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

Vlastimil Babka
2015-06-10T09:33:59Z

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@lge.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: David Rientjes <rientjes@google.com>
---
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