mm: page_alloc: only check the zone id check if pages are buddies

A node/zone index is used to check if pages are compatible for merging
but this happens unconditionally even if the buddy page is not free. Defer
the calculation as long as possible. Ideally we would check the zone boundary
but nodes can overlap.

Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: "Paul E. McKenney" <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -509,16 +509,26 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
 	if (!pfn_valid_within(page_to_pfn(buddy)))
 		return 0;
 
-	if (page_zone_id(page) != page_zone_id(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;
+
 		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
+		 * never merge.
+		 */
+		if (page_zone_id(page) != page_zone_id(buddy))
+			return 0;
+
 		return 1;
 	}
 	return 0;