[PATCH v3 2/8] mm, compaction: remove redundant watermark check in compact_finished()

[Vlastimil Babka]

When detecting whether compaction has succeeded in forming a high-order page,
__compact_finished() employs a watermark check, followed by an own search for
a suitable page in the freelists. This is not ideal for two reasons:

- The watermark check also searches high-order freelists, but has a less strict
  criteria wrt fallback. It's therefore redundant and waste of cycles. This was
  different in the past when high-order watermark check attempted to apply
  reserves to high-order pages.

- The watermark check might actually fail due to lack of order-0 pages.
  Compaction can't help with that, so there's no point in continuing because of
  that. It's possible that high-order page still exists and it terminates.

This patch therefore removes the watermark check. This should save some cycles
and terminate compaction sooner in some cases.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/compaction.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 81e1eaa2a2cf..9222ff362f33 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1280,7 +1280,6 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
 			    const int migratetype)
 {
 	unsigned int order;
-	unsigned long watermark;
 
 	if (cc->contended || fatal_signal_pending(current))
 		return COMPACT_CONTENDED;
@@ -1308,13 +1307,6 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_
 	if (is_via_compact_memory(cc->order))
 		return COMPACT_CONTINUE;
 
-	/* Compaction run is not finished if the watermark is not met */
-	watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];
-
-	if (!zone_watermark_ok(zone, cc->order, watermark, cc->classzone_idx,
-							cc->alloc_flags))
-		return COMPACT_CONTINUE;
-
 	/* Direct compactor: Is a suitable page free? */
 	for (order = cc->order; order < MAX_ORDER; order++) {
 		struct free_area *area = &zone->free_area[order];
-- 
2.12.0