[PATCH] mm, compaction: prevent nr_isolated_* 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.  putback_movable_pages()
decrements the NR_ISOLATED counts which acct_isolated() increments,
so isolate_migratepages_block() needs to acct before putback in that
special case. It's also useful to reset cc->nr_migratepages after putback
so we don't needlessly return too early on the COMPACT_CLUSTER_MAX check.

Also it's easier to track the life of cc->migratepages if we don't assign
it to a migratelist variable.
---
 mm/compaction.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 67f886ecd773..ab649fba3d88 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -638,7 +638,6 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 {
 	struct zone *zone = cc->zone;
 	unsigned long nr_scanned = 0, nr_isolated = 0;
-	struct list_head *migratelist = &cc->migratepages;
 	struct lruvec *lruvec;
 	unsigned long flags = 0;
 	bool locked = false;
@@ -817,7 +816,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		del_page_from_lru_list(page, lruvec, page_lru(page));
 
 isolate_success:
-		list_add(&page->lru, migratelist);
+		list_add(&page->lru, &cc->migratepages);
 		cc->nr_migratepages++;
 		nr_isolated++;
 
@@ -851,9 +850,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 				spin_unlock_irqrestore(&zone->lru_lock,	flags);
 				locked = false;
 			}
-			putback_movable_pages(migratelist);
-			nr_isolated = 0;
+			acct_isolated(zone, cc);
+			putback_movable_pages(&cc->migratepages);
+			cc->nr_migratepages = 0;
 			cc->last_migrated_pfn = 0;
+			nr_isolated = 0;
 		}
 
 		if (low_pfn < next_skip_pfn) {
-- 
2.8.1