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Date:	Mon, 18 Dec 2006 20:42:04 +1100
From:	Nick Piggin <>
To:	Andrew Morton <>
CC:	Linus Torvalds <>,,
	Linux Kernel Mailing List <>,
	Peter Zijlstra <>,
	Hugh Dickins <>,
	Florian Weimer <>,
	Marc Haber <>,
	Martin Michlmayr <>
Subject: Re: 2.6.19 file content corruption on ext3

Andrew Morton wrote:
> On Mon, 18 Dec 2006 18:22:42 +1100
> Nick Piggin <> wrote:

 >>Yes I could believe it the corruption is caused by something else
 > Think so.  We do have a problem here, but only on threaded apps, I believe.
 > rtorrent doesn't appear to be threaded, and the bug is hit on non-preempt
 > UP.

I think (see below) that it does not apply only to threaded apps. But
it would need one of SMP or PREEMPT to trigger.

>>After try_to_free_buffers detaches the buffers from the page, a
>>pagefault can come in, and mark the pte writeable, then set_page_dirty
>>(which finds no buffers, so only sets PG_dirty).
>>The page can now get dirtied through this mapping.
>>try_to_free_buffers then goes on to clean the page and ptes.
> try_to_free_buffers() isn't called against a page which doesn't have
> buffers.  It'll oops.

Sure. But I think the race exists... I'll try spelling it out in
the conventional way:

   drop_buffers() (succeeds)

** preempt here or run right-hand thread on 2nd CPU in SMP **


                                [now modify the page via this mapping
                                (from this process or a concurrent thread)]

   clear_page_dirty() (clears PG_dirty + pte dirty, oops)

SUSE Labs, Novell Inc.
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