[BUGFIX][PATCH] memcg: get/put parents at create/free

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

This version works well in my test.

Andrew, please pick up this one.

===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

The lifetime of struct cgroup and struct mem_cgroup is different and
mem_cgroup has its own reference count for handling references from swap_cgroup.

This causes strange problem that the parent mem_cgroup dies while
child mem_cgroup alive, and this problem causes a bug in case of use_hierarchy==1
because res_counter_uncharge climbs up the tree.

This patch is for avoiding it by getting the parent at create, and
putting it at freeing.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reviewed-by; KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fb62b43..45e1b51 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
 
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
+static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
+static void mem_cgroup_get_parent(struct mem_cgroup *mem);
 
 static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
 					 struct page_cgroup *pc,
@@ -2185,10 +2187,28 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
 
 static void mem_cgroup_put(struct mem_cgroup *mem)
 {
-	if (atomic_dec_and_test(&mem->refcnt))
+	if (atomic_dec_and_test(&mem->refcnt)) {
+		struct mem_cgroup *parent = parent_mem_cgroup(mem);
 		__mem_cgroup_free(mem);
+		if (parent)
+			mem_cgroup_put(parent);
+	}
+}
+
+static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
+{
+	if (!mem->res.parent)
+		return NULL;
+	return mem_cgroup_from_res_counter(mem->res.parent, res);
 }
 
+static void mem_cgroup_get_parent(struct mem_cgroup *mem)
+{
+	struct mem_cgroup *parent = parent_mem_cgroup(mem);
+
+	if (parent)
+		mem_cgroup_get(parent);
+}
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 static void __init enable_swap_cgroup(void)
@@ -2237,6 +2257,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	if (parent)
 		mem->swappiness = get_swappiness(parent);
 	atomic_set(&mem->refcnt, 1);
+	mem_cgroup_get_parent(mem);
 	return &mem->css;
 free_out:
 	__mem_cgroup_free(mem);

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