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Date:	Tue, 8 Feb 2011 10:45:05 +0900
From:	KAMEZAWA Hiroyuki <>
To:	Michel Lespinasse <>
Cc:, Lee Schermerhorn <>,
	Andrew Morton <>,
	Hugh Dickins <>,
	Rik van Riel <>,
	Andrea Arcangeli <>,
Subject: Re: [PATCH 1/2] mlock: fix race when munlocking pages in

On Mon,  7 Feb 2011 16:47:35 -0800
Michel Lespinasse <> 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 <>

Reviewed-by: KAMEZAWA Hiroyuki <>

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