linux-kernel - Re: 2.6.19 file content corruption on ext3

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Date:	Mon, 18 Dec 2006 20:42:04 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...l.org>
CC:	Linus Torvalds <torvalds@...l.org>, andrei.popa@...eo.ro,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hugh@...itas.com>,
	Florian Weimer <fw@...eb.enyo.de>,
	Marc Haber <mh+linux-kernel@...schlus.de>,
	Martin Michlmayr <tbm@...ius.com>
Subject: Re: 2.6.19 file content corruption on ext3

Andrew Morton wrote:
> On Mon, 18 Dec 2006 18:22:42 +1100
> Nick Piggin <nickpiggin@...oo.com.au> wrote:

 >>Yes I could believe it the corruption is caused by something else
 >>completely.
 >
 >
 > 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:

try_to_free_buffers()
   drop_buffers() (succeeds)

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

                                do_no_page()
                                  set_page_dirty()

                                [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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