[PATCH] Fix buddy list race that could lead to page lru list corruptions

Rohit found an obscure bug causing buddy list corruption.

page_is_buddy is using a non-atomic test (PagePrivate && page_count == 0)
to determine whether or not a free page's buddy is itself free and in the
buddy lists.

Each of the conjuncts may be true at different times due to unrelated
conditions, so the non-atomic page_is_buddy test may find each conjunct to
be true even if they were not both true at the same time (ie. the page was
not on the buddy lists).

Signed-off-by: Martin Bligh <mbligh@google.com>
Signed-off-by: Rohit Seth <rohitseth@google.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 2 insertions, 3 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6aa016f1d3ae..1154684209a4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -229,10 +229,9 @@ struct page {
                 unsigned long private;          /* Mapping-private opaque data:
                                                  * usually used for buffer_heads
                                                  * if PagePrivate set; used for
-                                                 * swp_entry_t if PageSwapCache.
-                                                 * When page is free, this
+                                                 * swp_entry_t if PageSwapCache;
                                                  * indicates order in the buddy
-                                                 * system.
+                                                 * system if PG_buddy is set.
                                                  */
                 struct address_space *mapping;  /* If low bit clear, points to
                                                  * inode address_space, or NULL.