[PATCH 3.11 109/144] memcg: reparent charges of children before processing parent

[Luis Henriques]

3.11.10.7 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Filipe Brandenburger <filbranden@xxxxxxxxxx>

commit 4fb1a86fb5e4209a7d4426d4e586c58e9edc74ac upstream.

Sometimes the cleanup after memcg hierarchy testing gets stuck in
mem_cgroup_reparent_charges(), unable to bring non-kmem usage down to 0.

There may turn out to be several causes, but a major cause is this: the
workitem to offline parent can get run before workitem to offline child;
parent's mem_cgroup_reparent_charges() circles around waiting for the
child's pages to be reparented to its lrus, but it's holding
cgroup_mutex which prevents the child from reaching its
mem_cgroup_reparent_charges().

Further testing showed that an ordered workqueue for cgroup_destroy_wq
is not always good enough: percpu_ref_kill_and_confirm's call_rcu_sched
stage on the way can mess up the order before reaching the workqueue.

Instead, when offlining a memcg, call mem_cgroup_reparent_charges() on
all its children (and grandchildren, in the correct order) to have their
charges reparented first.

Fixes: e5fca243abae ("cgroup: use a dedicated workqueue for cgroup destruction")
Signed-off-by: Filipe Brandenburger <filbranden@xxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[ Hugh:
  Yes, more differences, and the old mem_cgroup_reparent_charges line
  is intentionally left in for 3.10 whereas it was removed for 3.12+:
  that's because the css/cgroup iterator changed in between, it used
  not to supply the root of the subtree, but nowadays it does. ]
[ luis: backported to 3.11: used Hugh's backport to 3.10 ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index bb13089..4e4b328 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6346,11 +6346,25 @@ static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
 static void mem_cgroup_css_offline(struct cgroup *cont)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
+	struct cgroup *iter;
 
 	kmem_cgroup_css_offline(memcg);
 
 	mem_cgroup_invalidate_reclaim_iterators(memcg);
+
+	/*
+	 * This requires that offlining is serialized.  Right now that is
+	 * guaranteed because css_killed_work_fn() holds the cgroup_mutex.
+	 */
+	rcu_read_lock();
+	cgroup_for_each_descendant_post(iter, cont) {
+		rcu_read_unlock();
+		mem_cgroup_reparent_charges(mem_cgroup_from_cont(iter));
+		rcu_read_lock();
+	}
+	rcu_read_unlock();
 	mem_cgroup_reparent_charges(memcg);
+
 	mem_cgroup_destroy_all_caches(memcg);
 	vmpressure_cleanup(&memcg->vmpressure);
 }
-- 
1.9.1

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