[PATCH 5/6] memcg: clear memcg_params after removing cache from memcg_slab_caches list

[Vladimir Davydov <vdavydov@parallels.com>]

All caches of the same memory cgroup are linked in the memcg_slab_caches
list via kmem_cache::memcg_params::list. This list is traversed when we
read memory.kmem.slabinfo. Since the list actually consists of
memcg_cache_params objects, to convert an element of the list to a
kmem_cache object, we use memcg_params_to_cache(), which obtains the
pointer to the cache from the memcg_params::memcg_caches array of the
root cache, but on cache destruction this pointer is cleared before the
removal of the cache from the list, which potentially can result in a
NULL ptr dereference. Let's fix this by clearing the pointer to a cache
in the memcg_params::memcg_caches array of its parent only after it
cannot be accessed by the memcg_slab_caches list.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 62b9991..ad8de6a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3241,6 +3241,11 @@ void memcg_register_cache(struct kmem_cache *s)
 	 */
 	smp_wmb();
 
+	/*
+	 * Initialize the pointer to this cache in its parent's memcg_params
+	 * before adding it to the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
 	root->memcg_params->memcg_caches[id] = s;
 
 	mutex_lock(&memcg->slab_caches_mutex);
@@ -3265,15 +3270,20 @@ void memcg_release_cache(struct kmem_cache *s)
 		goto out;
 
 	memcg = s->memcg_params->memcg;
-	id  = memcg_cache_id(memcg);
-
+	id = memcg_cache_id(memcg);
 	root = s->memcg_params->root_cache;
-	root->memcg_params->memcg_caches[id] = NULL;
 
 	mutex_lock(&memcg->slab_caches_mutex);
 	list_del(&s->memcg_params->list);
 	mutex_unlock(&memcg->slab_caches_mutex);
 
+	/*
+	 * Clear the pointer to this cache in its parent's memcg_params only
+	 * after removing it from the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
+	root->memcg_params->memcg_caches[id] = NULL;
+
 	css_put(&memcg->css);
 out:
 	kfree(s->memcg_params);
-- 
1.7.10.4

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