linux-kernel - Re: [RFT][PATCH] mm: Fix race in kunmap

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Message-ID: <4CC86587.6010506@redhat.com>
Date:	Wed, 27 Oct 2010 13:46:47 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFT][PATCH] mm: Fix race in kunmap_atomic()

On 10/27/2010 10:54 AM, Peter Zijlstra wrote:

> Christoph reported a nice splat which illustrated a race in the new
> stack based kmap_atomic implementation.
>
> The problem is that we pop our stack slot before we're completely done
> resetting its state -- in particular clearing the PTE (sometimes that's
> CONFIG_DEBUG_HIGHMEM). If an interrupt happens before we actually clear
> the PTE used for the last slot, that interrupt can reuse the slot in a
> dirty state, which triggers a BUG in kmap_atomic().
>
> Fix this by introducing kmap_atomic_idx() which reports the current slot
> index without actually releasing it and use that to find the PTE and
> delay the _pop() until after we're completely done.
>
> Reported-by: Christoph Hellwig<hch@...radead.org>
> Signed-off-by: Peter Zijlstra<a.p.zijlstra@...llo.nl>

Acked-by: Rik van Riel <riel@...hat.com>

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