memcg, oom: notify on oom killer invocation from the charge path

commit 7056d3a37d2c6aaaab10c13e8e69adc67ec1fc65 upstream.

Burt Holzman has noticed that memcg v1 doesn't notify about OOM events via
eventfd anymore.  The reason is that 29ef680a ("memcg, oom: move
out_of_memory back to the charge path") has moved the oom handling back to
the charge path.  While doing so the notification was left behind in
mem_cgroup_oom_synchronize.

Fix the issue by replicating the oom hierarchy locking and the
notification.

Link: http://lkml.kernel.org/r/20181224091107.18354-1-mhocko@kernel.org
Fixes: 29ef680a ("memcg, oom: move out_of_memory back to the charge path")
Signed-off-by: NMichal Hocko <mhocko@suse.com>
Reported-by: NBurt Holzman <burt@fnal.gov>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com
Cc: <stable@vger.kernel.org>	[4.19+]
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/memcontrol.c

@@ -1666,6 +1666,9 @@ enum oom_status {
 
 static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int order)
 {
+	enum oom_status ret;
+	bool locked;
+
 	if (order > PAGE_ALLOC_COSTLY_ORDER)
 		return OOM_SKIPPED;
 
@@ -1698,10 +1701,23 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
 		return OOM_ASYNC;
 	}
 
+	mem_cgroup_mark_under_oom(memcg);
+
+	locked = mem_cgroup_oom_trylock(memcg);
+
+	if (locked)
+		mem_cgroup_oom_notify(memcg);
+
+	mem_cgroup_unmark_under_oom(memcg);
 	if (mem_cgroup_out_of_memory(memcg, mask, order))
-		return OOM_SUCCESS;
+		ret = OOM_SUCCESS;
+	else
+		ret = OOM_FAILED;
 
-	return OOM_FAILED;
+	if (locked)
+		mem_cgroup_oom_unlock(memcg);
+
+	return ret;
 }
 
 /**