path: root/mm/vmscan.c
diff options
authorJohannes Weiner <hannes@cmpxchg.org>2009-10-26 16:50:00 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2009-10-29 07:39:30 -0700
commit6a7b95481d49f73991d3dbf8c1e696a24684ac05 (patch)
tree32f80d8839daf939bcae8d7711e64b8e96d89ce1 /mm/vmscan.c
parentdo_mbind(): fix memory leak (diff)
vmscan: order evictable rescue in LRU putback
Isolators putting a page back to the LRU do not hold the page lock, and if the page is mlocked, another thread might munlock it concurrently. Expecting this, the putback code re-checks the evictability of a page when it just moved it to the unevictable list in order to correct its decision. The problem, however, is that ordering is not garuanteed between setting PG_lru when moving the page to the list and checking PG_mlocked afterwards: #0: #1 spin_lock() if (TestClearPageMlocked()) if (PageLRU()) move to evictable list SetPageLRU() spin_unlock() if (!PageMlocked()) move to evictable list The PageMlocked() check may get reordered before SetPageLRU() in #0, resulting in #0 not moving the still mlocked page, and in #1 failing to isolate and move the page as well. The page is now stranded on the unevictable list. The race condition is very unlikely. The consequence currently is one page falling off the reclaim grid and eventually getting freed with PG_unevictable set, which triggers a warning in the page allocator. TestClearPageMlocked() in #1 already provides full memory barrier semantics. This patch adds an explicit full barrier to force ordering between SetPageLRU() and PageMlocked() so that either one of the competitors rescues the page. Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Mel Gorman <mel@csn.ul.ie> Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Reviewed-by: Rik van Riel <riel@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to '')
1 files changed, 10 insertions, 0 deletions
diff --git a/mm/vmscan.c b/mm/vmscan.c
index fbcac3bdcf19..777af57fd8c8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -544,6 +544,16 @@ redo:
+ /*
+ * When racing with an mlock clearing (page is
+ * unlocked), make sure that if the other thread does
+ * not observe our setting of PG_lru and fails
+ * isolation, we see PG_mlocked cleared below and move
+ * the page back to the evictable list.
+ *
+ * The other side is TestClearPageMlocked().
+ */
+ smp_mb();