linux-kernel - Re: [PATCH] update ctime and mtime for mmaped write

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Message-Id: <20070221202615.a0a167f4.akpm@linux-foundation.org>
Date:	Wed, 21 Feb 2007 20:26:15 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	staubach@...hat.com, hugh@...itas.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] update ctime and mtime for mmaped write

On Wed, 21 Feb 2007 18:51:52 +0100 Miklos Szeredi <miklos@...redi.hu> wrote:

> This patch makes writing to shared memory mappings update st_ctime and
> st_mtime as defined by SUSv3:
> 
>    The st_ctime and st_mtime fields of a file that is mapped with
>    MAP_SHARED and PROT_WRITE shall be marked for update at some point
>    in the interval between a write reference to the mapped region and
>    the next call to msync() with MS_ASYNC or MS_SYNC for that portion
>    of the file by any process. If there is no such call and if the
>    underlying file is modified as a result of a write reference, then
>    these fields shall be marked for update at some time after the
>    write reference.
> 
> A new address_space flag is introduced: AS_CMTIME.  This is set each
> time a page is dirtied through a userspace memory mapping.  This
> includes write accesses via get_user_pages().
> 
> Note, the flag is set unconditionally, even if the page is already
> dirty.  This is important, because the page might have been dirtied
> earlier by a non-mmap write.
> 
> This flag is checked in msync() and __fput(), and if set, the file
> times are updated and the flag is cleared
> 
> The flag is also cleared, if the time update is triggered by a normal
> write.  This is not mandated by the standard, but seems to be a sane
> thing to do.

Why is the flag checked in __fput()?
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