linux-kernel - Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

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Date:	Thu, 04 Jan 2007 14:57:24 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrea Gelmini <gelma@...ma.net>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

Andrea Gelmini wrote:
> On Sun, Dec 31, 2006 at 02:55:58PM +1100, Nick Piggin wrote:
> 
>>This bug was only introduced in 2.6.19, due to a change that caused pte
> 
> no, Linus said that with 2.6.19 it's easier to trigger this bug...

Yhat's when the bug was introduced -- 2.6.19. 2.6.18 does not have
this bug, so it cannot be years old.

>>So if your corruption is years old, then it must be something else.
>>Maybe it is hidden by a timing change, or BDB isn't using msync properly.
> 
> I can give you a complete image where just changing kernel (everything
> is same, of course) corruptions goes away.
> we spent a lot, I mean a *lot*, of time looking for our code mistake,
> and so on.
> I don't want to seem rude, but I am sure that Berkeley DB corruption we
> have seen (not just Klibido, but I also think about postgrey, and so on)
> depends on this bug.
> I repeat, if you have time/interest I can give you a complete machine
> to see the problem.

You're not being rude, but I just wanted to point out that this patch
(nor the dirty page accounting also in 2.6.19) doesn't fix anything
that was in 2.6.18, AFAIKS.

I wouldn't discount a kernel bug, but it will be hard to track down
unless you can find an earlier kernel that did not cause the corruptions
and/or provide source for a minimal test case to reproduce.

-- 
SUSE Labs, Novell Inc.
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