[PATCH v4 2/9] memcg, kmem: fix reference count handling on the errorpath

[Li Zefan]

From: Michal Hocko <mhocko@xxxxxxx>

mem_cgroup_css_online calls mem_cgroup_put if memcg_init_kmem
fails. This is not correct because only memcg_propagate_kmem takes an
additional reference while mem_cgroup_sockets_init is allowed to fail as
well (although no current implementation fails) but it doesn't take any
reference. This all suggests that it should be memcg_propagate_kmem that
should clean up after itself so this patch moves mem_cgroup_put over
there.

Unfortunately this is not that easy (as pointed out by Li Zefan) because
memcg_kmem_mark_dead marks the group dead (KMEM_ACCOUNTED_DEAD) if it
is marked active (KMEM_ACCOUNTED_ACTIVE) which is the case even if
memcg_propagate_kmem fails so the additional reference is dropped in
that case in kmem_cgroup_destroy which means that the reference would be
dropped two times.

The easiest way then would be to simply remove mem_cgrroup_put from
mem_cgroup_css_online and rely on kmem_cgroup_destroy doing the right
thing.

Cc: <stable@xxxxxxxxxxxxxxx> # 3.8+
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/memcontrol.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0bacc0d..b5ec4da 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6325,14 +6325,6 @@ mem_cgroup_css_online(struct cgroup *cont)
 
 	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
 	mutex_unlock(&memcg_create_mutex);
-	if (error) {
-		/*
-		 * We call put now because our (and parent's) refcnts
-		 * are already in place. mem_cgroup_put() will internally
-		 * call __mem_cgroup_free, so return directly
-		 */
-		mem_cgroup_put(memcg);
-	}
 	return error;
 }
 
-- 
1.8.0.2
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