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

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>

1 files changed, 8 insertions, 2 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6273984f2e34..a294b7576070 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -760,7 +760,13 @@ 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);
@@ -1009,7 +1015,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
 	static char memcg_name[PATH_MAX];
 	int ret;
 
-	if (!memcg)
+	if (!memcg || !p)
 		return;