From 4c5018ce06c6be292d4ed96cecf2c8dda361a923 Mon Sep 17 00:00:00 2001 From: Weijie Yang Date: Tue, 10 Feb 2015 14:11:39 -0800 Subject: [PATCH] mm/page_alloc.c: place zone_id check before VM_BUG_ON_PAGE check 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 can'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 Acked-by: Mel Gorman Cc: Johannes Weiner Cc: Rik van Riel Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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 8e20f9c2fa5a..f121050e8530 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -552,17 +552,15 @@ 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 @@ -571,6 +569,8 @@ 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; -- 2.20.1