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

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Date:	Thu, 22 Feb 2007 08:49:12 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	akpm@...ux-foundation.org
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()?

It's because of this bit in the standard:

    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.

It could be done in munmap/mremap, but it seemed more difficult to
track down all the places where the vma is removed.  But yes, that may
be a nicer solution.

Miklos
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