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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a2955e10171546be4c91cf38479298c37d47e2db..da526905b4a571efee3dec741d50fd4020017e7a 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;