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

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Date:	Fri, 5 Oct 2007 14:58:28 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

On Fri, 5 Oct 2007 12:36:33 +0100 (BST)
Hugh Dickins <hugh@...itas.com> wrote:

> To my naive mind, your problem actually lies in those two stages:
> whatever marks the pages RO should not be keeping Xen in ignorance.

It does.  Telling Xen to pin the page as a page table page is
basically the first thing a Xen kernel does after marking the
page read-only.

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.

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.

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