memcg: avoid oom-killing innocent task in case of use_hierarchy

commit d31f56db upstream.

task_in_mem_cgroup(), which is called by select_bad_process() to check
whether a task can be a candidate for being oom-killed from memcg's limit,
checks "curr->use_hierarchy"("curr" is the mem_cgroup the task belongs
to).

But this check return true(it's false positive) when:

	<some path>/aa		use_hierarchy == 0	<- hitting limit
	  <some path>/aa/00	use_hierarchy == 1	<- the task belongs to

This leads to killing an innocent task in aa/00.  This patch is a fix for
this bug.  And this patch also fixes the arg for
mem_cgroup_print_oom_info().  We should print information of mem_cgroup
which the task being killed, not current, belongs to.
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

mm/memcontrol.c

@@ -758,7 +758,13 @@ int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem)
 	task_unlock(task);
 	if (!curr)
 		return 0;
-	if (curr->use_hierarchy)
+	/*
+	 * We should check use_hierarchy of "mem" not "curr". Because checking
+	 * use_hierarchy of "curr" here make this function true if hierarchy is
+	 * enabled in "curr" and "curr" is a child of "mem" in *cgroup*
+	 * hierarchy(even if use_hierarchy is disabled in "mem").
+	 */
+	if (mem->use_hierarchy)
 		ret = css_is_ancestor(&curr->css, &mem->css);
 	else
 		ret = (curr == mem);

mm/oom_kill.c

@@ -404,7 +404,7 @@ static int oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
 		cpuset_print_task_mems_allowed(current);
 		task_unlock(current);
 		dump_stack();
-		mem_cgroup_print_oom_info(mem, current);
+		mem_cgroup_print_oom_info(mem, p);
 		show_mem();
 		if (sysctl_oom_dump_tasks)
 			dump_tasks(mem);