linux-kernel - Re: [patch 01/22] update ctime and mtime for mmaped write

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Date:	Thu, 01 Mar 2007 08:25:12 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	staubach@...hat.com
CC:	miklos@...redi.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [patch 01/22] update ctime and mtime for mmaped write

> >> What happens if the application overwrites what it had written some
> >> time later?  Nothing.  The page is already read-write, the pte dirty,
> >> so even though the file was clearly modified, there's absolutely no
> >> way in which this can be used to force an update to the timestamp.
> >>     
> >
> > Which, I realize now, actually means, that the patch is wrong.  Msync
> > will have to write protect the page table entries, so that later
> > dirtyings may have an effect on the timestamp.
> 
> I thought that PeterZ's changes were to write-protect the page after
> cleaning it so that future modifications could be detected and tracked
> accordingly?  Does the right thing not happen already?

Yes, but MS_ASYNC does not clean the pages.

In fact a better solution may be to rely on the dirty bit in the page
tables, so that no more page faults are necessary.

Thanks,
Miklos
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