mm: memcontrol: drain memcg stock on force_empty

The per-cpu memcg stock can retain a charge of upto 32 pages.  On a
machine with large number of cpus, this can amount to a decent amount of
memory.  Additionally force_empty interface might be triggering unneeded
memcg reclaims.

Link: http://lkml.kernel.org/r/20180507201651.165879-1-shakeelb@google.com
Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2610,6 +2610,9 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
 
 	/* we call try-to-free pages for make this cgroup empty */
 	lru_add_drain_all();
+
+	drain_all_stock(memcg);
+
 	/* try to free all pages in this cgroup */
 	while (nr_retries && page_counter_read(&memcg->memory)) {
 		int progress;