mm: fix negative nr_isolated counts

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.

It is possible that the bug whcih this patch fixes could cause OOM kills
when the system still has a lot of reclaimable page cache.

Fixes: edc2ca6 ("mm, compaction: move pageblock checks up from isolate_migratepages_range()")
Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Joonsoo Kim <[email protected]>
Cc: <[email protected]>	[3.18+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/compaction.c

@@ -1103,8 +1103,10 @@ static isolate_migrate_t isolate_migratepages(struct zone *zone,
 		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