From: Filipe Brandenburger <filbranden@xxxxxxxxxx>
Subject: memcg: reparent charges of children before processing parent
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>
Cc: <stable@xxxxxxxxxxxxxxx> [v3.10+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff -puN mm/memcontrol.c~memcg-reparent-charges-of-children-before-processing-parent mm/memcontrol.c
--- a/mm/memcontrol.c~memcg-reparent-charges-of-children-before-processing-parent
+++ a/mm/memcontrol.c
@@ -6595,6 +6595,7 @@ static void mem_cgroup_css_offline(struc
{
struct mem_cgroup *memcg = mem_cgroup_from_css(css);
struct mem_cgroup_event *event, *tmp;
+ struct cgroup_subsys_state *iter;
/*
* Unregister events and notify userspace.
@@ -6611,7 +6612,14 @@ static void mem_cgroup_css_offline(struc
kmem_cgroup_css_offline(memcg);
mem_cgroup_invalidate_reclaim_iterators(memcg);
- mem_cgroup_reparent_charges(memcg);
+
+ /*
+ * This requires that offlining is serialized. Right now that is
+ * guaranteed because css_killed_work_fn() holds the cgroup_mutex.
+ */
+ css_for_each_descendant_post(iter, css)
+ mem_cgroup_reparent_charges(mem_cgroup_from_css(iter));
+
mem_cgroup_destroy_all_caches(memcg);
vmpressure_cleanup(&memcg->vmpressure);
}
_
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