memcg: fix OOM killer under memcg

This patch tries to fix OOM Killer problems caused by hierarchy.
Now, memcg itself has OOM KILL function (in oom_kill.c) and tries to
kill a task in memcg.

But, when hierarchy is used, it's broken and correct task cannot
be killed. For example, in following cgroup

	/groupA/	hierarchy=1, limit=1G,
		01	nolimit
		02	nolimit
All tasks' memory usage under /groupA, /groupA/01, groupA/02 is limited to
groupA's 1Gbytes but OOM Killer just kills tasks in groupA.

This patch provides makes the bad process be selected from all tasks
under hierarchy. BTW, currently, oom_jiffies is updated against groupA
in above case. oom_jiffies of tree should be updated.

To see how oom_jiffies is used, please check mem_cgroup_oom_called()
callers.

[akpm@linux-foundation.org: build fix]
[akpm@linux-foundation.org: const fix]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memcg_test.txt

@@ -1,5 +1,5 @@
 Memory Resource Controller(Memcg)  Implementation Memo.
-Last Updated: 2009/1/19
+Last Updated: 2009/1/20
 Base Kernel Version: based on 2.6.29-rc2.
 
 Because VM is getting complex (one of reasons is memcg...), memcg's behavior
@@ -360,3 +360,21 @@ Under below explanation, we assume CONFIG_MEM_RES_CTRL_SWAP=y.
 	# kill malloc task.
 
 	Of course, tmpfs v.s. swapoff test should be tested, too.
+
+ 9.8 OOM-Killer
+	Out-of-memory caused by memcg's limit will kill tasks under
+	the memcg. When hierarchy is used, a task under hierarchy
+	will be killed by the kernel.
+	In this case, panic_on_oom shouldn't be invoked and tasks
+	in other groups shouldn't be killed.
+
+	It's not difficult to cause OOM under memcg as following.
+	Case A) when you can swapoff
+	#swapoff -a
+	#echo 50M > /memory.limit_in_bytes
+	run 51M of malloc
+
+	Case B) when you use mem+swap limitation.
+	#echo 50M > memory.limit_in_bytes
+	#echo 50M > memory.memsw.limit_in_bytes
+	run 51M of malloc

include/linux/cgroup.h

@@ -503,7 +503,7 @@ struct cgroup_subsys_state *css_get_next(struct cgroup_subsys *ss, int id,
 
 /* Returns true if root is ancestor of cg */
 bool css_is_ancestor(struct cgroup_subsys_state *cg,
-		     struct cgroup_subsys_state *root);
+		     const struct cgroup_subsys_state *root);
 
 /* Get id and depth of css */
 unsigned short css_id(struct cgroup_subsys_state *css);

kernel/cgroup.c

@@ -3405,7 +3405,7 @@ unsigned short css_depth(struct cgroup_subsys_state *css)
 }
 
 bool css_is_ancestor(struct cgroup_subsys_state *child,
-		    struct cgroup_subsys_state *root)
+		    const struct cgroup_subsys_state *root)
 {
 	struct css_id *child_id = rcu_dereference(child->id);
 	struct css_id *root_id = rcu_dereference(root->id);

mm/memcontrol.c

@@ -295,6 +295,9 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
 static struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
 {
 	struct mem_cgroup *mem = NULL;
+
+	if (!mm)
+		return NULL;
 	/*
 	 * Because we have no locks, mm->owner's may be being moved to other
 	 * cgroup. We use css_tryget() here even if this looks
@@ -486,10 +489,20 @@ void mem_cgroup_move_lists(struct page *page,
 int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem)
 {
 	int ret;
+	struct mem_cgroup *curr = NULL;
 
 	task_lock(task);
-	ret = task->mm && mm_match_cgroup(task->mm, mem);
+	rcu_read_lock();
+	curr = try_get_mem_cgroup_from_mm(task->mm);
+	rcu_read_unlock();
 	task_unlock(task);
+	if (!curr)
+		return 0;
+	if (curr->use_hierarchy)
+		ret = css_is_ancestor(&curr->css, &mem->css);
+	else
+		ret = (curr == mem);
+	css_put(&curr->css);
 	return ret;
 }
 
@@ -820,6 +833,19 @@ bool mem_cgroup_oom_called(struct task_struct *task)
 	rcu_read_unlock();
 	return ret;
 }
+
+static int record_last_oom_cb(struct mem_cgroup *mem, void *data)
+{
+	mem->last_oom_jiffies = jiffies;
+	return 0;
+}
+
+static void record_last_oom(struct mem_cgroup *mem)
+{
+	mem_cgroup_walk_tree(mem, NULL, record_last_oom_cb);
+}
+
+
 /*
  * Unlike exported interface, "oom" parameter is added. if oom==true,
  * oom-killer can be invoked.
@@ -902,7 +928,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 				mutex_lock(&memcg_tasklist);
 				mem_cgroup_out_of_memory(mem_over_limit, gfp_mask);
 				mutex_unlock(&memcg_tasklist);
-				mem_over_limit->last_oom_jiffies = jiffies;
+				record_last_oom(mem_over_limit);
 			}
 			goto nomem;
 		}