Re: [PATCH 1/2] mlock: fix race when munlocking pages in do_wp_page()

[Hugh Dickins <hughd@google.com>]

On Mon, Feb 7, 2011 at 4:47 PM, Michel Lespinasse <walken@google.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@google.com>

Acked-by: Hugh Dickins <hughd@google.com>

(but I have to say, do_wp_page() grows even ughlier!)