[PATCH] mm, compaction: more reliably increase direct compaction priority

[Vlastimil Babka]

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 the lower priority due to parallel activity, so we might needlessly
struggle on the lower priority 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.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bb9b4fb66e85..aa2580a1bcf9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3155,13 +3155,8 @@ 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
@@ -3185,6 +3180,15 @@ 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
-- 
2.9.0