[PATCH 08/22] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

[Mel Gorman]

When pageblocks get fragmented, watermarks are artifically boosted to
reclaim pages to avoid further fragmentation events. However, compaction
is often either fragmentation-neutral or moving movable pages away from
unmovable/reclaimable pages. As the true watermarks are preserved, allow
compaction to ignore the boost factor.

The expected impact is very slight as the main benefit is that compaction
is slightly more likely to succeed when the system has been fragmented
very recently. On both 1-socket and 2-socket machines for THP-intensive
allocation during fragmentation the success rate was increased by less
than 1% which is marginal. However, detailed tracing indicated that
failure of migration due to a premature ENOMEM triggered by watermark
checks were eliminated.

Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fc769ff4fb2c..6607cb7131b0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3071,7 +3071,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
 		 * watermark, because we already know our high-order page
 		 * exists.
 		 */
-		watermark = min_wmark_pages(zone) + (1UL << order);
+		watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
 		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
 			return 0;
 
-- 
2.16.4