[PATCH 1/6] mm, compaction: more robust check for scanners meeting

[Vlastimil Babka]

Compaction should finish when the migration and free scanner meet, i.e. they
reach the same pageblock. Currently however, the test in compact_finished()
simply just compares the exact pfns, which may yield a false negative when the
free scanner position is in the middle of a pageblock and the migration scanner
reaches the beginning of the same pageblock.

This hasn't been a problem until commit e14c720efdd7 ("mm, compaction: remember
position within pageblock in free pages scanner") allowed the free scanner
position to be in the middle of a pageblock between invocations.  The hot-fix
1d5bfe1ffb5b ("mm, compaction: prevent infinite loop in compact_zone")
prevented the issue by adding a special check in the migration scanner to
satisfy the current detection of scanners meeting.

However, the proper fix is to make the detection more robust. This patch
introduces the compact_scanners_met() function that returns true when the free
scanner position is in the same or lower pageblock than the migration scanner.
The special case in isolate_migratepages() introduced by 1d5bfe1ffb5b is
removed.

Suggested-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/compaction.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 16e1b57..d46aaeb 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -902,6 +902,16 @@ static bool suitable_migration_target(struct page *page)
 }
 
 /*
+ * Test whether the free scanner has reached the same or lower pageblock than
+ * the migration scanner, and compaction should thus terminate.
+ */
+static inline bool compact_scanners_met(struct compact_control *cc)
+{
+	return (cc->free_pfn >> pageblock_order)
+		<= (cc->migrate_pfn >> pageblock_order);
+}
+
+/*
  * Based on information in the current compact_control, find blocks
  * suitable for isolating free pages from and then isolate them.
  */
@@ -1131,12 +1141,8 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 	}
 
 	acct_isolated(zone, cc);
-	/*
-	 * Record where migration scanner will be restarted. If we end up in
-	 * the same pageblock as the free scanner, make the scanners fully
-	 * meet so that compact_finished() terminates compaction.
-	 */
-	cc->migrate_pfn = (end_pfn <= cc->free_pfn) ? low_pfn : cc->free_pfn;
+	/* Record where migration scanner will be restarted. */
+	cc->migrate_pfn = low_pfn;
 
 	return cc->nr_migratepages ? ISOLATE_SUCCESS : ISOLATE_NONE;
 }
@@ -1151,7 +1157,7 @@ static int __compact_finished(struct zone *zone, struct compact_control *cc,
 		return COMPACT_PARTIAL;
 
 	/* Compaction run completes if the migrate and free scanner meet */
-	if (cc->free_pfn <= cc->migrate_pfn) {
+	if (compact_scanners_met(cc)) {
 		/* Let the next compaction start anew. */
 		zone->compact_cached_migrate_pfn[0] = zone->zone_start_pfn;
 		zone->compact_cached_migrate_pfn[1] = zone->zone_start_pfn;
@@ -1380,7 +1386,7 @@ static int compact_zone(struct zone *zone, struct compact_control *cc)
 			 * migrate_pages() may return -ENOMEM when scanners meet
 			 * and we want compact_finished() to detect it
 			 */
-			if (err == -ENOMEM && cc->free_pfn > cc->migrate_pfn) {
+			if (err == -ENOMEM && !compact_scanners_met(cc)) {
 				ret = COMPACT_PARTIAL;
 				goto out;
 			}
-- 
2.1.4

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