mm, vmscan: avoid thrashing anon lru when free + file is low

The purpose of the code that commit 6237625 ("revert 'mm: vmscan:
do not swap anon pages just because free+file is low'") reintroduces is
to prefer swapping anonymous memory rather than trashing the file lru.

If the anonymous inactive lru for the set of eligible zones is
considered low, however, or the length of the list for the given reclaim
priority does not allow for effective anonymous-only reclaiming, then
avoid forcing SCAN_ANON.  Forcing SCAN_ANON will end up thrashing the
small list and leave unreclaimed memory on the file lrus.

If the inactive list is insufficient, fallback to balanced reclaim so
the file lru doesn't remain untouched.

[[email protected]: fix build]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: David Rientjes <[email protected]>
Suggested-by: Minchan Kim <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmscan.c

@@ -2228,8 +2228,17 @@ static void get_scan_count(struct lruvec *lruvec, struct mem_cgroup *memcg,
 		}
 
 		if (unlikely(pgdatfile + pgdatfree <= total_high_wmark)) {
-			scan_balance = SCAN_ANON;
-			goto out;
+			/*
+			 * Force SCAN_ANON if there are enough inactive
+			 * anonymous pages on the LRU in eligible zones.
+			 * Otherwise, the small LRU gets thrashed.
+			 */
+			if (!inactive_list_is_low(lruvec, false, memcg, sc, false) &&
+			    lruvec_lru_size(lruvec, LRU_INACTIVE_ANON, sc->reclaim_idx)
+					>> sc->priority) {
+				scan_balance = SCAN_ANON;
+				goto out;
+			}
 		}
 	}