[PATCH 5/6] memcg: make mem_cgroup_reparent_charges non failing

[Michal Hocko <mhocko@suse.cz>]

Now that pre_destroy callbacks are called from within cgroup_lock and
the cgroup has been checked to be empty without any children then there
is no other way to fail.
mem_cgroup_pre_destroy doesn't have to take a reference to memcg's css
because all css' are marked dead already.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c |   18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f57ba4c..7c75da3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3738,14 +3738,12 @@ static void mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
  *
  * Caller is responsible for holding css reference on the memcg.
  */
-static int mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
+static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
 {
 	struct cgroup *cgrp = memcg->css.cgroup;
 	int node, zid;
 
 	do {
-		if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
-			return -EBUSY;
 		/* This is for making all *used* pages to be on LRU. */
 		lru_add_drain_all();
 		drain_all_stock_sync(memcg);
@@ -3771,8 +3769,6 @@ static int mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
 		 * charge before adding to the LRU.
 		 */
 	} while (res_counter_read_u64(&memcg->res, RES_USAGE) > 0);
-
-	return 0;
 }
 
 /*
@@ -3809,7 +3805,9 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
 
 	}
 	lru_add_drain();
-	return mem_cgroup_reparent_charges(memcg);
+	mem_cgroup_reparent_charges(memcg);
+
+	return 0;
 }
 
 static int mem_cgroup_force_empty_write(struct cgroup *cont, unsigned int event)
@@ -5013,13 +5011,9 @@ free_out:
 static int mem_cgroup_pre_destroy(struct cgroup *cont)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
-	int ret;
 
-	css_get(&memcg->css);
-	ret = mem_cgroup_reparent_charges(memcg);
-	css_put(&memcg->css);
-
-	return ret;
+	mem_cgroup_reparent_charges(memcg);
+	return 0;
 }
 
 static void mem_cgroup_destroy(struct cgroup *cont)
-- 
1.7.10.4