Subject: Do not try to migrate unmapped anonymous pages From: Mel Gorman 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 --- diff --git a/mm/migrate.c b/mm/migrate.c --- a/mm/migrate.c +++ b/mm/migrate.c @@ -606,6 +606,17 @@ static int unmap_and_move(new_page_t get if (PageAnon(page)) { rcu_read_lock(); 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); atomic_inc(&anon_vma->migrate_refcount); }