[PATCH v2] mm, memcg: fix inconsistent oom event behavior

[Yafang Shao <laoar.shao@gmail.com>]

A recent commit 9852ae3fe529 ("mm, memcg: consider subtrees in
memory.events") changes the behavior of memcg events, which will
consider subtrees in memory.events. But oom_kill event is a special one
as it is used in both cgroup1 and cgroup2. In cgroup1, it is displayed
in memory.oom_control. The file memory.oom_control is in both root memcg
and non root memcg, that is different with memory.event as it only in
non-root memcg. That commit is okay for cgroup2, but it is not okay for
cgroup1 as it will cause inconsistent behavior between root memcg and
non-root memcg.

Here's an example on why this behavior is inconsistent in cgroup1.
     root memcg
     /
  memcg foo
   /
memcg bar

Suppose there's an oom_kill in memcg bar, then the oon_kill will be

     root memcg : memory.oom_control(oom_kill)  0
     /
  memcg foo : memory.oom_control(oom_kill)  1
   /
memcg bar : memory.oom_control(oom_kill)  1

For the non-root memcg, its memory.oom_control(oom_kill) includes its
descendants' oom_kill, but for root memcg, it doesn't include its
descendants' oom_kill. That means, memory.oom_control(oom_kill) has
different meanings in different memcgs. That is inconsistent. Then the user
has to know whether the memcg is root or not.

If we can't fully support it in cgroup1, for example by adding
memory.events.local into cgroup1 as well, then let's don't touch
its original behavior. So let's recover the original behavior for cgroup1.

Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
Cc: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: stable@vger.kernel.org
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 include/linux/memcontrol.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 8c340e6b347f..a0ae080a67d1 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -798,7 +798,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
 		atomic_long_inc(&memcg->memory_events[event]);
 		cgroup_file_notify(&memcg->events_file);
 
-		if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
+		if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS ||
+		    !cgroup_subsys_on_dfl(memory_cgrp_subsys))
 			break;
 	} while ((memcg = parent_mem_cgroup(memcg)) &&
 		 !mem_cgroup_is_root(memcg));
-- 
2.18.2