mm, compaction: create compact_gap wrapper

Compaction uses a watermark gap of (2UL << order) pages at various
places and it's not immediately obvious why.  Abstract it through a
compact_gap() wrapper to create a single place with a thorough
explanation.

[vbabka@suse.cz: clarify the comment of compact_gap()]
 Link: http://lkml.kernel.org/r/7b6aed1f-fdf8-2063-9ff4-bbe4de712d37@suse.cz
Link: http://lkml.kernel.org/r/20160810091226.6709-9-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
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>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index e2618ac062a6..bbf41ee99142 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1391,11 +1391,10 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
                 return COMPACT_SUCCESS;
 
         /*
-         * Watermarks for order-0 must be met for compaction. Note the 2UL.
-         * This is because during migration, copies of pages need to be
-         * allocated and for a short time, the footprint is higher
+         * Watermarks for order-0 must be met for compaction to be able to
+         * isolate free pages for migration targets.
          */
-        watermark = low_wmark_pages(zone) + (2UL << order);
+        watermark = low_wmark_pages(zone) + compact_gap(order);
         if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
                                  alloc_flags, wmark_target))
                 return COMPACT_SKIPPED;