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 <weijie.yang@samsung.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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;