mm, compaction: use proper alloc_flags in __compaction_suitable()

The __compaction_suitable() function checks the low watermark plus a
compact_gap() gap to decide if there's enough free memory to perform
compaction.  This check uses direct compactor's alloc_flags, but that's
wrong, since these flags are not applicable for freepage isolation.

For example, alloc_flags may indicate access to memory reserves, making
compaction proceed, and then fail watermark check during the isolation.

A similar problem exists for ALLOC_CMA, which may be part of
alloc_flags, but not during freepage isolation.  In this case however it
makes sense to use ALLOC_CMA both in __compaction_suitable() and
__isolate_free_page(), since there's actually nothing preventing the
freepage scanner to isolate from CMA pageblocks, with the assumption
that a page that could be migrated once by compaction can be migrated
also later by CMA allocation.  Thus we should count pages in CMA
pageblocks when considering compaction suitability and when isolating
freepages.

To sum up, this patch should remove some false positives from
__compaction_suitable(), and allow compaction to proceed when free pages
required for compaction reside in the CMA pageblocks.

Link: http://lkml.kernel.org/r/20160810091226.6709-10-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 2 deletions

diff --git a/mm/compaction.c b/mm/compaction.c
index bbf41ee99142..658c009d60cc 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1392,11 +1392,19 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 
         /*
          * Watermarks for order-0 must be met for compaction to be able to
-         * isolate free pages for migration targets.
+         * isolate free pages for migration targets. This means that the
+         * watermark and alloc_flags have to match, or be more pessimistic than
+         * the check in __isolate_free_page(). We don't use the direct
+         * compactor's alloc_flags, as they are not relevant for freepage
+         * isolation. We however do use the direct compactor's classzone_idx to
+         * skip over zones where lowmem reserves would prevent allocation even
+         * if compaction succeeds.
+         * ALLOC_CMA is used, as pages in CMA pageblocks are considered
+         * suitable migration targets
          */
         watermark = low_wmark_pages(zone) + compact_gap(order);
         if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
-                                 alloc_flags, wmark_target))
+                                                ALLOC_CMA, wmark_target))
                 return COMPACT_SKIPPED;
 
         /*