[PATCH v2] mm: compaction: avoid migrating non-cma pages to a cma area

[Roman Gushchin]

Compaction does treat cma pageblocks on pair with any movable
pageblocks. It means it can easily move non-cma pages into a cma zone.

It can create problems for the cma allocator.

The particular problem I'm looking at is related to btrfs metadata
pages, which are allocated without __GFP_MOVABLE, but beside that
are generic pagecache pages. In fact, they are sometimes movable
and sometimes not, depending on whether they are dirty and also
on the extent buffer reference counter.

Compaction moves them to the hugetlb_cma area, and then sometimes
the cma allocator fails to move them back from the cma area. It
results in failures of gigantic hugepages allocations.

Also in general cma areas are reserved close to the end of a zone,
and it's where compaction tries to migrate pages. It means
compaction will aggressively fill cma areas, which makes not much
sense.

So to avoid it, let's preserve non-cma pages from being moved into
a cma area. Because cma areas are usually quite large and the number
of areas is small, it should not significantly affect the memory
fragmentation.

v2:
  1) adjusted the __zone_watermark_ok() check, suggested by
     Vlastimil Babka
  2) fixed a typo in a comment

Signed-off-by: Roman Gushchin <guro@xxxxxx>
---
 mm/compaction.c | 10 ++++++++--
 mm/internal.h   |  1 +
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 46f0fcc93081..9ca036cb148a 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1159,6 +1159,10 @@ static bool suitable_migration_target(struct compact_control *cc,
 			return false;
 	}
 
+	/* Do not bring non-cma pages into a cma area */
+	if (is_migrate_cma(get_pageblock_migratetype(page)) && !cc->cma)
+		return false;
+
 	if (cc->ignore_block_suitable)
 		return true;
 
@@ -1832,6 +1836,8 @@ static isolate_migrate_t isolate_migratepages(struct compact_control *cc)
 		if (!low_pfn)
 			return ISOLATE_ABORT;
 
+		cc->cma = is_migrate_cma(get_pageblock_migratetype(page));
+
 		/*
 		 * Either we isolated something and proceed with migration. Or
 		 * we failed and compact_zone should decide if we should
@@ -2000,8 +2006,8 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 	watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
 				low_wmark_pages(zone) : min_wmark_pages(zone);
 	watermark += compact_gap(order);
-	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
-						ALLOC_CMA, wmark_target))
+	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx, 0,
+				 wmark_target))
 		return COMPACT_SKIPPED;
 
 	return COMPACT_CONTINUE;
diff --git a/mm/internal.h b/mm/internal.h
index b5634e78f01d..0ce649da824b 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -232,6 +232,7 @@ struct compact_control {
 	bool contended;			/* Signal lock or sched contention */
 	bool rescan;			/* Rescanning the same pageblock */
 	bool alloc_contig;		/* alloc_contig_range allocation */
+	bool cma;			/* migratepages contains cma pages */
 };
 
 /*
-- 
2.25.2