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    Subject[PATCH 02/12] mm,migration: Do not try to migrate unmapped anonymous pages
    Date
    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>
    ---
    mm/migrate.c | 10 ++++++++++
    1 files changed, 10 insertions(+), 0 deletions(-)

    diff --git a/mm/migrate.c b/mm/migrate.c
    index 63addfa..1ce6a2f 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -606,6 +606,16 @@ 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))
    + goto uncharge;
    +
    rcu_read_lock();
    rcu_locked = 1;
    anon_vma = page_anon_vma(page);
    --
    1.6.5


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