mm: fix oom_kill event handling

Commit e27be240df53 ("mm: memcg: make sure memory.events is uptodate
when waking pollers") converted most of memcg event counters to
per-memcg atomics, which made them less confusing for a user.  The
"oom_kill" counter remained untouched, so now it behaves differently
than other counters (including "oom").  This adds nothing but confusion.

Let's fix this by adding the MEMCG_OOM_KILL event, and follow the
MEMCG_OOM approach.

This also removes a hack from count_memcg_event_mm(), introduced earlier
specially for the OOM_KILL counter.

[akpm@linux-foundation.org: fix for droppage of memcg-replace-mm-owner-with-mm-memcg.patch]
Link: http://lkml.kernel.org/r/20180508124637.29984-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 6694348..84081e7 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -913,7 +913,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 
 	/* Raise event before sending signal: task reaper must see this */
 	count_vm_event(OOM_KILL);
-	count_memcg_event_mm(mm, OOM_KILL);
+	memcg_memory_event_mm(mm, MEMCG_OOM_KILL);
 
 	/*
 	 * We should send SIGKILL before granting access to memory reserves