linux-kernel - Re: [PATCH][RFC][BUG] updating the ctime and mtime time stamps in msync()

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Message-ID: <4df4ef0c0801091603y2bf507e1q2b99971c6028d1f3@mail.gmail.com>
Date:	Thu, 10 Jan 2008 03:03:03 +0300
From:	"Anton Salikhmetov" <salikhmetov@...il.com>
To:	"Rik van Riel" <riel@...hat.com>
Cc:	"Jakob Oestergaard" <jakob@...hought.net>, Valdis.Kletnieks@...edu,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][RFC][BUG] updating the ctime and mtime time stamps in msync()

2008/1/10, Rik van Riel <riel@...hat.com>:
> On Wed, 9 Jan 2008 23:33:40 +0100
> Jakob Oestergaard <jakob@...hought.net> wrote:
> > On Wed, Jan 09, 2008 at 05:06:33PM -0500, Rik van Riel wrote:
>
> > > Can we get by with simply updating the ctime and mtime every time msync()
> > > is called, regardless of whether or not the mmaped pages were still dirty
> > > by the time we called msync() ?
> >
> > The update must still happen, eventually, after a write to the mapped region
> > followed by an unmap/close even if no msync is ever called.
> >
> > The msync only serves as a "no later than" deadline. The write to the region
> > triggers the need for the update.
> >
> > At least this is how I read the standard - please feel free to correct me if I
> > am mistaken.
>
> You are absolutely right.  If we wrote dirty pages to disk, the ctime
> and mtime updates must happen no later than msync or close time.
>
> I guess a third possible time (if we want to minimize the number of
> updates) would be when natural syncing of the file data to disk, by
> other things in the VM, would be about to clear the I_DIRTY_PAGES
> flag on the inode.  That way we do not need to remember any special
> "we already flushed all dirty data, but we have not updated the mtime
> and ctime yet" state.
>
> Does this sound reasonable?

No, it doesn't. The msync() system call called with the MS_ASYNC flag
should (the POSIX standard requires that) update the st_ctime and
st_mtime stamps in the same manner as for the MS_SYNC flag. However,
the current implementation of msync() doesn't call the do_fsync()
function for the MS_ASYNC case. The msync() function may be called
with the MS_ASYNC flag before "natural syncing".
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