mm, compaction: restrict full priority to non-costly orders

The new ultimate compaction priority disables some heuristics, which may
result in excessive cost.  This is fine for non-costly orders where we
want to try hard before resulting for OOM, but might be disruptive for
costly orders which do not trigger OOM and should generally have some
fallback.  Thus, we disable the full priority for costly orders.

Suggested-by: Michal Hocko <mhocko@kernel.org>
Link: http://lkml.kernel.org/r/20160906135258.18335-4-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compaction.h

@@ -9,6 +9,7 @@ enum compact_priority {
 	COMPACT_PRIO_SYNC_FULL,
 	MIN_COMPACT_PRIORITY = COMPACT_PRIO_SYNC_FULL,
 	COMPACT_PRIO_SYNC_LIGHT,
+	MIN_COMPACT_COSTLY_PRIORITY = COMPACT_PRIO_SYNC_LIGHT,
 	DEF_COMPACT_PRIORITY = COMPACT_PRIO_SYNC_LIGHT,
 	COMPACT_PRIO_ASYNC,
 	INIT_COMPACT_PRIORITY = COMPACT_PRIO_ASYNC

mm/page_alloc.c

@@ -3163,6 +3163,7 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 		     int *compaction_retries)
 {
 	int max_retries = MAX_COMPACT_RETRIES;
+	int min_priority;
 
 	if (!order)
 		return false;
@@ -3205,7 +3206,9 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
 	 * all retries or failed at the lower priorities.
 	 */
 check_priority:
-	if (*compact_priority > MIN_COMPACT_PRIORITY) {
+	min_priority = (order > PAGE_ALLOC_COSTLY_ORDER) ?
+			MIN_COMPACT_COSTLY_PRIORITY : MIN_COMPACT_PRIORITY;
+	if (*compact_priority > min_priority) {
 		(*compact_priority)--;
 		*compaction_retries = 0;
 		return true;