Re: [PATCH v3 2/3] sha1_file: open window into packfiles with O_CLOEXEC

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Oct 27, 2016, 23:09 UTC
Message-ID: <CA+55aFxTHF4BRfcrCiV1D26-be+_rPhwAV+Vq8Roz-NMpPBadg@mail.gmail.com>
In-Reply-To: <xmqqoa254czs.fsf@gitster.mtv.corp.google.com>

On Thu, Oct 27, 2016 at 3:56 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 8 quoted lines

>
> I also thought O_NOATIME shouldn't be an effective fcntl(2) thing,
> for the reasons you stated above, when I was studying the area
> because of the discussion on these patches.  I was surprised to see
> that http://man7.org/linux/man-pages/man2/fcntl.2.html contradicts
> by saying F_SETFL can take O_NOATIME.
>
> Perhaps it deserves a bugreport?

It's not a bug per se.

You can indeed change O_NOATIME with F_SETFL. And it actually does matter, since atime is normally changed by mmap(), read() and write() calls.

So F_SETFL O_NOATIME actually does make sense, but it doesn't cover the atime update done by open() itself.

That said, now that I think about it, I should double-check: maybe open() doesn't actually set atime at all, and we *could* do NOATIME with SETFL after all.

The exact [acm]time semantics are damn subtle.

             Linus