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

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Message-ID: <4705FE5C.4060807@goop.org>
Date:	Fri, 05 Oct 2007 02:05:32 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Keir Fraser <Keir.Fraser@...source.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickens <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Zachary Amsden <zach@...are.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	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

Keir Fraser wrote:
>
> Hang on! How is the access unlocked? By my reading
> page_referenced_one()->page_check_address()->spin_lock(pte_lockptr()).
>

Ah, OK.  I'd overlooked that.

> The problem here is most likely insufficient locking in the pin/unpin
> table-walking code, in light of the fact that you are probably running
> with
> per-page spinlocks (SPLIT_PTLOCK_CPUS). Because we nobble that option
> in our
> own kernel ports it suffices to take the page_table_lock when doing the
> walk-[un]pin-remap routine. This is *not* true with SPLIT_PTLOCK_CPUS.
>

Hm, I see.

> Fortuitously, Jan Beulich has a patch to fix this. It's not going to be
> directly applicable to 2.6.23-rc series, but should be easily ported:
> <http://lists.xensource.com/archives/html/xen-devel/2007-03/msg01200.html>.
>

OK, I can use that.


Andi says:
> Do I misread that patch or does it really walk the complete address
> space and try to take all possible locks? Isn't that very slow?
>   

That's pretty much what it has to do.  Pinning/unpinning walks the whole
pagetable anyway, so it shouldn't be much more expensive.  And they're
relatively rare operations (fork, exec, exit).

    J
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