Re: [PATCH v3 13/17] mm, compaction: use correct watermark when checking allocation success

From: Vlastimil Babka
Date: Mon Jul 18 2016 - 05:23:52 EST

On 07/06/2016 07:47 AM, Joonsoo Kim wrote:
On Fri, Jun 24, 2016 at 11:54:33AM +0200, Vlastimil Babka wrote:
The __compact_finished() function uses low watermark in a check that has to
pass if the direct compaction is to finish and allocation should succeed. This
is too pessimistic, as the allocation will typically use min watermark. It may
happen that during compaction, we drop below the low watermark (due to parallel
activity), but still form the target high-order page. By checking against low
watermark, we might needlessly continue compaction.

Similarly, __compaction_suitable() uses low watermark in a check whether
allocation can succeed without compaction. Again, this is unnecessarily

After this patch, these check will use direct compactor's alloc_flags to
determine the watermark, which is effectively the min watermark.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
mm/compaction.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 76897850c3c2..371760a85085 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1320,7 +1320,7 @@ static enum compact_result __compact_finished(struct zone *zone, struct compact_

/* Compaction run is not finished if the watermark is not met */
- watermark = low_wmark_pages(zone);
+ watermark = zone->watermark[cc->alloc_flags & ALLOC_WMARK_MASK];

finish condition is changed. We have two more watermark checks in
try_to_compact_pages() and kcompactd_do_work() and they should be
changed too.

Ugh, I've completely missed them. Thanks for catching this, hopefully fixing that will improve the results.