[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>

3 files changed, 27 insertions, 17 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.
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 9ea629c02a4b..547aac7696cd 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -74,7 +74,9 @@
 #define PG_mappedtodisk         16      /* Has blocks allocated on-disk */
 #define PG_reclaim              17      /* To be reclaimed asap */
 #define PG_nosave_free          18      /* Free, should not be written */
-#define PG_uncached             19      /* Page has been mapped as uncached */
+#define PG_buddy                19      /* Page is free, on buddy lists */
+
+#define PG_uncached             20      /* Page has been mapped as uncached */
 
 /*
  * Global page accounting.  One instance per CPU.  Only unsigned longs are
@@ -317,6 +319,10 @@ extern void __mod_page_state_offset(unsigned long offset, unsigned long delta);
 #define SetPageNosaveFree(page) set_bit(PG_nosave_free, &(page)->flags)
 #define ClearPageNosaveFree(page)               clear_bit(PG_nosave_free, &(page)->flags)
 
+#define PageBuddy(page)         test_bit(PG_buddy, &(page)->flags)
+#define __SetPageBuddy(page)    __set_bit(PG_buddy, &(page)->flags)
+#define __ClearPageBuddy(page)  __clear_bit(PG_buddy, &(page)->flags)
+
 #define PageMappedToDisk(page)  test_bit(PG_mappedtodisk, &(page)->flags)
 #define SetPageMappedToDisk(page) set_bit(PG_mappedtodisk, &(page)->flags)
 #define ClearPageMappedToDisk(page) clear_bit(PG_mappedtodisk, &(page)->flags)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index dc523a1f270d..b8165e037dee 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -151,7 +151,8 @@ static void bad_page(struct page *page)
                         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 unsigned long page_order(struct page *page) {
 
 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 @@ __find_combined_index(unsigned long page_idx, unsigned int order)
  * 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 @@ static inline int page_is_buddy(struct page *page, int order)
                 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 @@ static inline int page_is_buddy(struct page *page, int order)
  * 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 @@ static inline int free_pages_check(struct page *page)
                         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 @@ static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
                         1 << PG_slab    |
                         1 << PG_swapcache |
                         1 << PG_writeback |
-                        1 << PG_reserved ))))
+                        1 << PG_reserved |
+                        1 << PG_buddy ))))
                 bad_page(page);
 
         /*