[RESEND PATCH ] mm: page_alloc: place zone_id check before VM_BUG_ON_PAGE check

[Weijie Yang]

If the freeing page and its buddy page are not at the same zone, the current
holding zone->lock for the freeing page cann't prevent buddy page getting
allocated, this could trigger VM_BUG_ON_PAGE in page_is_buddy() at a
very tiny chance, such as:

cpu 0:						cpu 1:
hold zone_1 lock
check page and it buddy
PageBuddy(buddy) is true			hold zone_2 lock
page_order(buddy) == order is true		alloc buddy
trigger VM_BUG_ON_PAGE(page_count(buddy) != 0)

zone_1->lock prevents the freeing page getting allocated
zone_2->lock prevents the buddy page getting allocated
they are not the same zone->lock.

If we cann't remove the zone_id check statement, it's better handle
this rare race. This patch fixes this by placing the zone_id check
before the VM_BUG_ON_PAGE check.

Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
---
 mm/page_alloc.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 616a2c9..491d055 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -488,17 +488,15 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 		return 0;
 
 	if (page_is_guard(buddy) && page_order(buddy) == order) {
-		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
-
 		if (page_zone_id(page) != page_zone_id(buddy))
 			return 0;
 
+		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
 		return 1;
 	}
 
 	if (PageBuddy(buddy) && page_order(buddy) == order) {
-		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
-
 		/*
 		 * zone check is done late to avoid uselessly
 		 * calculating zone/node ids for pages that could
@@ -507,6 +505,8 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 		if (page_zone_id(page) != page_zone_id(buddy))
 			return 0;
 
+		VM_BUG_ON_PAGE(page_count(buddy) != 0, buddy);
+
 		return 1;
 	}
 	return 0;
-- 
1.7.10.4


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