[PATCH] mm, cma: prevent nr_isolated_* counters from going negative

[Hugh Dickins]

/proc/sys/vm/stat_refresh warns nr_isolated_anon and nr_isolated_file
go increasingly negative under compaction: which would add delay when
should be none, or no delay when should delay. The bug in compaction was
due to a recent mmotm patch, but much older instance of the bug was also
noticed in isolate_migratepages_range() which is used for CMA and
gigantic hugepage allocations.

The bug is caused by putback_movable_pages() in an error path decrementing
the isolated counters without them being previously incremented by
acct_isolated(). Fix isolate_migratepages_range() by removing the error-path
putback, thus reaching acct_isolated() with migratepages still isolated, and
leaving putback to caller like most other places do.

[vbabka@xxxxxxx: expanded the changelog]
Fixes: edc2ca612496 ("mm, compaction: move pageblock checks up from isolate_migratepages_range()")
Cc: stable@xxxxxxxxxxxxxxx #3.18+
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/compaction.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ab649fba3d88..c0be38f634d3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -930,16 +930,8 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
 		pfn = isolate_migratepages_block(cc, pfn, block_end_pfn,
 							ISOLATE_UNEVICTABLE);
 
-		/*
-		 * In case of fatal failure, release everything that might
-		 * have been isolated in the previous iteration, and signal
-		 * the failure back to caller.
-		 */
-		if (!pfn) {
-			putback_movable_pages(&cc->migratepages);
-			cc->nr_migratepages = 0;
+		if (!pfn)
 			break;
-		}
 
 		if (cc->nr_migratepages == COMPACT_CLUSTER_MAX)
 			break;
-- 
2.8.1