linux-kernel - Re: [patch 2/8] update ctime and mtime for mmaped write

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Message-Id: <E1HOiGt-0006f3-00@dorka.pomaz.szeredi.hu>
Date:	Tue, 06 Mar 2007 23:36:39 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	a.p.zijlstra@...llo.nl
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, staubach@...hat.com,
	hugh@...itas.com
Subject: Re: [patch 2/8] update ctime and mtime for mmaped write

> > AS_CMTIME is only for the case, when the "file modified since the last
> > msync" info is lost from the ptes, e.g. because of page reclaim.
> > 
> > So it doesn't matter if AS_CMTIME is not set, is_page_modified() will
> > provide the necessary barrier.
> 
> The trouble is, we went from a pull to a push model, and now you're
> adding pull code again.
> 
> We have PG_dirty correct at all times, I think its no less reasonable to
> have AS_CMTIME correct in the same fashion.

There's a very important difference.  PG_dirty correctness is needed,
because the _number_ of dirty pages needs to be accounted.  But there
is no such requirement for AS_CMTIME, nobody cares _how_many_ pages
have been modified since the last msync() call.  All we need to know
is _if_ the file has been modified or not.

Thanks,
Miklos
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