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| author | Mel Gorman <mel@csn.ul.ie> | 2010-05-24 23:32:19 +0200 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2010-05-25 17:06:58 +0200 |
| commit | 67b9509b2c68ae38cecb83a239881cb0ddf087dc (patch) | |
| tree | a8cee4f4bec57ab44a993ce81df2a10a4980f0f8 /mm | |
| parent | mm: migration: share the anon_vma ref counts between KSM and page migration (diff) | |
| download | linux-67b9509b2c68ae38cecb83a239881cb0ddf087dc.tar.xz linux-67b9509b2c68ae38cecb83a239881cb0ddf087dc.zip | |
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 <mel@csn.ul.ie>
Acked-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/migrate.c | 11 |
1 files changed, 11 insertions, 0 deletions
diff --git a/mm/migrate.c b/mm/migrate.c index 42a3d24d1107..b114635962dc 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -600,6 +600,17 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private, 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->external_refcount); } |