vmscan: do not force-scan file lru if its absolute size is small

We assume there is enough inactive page cache if the size of inactive
file lru is greater than the size of active file lru, in which case we
force-scan file lru ignoring anonymous pages.  While this logic works
fine when there are plenty of page cache pages, it fails if the size of
file lru is small (several MB): in this case (lru_size >> prio) will be
0 for normal scan priorities, as a result, if inactive file lru happens
to be larger than active file lru, anonymous pages of a cgroup will
never get evicted unless the system experiences severe memory pressure,
even if there are gigabytes of unused anonymous memory there, which is
unfair in respect to other cgroups, whose workloads might be page cache
oriented.

This patch attempts to fix this by elaborating the "enough inactive page
cache" check: it makes it not only check that inactive lru size > active
lru size, but also that we will scan something from the cgroup at the
current scan priority.  If these conditions do not hold, we proceed to
SCAN_FRACT as usual.
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2043,10 +2043,16 @@ static void get_scan_count(struct lruvec *lruvec, int swappiness,
 	}
 
 	/*
-	 * There is enough inactive page cache, do not reclaim
-	 * anything from the anonymous working set right now.
-	 */
-	if (!inactive_file_is_low(lruvec)) {
+	 * If there is enough inactive page cache, i.e. if the size of the
+	 * inactive list is greater than that of the active list *and* the
+	 * inactive list actually has some pages to scan on this priority, we
+	 * do not reclaim anything from the anonymous working set right now.
+	 * Without the second condition we could end up never scanning an
+	 * lruvec even if it has plenty of old anonymous pages unless the
+	 * system is under heavy pressure.
+	 */
+	if (!inactive_file_is_low(lruvec) &&
+	    get_lru_size(lruvec, LRU_INACTIVE_FILE) >> sc->priority) {
 		scan_balance = SCAN_FILE;
 		goto out;
 	}