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>

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 1e2f4fe12773bdaf9c5ffddd9b5eb77d42b665a0..a060142aa5f53daa9bbfe147b0834a1bcb079c63 100644 (file)
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -513,7 +513,9 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
  *
  * Both the root cache and the child caches will have it. For the root cache,
  * this will hold a dynamically allocated array large enough to hold
- * information about the currently limited memcgs in the system.
+ * information about the currently limited memcgs in the system. To allow the
+ * array to be accessed without taking any locks, on relocation we free the old
+ * version only after a grace period.
  *
  * Child caches will hold extra metadata needed for its operation. Fields are:
  *
@@ -528,7 +530,10 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 struct memcg_cache_params {
        bool is_root_cache;
        union {
-               struct kmem_cache *memcg_caches[0];
+               struct {
+                       struct rcu_head rcu_head;
+                       struct kmem_cache *memcg_caches[0];
+               };
                struct {
                        struct mem_cgroup *memcg;
                        struct list_head list;

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 80197e544764d8b305255ed2759e03c2f441aba5..216659d4441ad36232d5558979a620d27278837c 100644 (file)
--- 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 72d1f9df71bddba1463a1e4851918bb301fdad4c..8184a7cde272b8aa9d6e08e6407482ca5d7bd7df 100644 (file)
--- 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