[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>
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dc523a1..b8165e0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -151,7 +151,8 @@
1 << PG_reclaim |
1 << PG_slab |
1 << PG_swapcache |
- 1 << PG_writeback );
+ 1 << PG_writeback |
+ 1 << PG_buddy );
set_page_count(page, 0);
reset_page_mapcount(page);
page->mapping = NULL;
@@ -236,12 +237,12 @@
static inline void set_page_order(struct page *page, int order) {
set_page_private(page, order);
- __SetPagePrivate(page);
+ __SetPageBuddy(page);
}
static inline void rmv_page_order(struct page *page)
{
- __ClearPagePrivate(page);
+ __ClearPageBuddy(page);
set_page_private(page, 0);
}
@@ -280,11 +281,13 @@
* This function checks whether a page is free && is the buddy
* we can do coalesce a page and its buddy if
* (a) the buddy is not in a hole &&
- * (b) the buddy is free &&
- * (c) the buddy is on the buddy system &&
- * (d) a page and its buddy have the same order.
- * for recording page's order, we use page_private(page) and PG_private.
+ * (b) the buddy is in the buddy system &&
+ * (c) a page and its buddy have the same order.
*
+ * For recording whether a page is in the buddy system, we use PG_buddy.
+ * Setting, clearing, and testing PG_buddy is serialized by zone->lock.
+ *
+ * For recording page's order, we use page_private(page).
*/
static inline int page_is_buddy(struct page *page, int order)
{
@@ -293,10 +296,10 @@
return 0;
#endif
- if (PagePrivate(page) &&
- (page_order(page) == order) &&
- page_count(page) == 0)
+ if (PageBuddy(page) && page_order(page) == order) {
+ BUG_ON(page_count(page) != 0);
return 1;
+ }
return 0;
}
@@ -313,7 +316,7 @@
* as necessary, plus some accounting needed to play nicely with other
* parts of the VM system.
* At each level, we keep a list of pages, which are heads of continuous
- * free pages of length of (1 << order) and marked with PG_Private.Page's
+ * free pages of length of (1 << order) and marked with PG_buddy. Page's
* order is recorded in page_private(page) field.
* So when we are allocating or freeing one, we can derive the state of the
* other. That is, if we allocate a small block, and both were
@@ -376,7 +379,8 @@
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |
- 1 << PG_reserved ))))
+ 1 << PG_reserved |
+ 1 << PG_buddy ))))
bad_page(page);
if (PageDirty(page))
__ClearPageDirty(page);
@@ -524,7 +528,8 @@
1 << PG_slab |
1 << PG_swapcache |
1 << PG_writeback |
- 1 << PG_reserved ))))
+ 1 << PG_reserved |
+ 1 << PG_buddy ))))
bad_page(page);
/*