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

Anton Salikhmetov
2008-01-10T00:03:23Z

2008/1/10, Rik van Riel <riel@redhat.com>:
> On Wed, 9 Jan 2008 23:33:40 +0100
> Jakob Oestergaard <jakob@unthought.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".