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    Subject[PATCH 02/14] mm,migration: Do not try to migrate unmapped anonymous pages
    rmap_walk_anon() was triggering errors in memory compaction that look like
    use-after-free errors. The problem is 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 gets freed. This can happen during memory
    compaction if pages being migrated belong to a process that exits before
    migration completes. Hence, the use-after-free race looks like

    1. Page isolated for migration
    2. Process exits
    3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
    4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
    4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
    is garbage.

    This patch checks the mapcount after the rcu lock is taken. If the
    mapcount is zero, the anon_vma is assumed to be freed and no further
    action is taken.

    Signed-off-by: Mel Gorman <>
    Acked-by: Rik van Riel <>
    Reviewed-by: Minchan Kim <>
    Reviewed-by: KAMEZAWA Hiroyuki <>
    mm/migrate.c | 11 +++++++++++
    1 files changed, 11 insertions(+), 0 deletions(-)

    diff --git a/mm/migrate.c b/mm/migrate.c
    index 06e6316..5c5c1bd 100644
    --- a/mm/migrate.c
    +++ b/mm/migrate.c
    @@ -599,6 +599,17 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
    if (PageAnon(page)) {
    rcu_locked = 1;
    + /*
    + * 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_mapped(page))
    + goto rcu_unlock;
    anon_vma = page_anon_vma(page);

     \ /
      Last update: 2010-03-30 11:17    [W:0.019 / U:16.708 seconds]
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