linux-kernel - Re: [ 08/82] mm: pmd_read_atomic: fix 32bit PAE pmd walk vs pmd

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Message-ID: <CA+5PVA6A0K3WYBQJH8z3ddd2X3=fQm1a34z+fWSYGcUg4TR9+A@mail.gmail.com>
Date:	Thu, 7 Jun 2012 09:42:55 -0400
From:	Josh Boyer <jwboyer@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Ulrich Obergfell <uobergfe@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Petr Matousek <pmatouse@...hat.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [ 08/82] mm: pmd_read_atomic: fix 32bit PAE pmd walk vs
 pmd_populate SMP race condition

On Thu, Jun 7, 2012 at 12:03 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
> 3.4-stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andrea Arcangeli <aarcange@...hat.com>
>
> commit 26c191788f18129af0eb32a358cdaea0c7479626 upstream.
>
> When holding the mmap_sem for reading, pmd_offset_map_lock should only
> run on a pmd_t that has been read atomically from the pmdp pointer,
> otherwise we may read only half of it leading to this crash.

This one is important, but it can break Xen apparently:

http://permalink.gmane.org/gmane.comp.emulators.xen.devel/132522
https://bugzilla.redhat.com/show_bug.cgi?id=829016

Not sure if you want to hold off on it or see if Andrea comes up with
a follow up fix?

josh
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