memcg: fix memory.high target

When the memory.high threshold is exceeded, try_charge() schedules a
task_work to reclaim the excess.  The reclaim target is set to the
number of pages requested by try_charge().

This is wrong, because try_charge() usually charges more pages than
requested (batch > nr_pages) in order to refill per cpu stocks.  As a
result, a process in a cgroup can easily exceed memory.high
significantly when doing a lot of charges w/o returning to userspace
(e.g.  reading a file in big chunks).

Fix this issue by assuring that when exceeding memory.high a process
reclaims as many pages as were actually charged (i.e.  batch).

Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2128,7 +2128,7 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	 */
 	do {
 		if (page_counter_read(&memcg->memory) > memcg->high) {
-			current->memcg_nr_pages_over_high += nr_pages;
+			current->memcg_nr_pages_over_high += batch;
 			set_notify_resume(current);
 			break;
 		}