linux-kernel - Re: race with page_referenced_one->ptep_test_and_clear

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Date:	Fri, 05 Oct 2007 12:40:05 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Rik van Riel <riel@...hat.com>
CC:	Hugh Dickins <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Zachary Amsden <zach@...are.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Keir Fraser <keir@...source.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: race with page_referenced_one->ptep_test_and_clear_young and
 pagetable setup/pulldown

Rik van Riel wrote:
> This makes for a narrow race window, during which ptep_test_and_clear_young
> cannot clear the referenced bit and may end up causing a crash.  We do not
> care about it not clearing the referenced bit during that window, since it
> will be cleared during the next go-around and the race is very rare.
>   

Is this the only possible case?  It's just the one I found while testing
under high memory pressure.

> Hence, the only thing we need to fix is the crash.
>
> We can do that by adding an entry for ptep_test_and_clear_young to the
> exception table.  This way we do not need to turn this into a new paravirt
> ops hook (since the fast path is exactly the same as x86 native) and there
> is no need for added complexity.
>
> Also, Xen would not conflict with SPLIT_PTLOCK_CPUS.
>   

Well, isn't the correct fix to make Xen take all the pagetable locks
while pinning/unpinning?  Adding exception handling to
test_and_clear_bit would solve this particular race, but are there
others (either now or potentially)?  Seems fragile.

    J
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