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 <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b09f0a56fcf0174c7e574d6a593abb90acc5612b..89774ba24888df4ca6d80baf440d009a822e602d 100644 (file)
--- a/mm/page_alloc.c
+++ b/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;