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

Anton Salikhmetov
2008-01-10T15:56:18Z

2008/1/10, Rik van Riel <riel@redhat.com>:
> On Thu, 10 Jan 2008 13:53:59 +0300
> "Anton Salikhmetov" <salikhmetov@gmail.com> wrote:
>
> > Indeed, if msync() is called with MS_SYNC an explicit sync is
> > triggered, and Rik's suggestion would work. However, the POSIX
> > standard requires a call to msync() with MS_ASYNC to update the
> > st_ctime and st_mtime stamps too. No explicit sync of the inode data
> > is triggered in the current implementation of msync(). Hence Rik's
> > suggestion would fail to satisfy POSIX in the latter case.
>
> Since your patch is already changing msync(), has it occurred
> to you that your patch could change msync() to do the right
> thing?

No, not quite. Peter Staubach mentioned an issue in my solution:

>>>

> The patch adds a call to the file_update_time() function to change
> the file metadata before syncing. The patch also contains
> substantial code cleanup: consolidated error check
> for function parameters, using the PAGE_ALIGN() macro instead of
> "manual" alignment check, improved readability of the loop,
> which traverses the process memory regions, updated comments.
>
>

These changes catch the simple case, where the file is mmap'd,
modified via the mmap'd region, and then an msync is done,
all on a mostly quiet system.

However, I don't see how they will work if there has been
something like a sync(2) done after the mmap'd region is
modified and the msync call. When the inode is written out
as part of the sync process, I_DIRTY_PAGES will be cleared,
thus causing a miss in this code.

The I_DIRTY_PAGES check here is good, but I think that there
needs to be some code elsewhere too, to catch the case where
I_DIRTY_PAGES is being cleared, but the time fields still need
to be updated.

<<<

So I'm working on my next solution for this bug now.

>
> --
> All rights reversed.
>