memcg, slab: RCU protect memcg_params for root caches

We relocate root cache's memcg_params whenever we need to grow the
memcg_caches array to accommodate all kmem-active memory cgroups.
Currently on relocation we free the old version immediately, which can
lead to use-after-free, because the memcg_caches array is accessed
lock-free (see cache_from_memcg_idx()).  This patch fixes this by making
memcg_params RCU-protected for root caches.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 23 insertions, 8 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 80197e544764..216659d4441a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3178,18 +3178,17 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 
         if (num_groups > memcg_limited_groups_array_size) {
                 int i;
+                struct memcg_cache_params *new_params;
                 ssize_t size = memcg_caches_array_size(num_groups);
 
                 size *= sizeof(void *);
                 size += offsetof(struct memcg_cache_params, memcg_caches);
 
-                s->memcg_params = kzalloc(size, GFP_KERNEL);
-                if (!s->memcg_params) {
-                        s->memcg_params = cur_params;
+                new_params = kzalloc(size, GFP_KERNEL);
+                if (!new_params)
                         return -ENOMEM;
-                }
 
-                s->memcg_params->is_root_cache = true;
+                new_params->is_root_cache = true;
 
                 /*
                  * There is the chance it will be bigger than
@@ -3203,7 +3202,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
                 for (i = 0; i < memcg_limited_groups_array_size; i++) {
                         if (!cur_params->memcg_caches[i])
                                 continue;
-                        s->memcg_params->memcg_caches[i] =
+                        new_params->memcg_caches[i] =
                                                 cur_params->memcg_caches[i];
                 }
 
@@ -3216,7 +3215,9 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
                  * bigger than the others. And all updates will reset this
                  * anyway.
                  */
-                kfree(cur_params);
+                rcu_assign_pointer(s->memcg_params, new_params);
+                if (cur_params)
+                        kfree_rcu(cur_params, rcu_head);
         }
         return 0;
 }
diff --git a/mm/slab.h b/mm/slab.h
index 72d1f9df71bd..8184a7cde272 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -160,14 +160,28 @@ static inline const char *cache_name(struct kmem_cache *s)
         return s->name;
 }
 
+/*
+ * Note, we protect with RCU only the memcg_caches array, not per-memcg caches.
+ * That said the caller must assure the memcg's cache won't go away. Since once
+ * created a memcg's cache is destroyed only along with the root cache, it is
+ * true if we are going to allocate from the cache or hold a reference to the
+ * root cache by other means. Otherwise, we should hold either the slab_mutex
+ * or the memcg's slab_caches_mutex while calling this function and accessing
+ * the returned value.
+ */
 static inline struct kmem_cache *
 cache_from_memcg_idx(struct kmem_cache *s, int idx)
 {
         struct kmem_cache *cachep;
+        struct memcg_cache_params *params;
 
         if (!s->memcg_params)
                 return NULL;
-        cachep = s->memcg_params->memcg_caches[idx];
+
+        rcu_read_lock();
+        params = rcu_dereference(s->memcg_params);
+        cachep = params->memcg_caches[idx];
+        rcu_read_unlock();
 
         /*
          * Make sure we will access the up-to-date value. The code updating