linux-kernel - Re: [PATCH 1/2] mlock: fix race when munlocking pages in do_wp

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Date:	Tue, 8 Feb 2011 10:45:05 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	linux-mm@...ck.org, Lee Schermerhorn <lee.schermerhorn@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mlock: fix race when munlocking pages in
 do_wp_page()

On Mon,  7 Feb 2011 16:47:35 -0800
Michel Lespinasse <walken@...gle.com> wrote:

> vmscan can lazily find pages that are mapped within VM_LOCKED vmas,
> and set the PageMlocked bit on these pages, transfering them onto the
> unevictable list. When do_wp_page() breaks COW within a VM_LOCKED vma,
> it may need to clear PageMlocked on the old page and set it on the
> new page instead.
> 
> This change fixes an issue where do_wp_page() was clearing PageMlocked on
> the old page while the pte was still pointing to it (as well as rmap).
> Therefore, we were not protected against vmscan immediately trasnfering
> the old page back onto the unevictable list. This could cause pages to
> get stranded there forever.
> 
> I propose to move the corresponding code to the end of do_wp_page(),
> after the pte (and rmap) have been pointed to the new page. Additionally,
> we can use munlock_vma_page() instead of clear_page_mlock(), so that
> the old page stays mlocked if there are still other VM_LOCKED vmas
> mapping it.
> 
> Signed-off-by: Michel Lespinasse <walken@...gle.com>

Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>


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