memcg, slab: unregister cache from memcg before starting to destroy it

Currently, memcg_unregister_cache(), which deletes the cache being
destroyed from the memcg_slab_caches list, is called after
__kmem_cache_shutdown() (see kmem_cache_destroy()), which starts to
destroy the cache.

As a result, one can access a partially destroyed cache while traversing
a memcg_slab_caches list, which can have deadly consequences (for
instance, cache_show() called for each cache on a memcg_slab_caches list
from mem_cgroup_slabinfo_read() will dereference pointers to already
freed data).

To fix this, let's move memcg_unregister_cache() before the cache
destruction process beginning, issuing memcg_register_cache() on failure.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Glauber Costa <glommer@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 8 insertions, 7 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 451523c3bd4e..c22d8bf42d9a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3140,6 +3140,7 @@ int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
                 s->memcg_params->root_cache = root_cache;
                 INIT_WORK(&s->memcg_params->destroy,
                                 kmem_cache_destroy_work_func);
+                css_get(&memcg->css);
         } else
                 s->memcg_params->is_root_cache = true;
 
@@ -3148,6 +3149,10 @@ int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
 
 void memcg_free_cache_params(struct kmem_cache *s)
 {
+        if (!s->memcg_params)
+                return;
+        if (!s->memcg_params->is_root_cache)
+                css_put(&s->memcg_params->memcg->css);
         kfree(s->memcg_params);
 }
 
@@ -3170,9 +3175,6 @@ void memcg_register_cache(struct kmem_cache *s)
         memcg = s->memcg_params->memcg;
         id = memcg_cache_id(memcg);
 
-        css_get(&memcg->css);
-
-
         /*
          * Since readers won't lock (see cache_from_memcg_idx()), we need a
          * barrier here to ensure nobody will see the kmem_cache partially
@@ -3221,10 +3223,8 @@ void memcg_unregister_cache(struct kmem_cache *s)
          * after removing it from the memcg_slab_caches list, otherwise we can
          * fail to convert memcg_params_to_cache() while traversing the list.
          */
-        VM_BUG_ON(!root->memcg_params->memcg_caches[id]);
+        VM_BUG_ON(root->memcg_params->memcg_caches[id] != s);
         root->memcg_params->memcg_caches[id] = NULL;
-
-        css_put(&memcg->css);
 }
 
 /*
diff --git a/mm/slab_common.c b/mm/slab_common.c
index ccc012f00126..0c2879ff414c 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -313,9 +313,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
         s->refcount--;
         if (!s->refcount) {
                 list_del(&s->list);
+                memcg_unregister_cache(s);
 
                 if (!__kmem_cache_shutdown(s)) {
-                        memcg_unregister_cache(s);
                         mutex_unlock(&slab_mutex);
                         if (s->flags & SLAB_DESTROY_BY_RCU)
                                 rcu_barrier();
@@ -325,6 +325,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
                         kmem_cache_free(kmem_cache, s);
                 } else {
                         list_add(&s->list, &slab_caches);
+                        memcg_register_cache(s);
                         mutex_unlock(&slab_mutex);
                         printk(KERN_ERR "kmem_cache_destroy %s: Slab cache still has objects\n",
                                 s->name);