b/packfile.c index f0dc63e92f..1821cb7a3d 100644 --- a/packfile.c +++ b/packfile.c @@ -510,7 +510,6 @@ static int open_packed_git_1(struct packed_git *p) struct pack_header hdr; unsigned char hash[GIT_MAX_RAWSZ]; unsigned char *idx_hash; - long fd_flag; ssize_t read_result; const unsigned hashsz = the_hash_algo->rawsz; @@ -554,16 +553,6 @@ static int open_packed_git_1(struct packed_git *p) } else if (p->pack_size != st.st_size) return error("packfile %s size changed", p->pack_name); - /* We leave these file descriptors open with sliding mmap; - * there is no point keeping them open across exec(), though. - */ - fd_flag = fcntl(p->pack_fd, F_GETFD, 0); - if (fd_flag < 0) - return error("cannot determine file descriptor flags"); - fd_flag |= FD_CLOEXEC; - if (fcntl(p->pack_fd, F_SETFD, fd_flag) == -1) - return error("cannot set FD_CLOEXEC"); - /* Verify we recognize this pack file format. */ read_result = read_in_full(p->pack_fd, &hdr, sizeof(hdr)); if (read_result < 0) ``` ## Eric Wong, 2019-12-26 10:42 Subject: [PATCH 2/2] packfile: replace lseek+read with pread Message-ID: <20191226104220.27325-3-e@80x24.org> URL: https://gitlist.dev/e/20191226104220.27325-3-e%4080x24.org In-Reply-To: <20191226104220.27325-1-e@80x24.org> ``` We already have pread emulation for portability, so there's there's no reason to make two syscalls where one suffices. Furthermore, readers of the packfile will be using mmap (or pread to emulate mmap), anyways, so the file description offset does not matter in this case. Signed-off-by: Eric Wong --- packfile.c | 5 ++--- 1 file changed, 2 insertions(+), 3 deletions(-) diff --git a/packfile.c b/packfile.c index 1821cb7a3d..7e7c04e4d8 100644 --- a/packfile.c +++ b/packfile.c @@ -576,9 +576,8 @@ static int open_packed_git_1(struct packed_git *p) " while index indicates %"PRIu32" objects", p->pack_name, ntohl(hdr.hdr_entries), p->num_objects); - if (lseek(p->pack_fd, p->pack_size - hashsz, SEEK_SET) == -1) - return error("end of packfile %s is unavailable", p->pack_name); - read_result = read_in_full(p->pack_fd, hash, hashsz); + read_result = pread_in_full(p->pack_fd, hash, hashsz, + p->pack_size - hashsz); if (read_result < 0) return error_errno("error reading from %s", p->pack_name); if (read_result != hashsz) ``` ## Junio C Hamano, 2019-12-26 18:23 Subject: Re: [PATCH 2/2] packfile: replace lseek+read with pread Message-ID: URL: https://gitlist.dev/e/xmqqzhffkku2.fsf%40gitster-ct.c.googlers.com In-Reply-To: <20191226104220.27325-3-e@80x24.org> ``` Eric Wong writes: > We already have pread emulation for portability, so there's > there's no reason to make two syscalls where one suffices. > > Furthermore, readers of the packfile will be using mmap > (or pread to emulate mmap), anyways, so the file description > offset does not matter in this case. s/description/descriptor/ probably. After seeking to the packfile trailer and reading the pack id hash using lseek+read, this helper function does not read from the file descriptor, and the sole caller of it closes the file descriptor immediately after it returns, which means the read file offset after reading the packfile trailer does not matter. So this conversion is correct. Thanks for a careful analysis. Will queue both patches. > Signed-off-by: Eric Wong > --- > packfile.c | 5 ++--- > 1 file changed, 2 insertions(+), 3 deletions(-) > > diff --git a/packfile.c b/packfile.c > index 1821cb7a3d..7e7c04e4d8 100644 > --- a/packfile.c > +++ b/packfile.c > @@ -576,9 +576,8 @@ static int open_packed_git_1(struct packed_git *p) > " while index indicates %"PRIu32" objects", > p->pack_name, ntohl(hdr.hdr_entries), > p->num_objects); > - if (lseek(p->pack_fd, p->pack_size - hashsz, SEEK_SET) == -1) > - return error("end of packfile %s is unavailable", p->pack_name); > - read_result = read_in_full(p->pack_fd, hash, hashsz); > + read_result = pread_in_full(p->pack_fd, hash, hashsz, > + p->pack_size - hashsz); > if (read_result < 0) > return error_errno("error reading from %s", p->pack_name); > if (read_result != hashsz) ``` ## Junio C Hamano, 2019-12-26 18:32 Subject: Re: [PATCH 2/2] packfile: replace lseek+read with pread Message-ID: URL: https://gitlist.dev/e/xmqqv9q3kke4.fsf%40gitster-ct.c.googlers.com In-Reply-To: ``` Junio C Hamano writes: > Eric Wong writes: > >> We already have pread emulation for portability, so there's >> there's no reason to make two syscalls where one suffices. >> >> Furthermore, readers of the packfile will be using mmap >> (or pread to emulate mmap), anyways, so the file description >> offset does not matter in this case. > > s/description/descriptor/ probably. > > After seeking to the packfile trailer and reading the pack id hash > using lseek+read, this helper function does not read from the file > descriptor, and the sole caller of it closes the file descriptor > immediately after it returns, which means the read file offset after > reading the packfile trailer does not matter. Oops, that was not right. When we successfully open the packfile, we leave the file descriptor open, so we do need the "we never read using read(2) from the file descriptor" guarantee for this change to be correct. But we do have the guarantee, and existing code does depend on the guarantee, so the patch is good. Thanks. ``` ## Eric Wong, 2019-12-26 18:59 Subject: Re: [PATCH 2/2] packfile: replace lseek+read with pread Message-ID: <20191226185932.GA19165@dcvr> URL: https://gitlist.dev/e/20191226185932.GA19165%40dcvr In-Reply-To: ``` Junio C Hamano wrote: > Eric Wong writes: > > > > Furthermore, readers of the packfile will be using mmap > > (or pread to emulate mmap), anyways, so the file description > > offset does not matter in this case. > > s/description/descriptor/ probably. No, I meant "description" :) The offset is shared in case of dup{,2,3} syscalls, which only creates a new descriptor, not a new description. Both the Linux and POSIX lseek(2) manpages say "description". ```