mm, memcg: fix inconsistent oom event behavior

A recent commit 9852ae3 ("mm, memcg: consider subtrees in
memory.events") changed the behavior of memcg events, which will now
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.

Fixes: 9852ae3 ("mm, memcg: consider subtrees in memory.events")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Chris Down <chris@chrisdown.name>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200502141055.7378-1-laoar.shao@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -783,6 +783,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
 		atomic_long_inc(&memcg->memory_events[event]);
 		cgroup_file_notify(&memcg->events_file);
 
+		if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
+			break;
 		if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
 			break;
 	} while ((memcg = parent_mem_cgroup(memcg)) &&