[PATCH] mm: fix negative nr_isolated counts

[Hugh Dickins <hughd@google.com>]

The vmstat interfaces are good at hiding negative counts (at least
when CONFIG_SMP); but if you peer behind the curtain, you find that
nr_isolated_anon and nr_isolated_file soon go negative, and grow ever
more negative: so they can absorb larger and larger numbers of isolated
pages, yet still appear to be zero.

I'm happy to avoid a congestion_wait() when too_many_isolated() myself;
but I guess it's there for a good reason, in which case we ought to get
too_many_isolated() working again.

The imbalance comes from isolate_migratepages()'s ISOLATE_ABORT case:
putback_movable_pages() decrements the NR_ISOLATED counts, but we forgot
to call acct_isolated() to increment them.

Fixes: edc2ca612496 ("mm, compaction: move pageblock checks up from isolate_migratepages_range()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v3.18+
---

 mm/compaction.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- v3.19/mm/compaction.c	2015-02-08 18:54:22.000000000 -0800
+++ linux/mm/compaction.c	2015-02-10 22:25:04.613907871 -0800
@@ -1015,8 +1015,10 @@ static isolate_migrate_t isolate_migrate
 		low_pfn = isolate_migratepages_block(cc, low_pfn, end_pfn,
 								isolate_mode);
 
-		if (!low_pfn || cc->contended)
+		if (!low_pfn || cc->contended) {
+			acct_isolated(zone, cc);
 			return ISOLATE_ABORT;
+		}
 
 		/*
 		 * Either we isolated something and proceed with migration. Or

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