On 03/20/2014 05:16 AM, Bob Liu wrote:
> From: Vlastimil Babka <vbabka@xxxxxxx>
>
> A BUG_ON(!PageLocked) was triggered in mlock_vma_page() by Sasha Levin fuzzing
> with trinity. The call site try_to_unmap_cluster() does not lock the pages
> other than its check_page parameter (which is already locked).
>
> The BUG_ON in mlock_vma_page() is not documented and its purpose is somewhat
> unclear, but apparently it serializes against page migration, which could
> otherwise fail to transfer the PG_mlocked flag. This would not be fatal, as the
> page would be eventually encountered again, but NR_MLOCK accounting would
> become distorted nevertheless. This patch adds a comment to the BUG_ON in
> mlock_vma_page() and munlock_vma_page() to that effect.
>
> The call site try_to_unmap_cluster() is fixed so that for page != check_page,
> trylock_page() is attempted (to avoid possible deadlocks as we already have
> check_page locked) and mlock_vma_page() is performed only upon success. If the
> page lock cannot be obtained, the page is left without PG_mlocked, which is
> again not a problem in the whole unevictable memory design.
>
> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> Cc: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 4e1a6816..b1eb536 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -79,6 +79,7 @@ void clear_page_mlock(struct page *page)
> */
> void mlock_vma_page(struct page *page)
> {
> + /* Serialize with page migration */
> BUG_ON(!PageLocked(page));
>
> if (!TestSetPageMlocked(page)) {
> @@ -174,6 +175,7 @@ unsigned int munlock_vma_page(struct page *page)
> unsigned int nr_pages;
> struct zone *zone = page_zone(page);
>
> + /* For try_to_munlock() and to serialize with page migration */
> BUG_ON(!PageLocked(page));
>
> /*
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d9d4231..43d429b 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1322,9 +1322,19 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
> BUG_ON(!page || PageAnon(page));
>
> if (locked_vma) {
> - mlock_vma_page(page); /* no-op if already mlocked */
> - if (page == check_page)
> + if (page == check_page) {
> + /* we know we have check_page locked */
> + mlock_vma_page(page);
> ret = SWAP_MLOCK;
> + } else if (trylock_page(page)) {
> + /*
> + * If we can lock the page, perform mlock.
> + * Otherwise leave the page alone, it will be
> + * eventually encountered again later.
> + */
> + mlock_vma_page(page);
> + unlock_page(page);
> + }
> continue; /* don't unmap */
> }
>
>
--
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