[PATCH v2] mm/page_alloc: optimize find_suitable_fallback() and fallbacks array

[Yajun Deng]

There is no need to execute the next loop if it not return in the first
loop. So add a break at the end of the loop.

At the same time, add !migratetype_is_mergeable() before the loop and
reduce the first index size from MIGRATE_TYPES to MIGRATE_PCPTYPES in
fallbacks array.

Signed-off-by: Yajun Deng <yajun.deng@xxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 include/linux/mmzone.h |  2 +-
 mm/page_alloc.c        | 11 +++++------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ab94985ee7d9..0a817b8c7fb2 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -85,7 +85,7 @@ static inline bool is_migrate_movable(int mt)
  * Check whether a migratetype can be merged with another migratetype.
  *
  * It is only mergeable when it can fall back to other migratetypes for
- * allocation. See fallbacks[MIGRATE_TYPES][3] in page_alloc.c.
+ * allocation. See fallbacks[][] array in page_alloc.c.
  */
 static inline bool migratetype_is_mergeable(int mt)
 {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1113483fa6c5..536e8d838fb5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2603,7 +2603,7 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
  *
  * The other migratetypes do not have fallbacks.
  */
-static int fallbacks[MIGRATE_TYPES][MIGRATE_PCPTYPES - 1] = {
+static int fallbacks[MIGRATE_PCPTYPES][MIGRATE_PCPTYPES - 1] = {
 	[MIGRATE_UNMOVABLE]   = { MIGRATE_RECLAIMABLE, MIGRATE_MOVABLE   },
 	[MIGRATE_MOVABLE]     = { MIGRATE_RECLAIMABLE, MIGRATE_UNMOVABLE },
 	[MIGRATE_RECLAIMABLE] = { MIGRATE_UNMOVABLE,   MIGRATE_MOVABLE   },
@@ -2861,7 +2861,7 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
 	int i;
 	int fallback_mt;
 
-	if (area->nr_free == 0)
+	if (area->nr_free == 0 || !migratetype_is_mergeable(migratetype))
 		return -1;
 
 	*can_steal = false;
@@ -2873,11 +2873,10 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
 		if (can_steal_fallback(order, migratetype))
 			*can_steal = true;
 
-		if (!only_stealable)
-			return fallback_mt;
-
-		if (*can_steal)
+		if (!only_stealable || *can_steal)
 			return fallback_mt;
+		else
+			break;
 	}
 
 	return -1;
-- 
2.25.1