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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <200806181519.46661.nickpiggin@yahoo.com.au>
Date:	Wed, 18 Jun 2008 15:19:46 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Lee Schermerhorn <Lee.Schermerhorn@...com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	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 Wednesday 18 June 2008 05:28, Lee Schermerhorn wrote:
> 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.

I don't know if I've added a summary, which is something I should
do.

The best thing to do is never use page_count, but just use get
and put to refcount it. If you really must use it:

- If there are X references to a page, page_count will return >= X.
- If page_count returns Y, there are no more than Y references to the page.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/