Okay, so after recent discussions, I am proposing the following
patch. It won't remove hierarchy, or anything like that. Just default
to true in the root cgroup, and print a warning once if you try
to set it back to 0.

I am not adding it to feature-removal-schedule.txt because I don't
view it as a consensus. Rather, changing the default would allow us
to give it a time around in the open, and see if people complain
and what we can learn about that.

Signed-off-by: Glauber Costa <redacted>
CC: Michal Hocko <redacted>
CC: Kamezawa Hiroyuki <redacted>
CC: Johannes Weiner <redacted>
CC: Tejun Heo <redacted>
---
 mm/memcontrol.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9e710bc..037ddd4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c

@@ -3949,6 +3949,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
 	if (memcg->use_hierarchy == val)
 		goto out;
 
+	WARN_ONCE((!parent_memcg && memcg->use_hierarchy && val == false),
+		"Non-hierarchical memcg is considered for deprecation");
 	/*
 	 * If parent's use_hierarchy is set, we can't make any modifications
 	 * in the child subtrees. If it is unset, then the change can

@@ -5175,6 +5177,7 @@ mem_cgroup_create(struct cgroup *cont)
 			INIT_WORK(&stock->work, drain_local_stock);
 		}
 		hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
+		memcg->use_hierarchy = true;