memcg: make oom killer a no-op when no killable task can be found

It's pointless to try to kill current if select_bad_process() did not find
an eligible task to kill in mem_cgroup_out_of_memory() since it's
guaranteed that current is a member of the memcg that is oom and it is, by
definition, unkillable.
Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/oom_kill.c

@@ -479,12 +479,9 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
 	read_lock(&tasklist_lock);
 retry:
 	p = select_bad_process(&points, mem);
-	if (PTR_ERR(p) == -1UL)
+	if (!p || PTR_ERR(p) == -1UL)
 		goto out;
 
-	if (!p)
-		p = current;
-
 	if (oom_kill_process(p, gfp_mask, 0, points, mem,
 				"Memory cgroup out of memory"))
 		goto retry;