linux-kernel - Re: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in 2.6.26-rc5-mm3

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Date:	Tue, 17 Jun 2008 15:28:42 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org
Subject: Re: [PATCH][RFC] fix kernel BUG at mm/migrate.c:719! in
	2.6.26-rc5-mm3

On Tue, 2008-06-17 at 19:33 +0100, Hugh Dickins wrote:
> On Tue, 17 Jun 2008, Lee Schermerhorn wrote:
> > 
> > Now I wonder if the assertion that newpage count == 1 could be violated?
> > I don't see how.  We've just allocated and filled it and haven't
> > unlocked it yet, so we should hold the only reference.  Do you agree?
> 
> Disagree: IIRC, excellent example of the kind of assumption
> that becomes invalid with Nick's speculative page references.
> 
> Someone interested in the previous use of the page may have
> incremented the refcount, and in due course will find that
> it's got reused for something else, and will then back off.
> 

Yeah.  Kosaki-san mentioned that we'd need some rework for the
speculative page cache work.  Looks like we'll need to drop the
VM_BUG_ON().  

I need to go read up on the new invariants we can trust with the
speculative page cache.  

Thanks,
Lee

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