mm, compaction: more reliably increase direct compaction priority

During reclaim/compaction loop, compaction priority can be increased by
the should_compact_retry() function, but the current code is not optimal.
Priority is only increased when compaction_failed() is true, which means
that compaction has scanned the whole zone.  This may not happen even
after multiple attempts with a lower priority due to parallel activity, so
we might needlessly struggle on the lower priorities and possibly run out
of compaction retry attempts in the process.

After this patch we are guaranteed at least one attempt at the highest
compaction priority even if we exhaust all retries at the lower
priorities.

Link: http://lkml.kernel.org/r/20160906135258.18335-3-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>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bc0d3c78056c43079e08fb7f50dd8694bb2dbdf8..cf2c989528bb1b7fe7816430f2f6bf8168621cbf 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3172,13 +3172,8 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
         * so it doesn't really make much sense to retry except when the
         * failure could be caused by insufficient priority
         */
-       if (compaction_failed(compact_result)) {
-               if (*compact_priority > MIN_COMPACT_PRIORITY) {
-                       (*compact_priority)--;
-                       return true;
-               }
-               return false;
-       }
+       if (compaction_failed(compact_result))
+               goto check_priority;
 
        /*
         * make sure the compaction wasn't deferred or didn't bail out early
@@ -3202,6 +3197,15 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
        if (compaction_retries <= max_retries)
                return true;
 
+       /*
+        * Make sure there is at least one attempt at the highest priority
+        * if we exhausted all retries at the lower priorities
+        */
+check_priority:
+       if (*compact_priority > MIN_COMPACT_PRIORITY) {
+               (*compact_priority)--;
+               return true;
+       }
        return false;
 }
 #else