Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Mel.
On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman <mel@csn.ul.ie> wrote:
> rmap_walk_anon() was triggering errors in memory compaction that looks like
> use-after-free errors in anon_vma. The problem appears to be that between
> the page being isolated from the LRU and rcu_read_lock() being taken, the
> mapcount of the page dropped to 0 and the anon_vma was freed. This patch
> skips the migration of anon pages that are not mapped by anyone.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> Acked-by: Rik van Riel <riel@redhat.com>
> ---
>  mm/migrate.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 98eaaf2..3c491e3 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -602,6 +602,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>         * just care Anon page here.
>         */
>        if (PageAnon(page)) {
> +               /*
> +                * If the page has no mappings any more, just bail. An
> +                * unmapped anon page is likely to be freed soon but worse,
> +                * it's possible its anon_vma disappeared between when
> +                * the page was isolated and when we reached here while
> +                * the RCU lock was not held
> +                */
> +               if (!page_mapcount(page))

As looking code about mapcount of page, I got confused.
I think mapcount of page is protected by pte lock.
But I can't find pte lock in unmap_and_move.
If I am right, what protects race between this condition check and
rcu_read_lock?
This patch makes race window very small but It can't remove race totally.

I think I am missing something.
Pz, point me out. :)


> +                       goto uncharge;
> +
>                rcu_read_lock();
>                rcu_locked = 1;
>                anon_vma = page_anon_vma(page);
> --
> 1.6.5
>




-- 
Kind regards,
Minchan Kim

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