linux-kernel - Re: [PATCH] mm: fix page_mkclean

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Date:	Thu, 28 Dec 2006 14:38:15 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...l.org
Cc:	akpm@...l.org, guichaz@...oo.fr, ranma@...edrich.de,
	gordonfarquharson@...il.com, tbm@...ius.com,
	a.p.zijlstra@...llo.nl, andrei.popa@...eo.ro, hugh@...itas.com,
	nickpiggin@...oo.com.au, arjan@...radead.org,
	linux-kernel@...r.kernel.org, kenneth.w.chen@...el.com
Subject: Re: [PATCH] mm: fix page_mkclean_one

From: Linus Torvalds <torvalds@...l.org>
Date: Thu, 28 Dec 2006 12:14:31 -0800 (PST)

> I get corruption - but the whole point is that it's very much pdflush that 
> should be writing these pages out.

I think what might be happening is that pdflush writes them out fine,
however we don't trap writes by the application _during_ that writeout.

These corruptions look exactly as if:

1) pdflush begins writeback of page X
2) page goes to disk
3) application writes a chunk to the page
4) pdflush et al. think the page is clean, so it gets tossed, losing
   the writes done in #3

So there's a missing PTE change in there, so that we never get proper
re-dirtying of the page if the application tries to write to the page
during the writeback.

It's something that will only occur with writeback and MAP_SHARED
writable access to the file pages.  That's why we never see this
with normal filesystem writes, since those explicitly manage the
page dirty state.

I think the dirty balancing logic etc. isn't where the problems are,
to me it's a PTE state update issue for sure.
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