mm: drain memcg stocks on css offlining

mainline inclusion
from mainline-4.20-rc1
commit 591edfb1
category: bugfix
bugzilla: 34611
CVE: NA

-------------------------------------------------
Memcg charge is batched using per-cpu stocks, so an offline memcg can be
pinned by a cached charge up to a moment, when a process belonging to some
other cgroup will charge some memory on the same cpu.  In other words,
cached charges can prevent a memory cgroup from being reclaimed for some
time, without any clear need.

Let's optimize it by explicit draining of all stocks on css offlining.  As
draining is performed asynchronously, and is skipped if any parallel
draining is happening, it's cheap.

Link: http://lkml.kernel.org/r/20180827162621.30187-2-guro@fb.comSigned-off-by: NRoman Gushchin <guro@fb.com>
Reviewed-by: NShakeel Butt <shakeelb@google.com>
Acked-by: NMichal Hocko <mhocko@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 591edfb1)
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NLiu Shixin <liushixin2@huawei.com>
Reviewed-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

mm/memcontrol.c

@@ -4727,6 +4727,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	memcg_offline_kmem(memcg);
 	wb_memcg_offline(memcg);
 
+	drain_all_stock(memcg);
+
 	mem_cgroup_id_put(memcg);
 }