[PATCH -mm v3 4/7] memcg, slab: unregister cache from memcg before starting to destroy it

[Vladimir Davydov]

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 <vdavy...@parallels.com>
---
 mm/memcontrol.c  |   12 ++++++------
 mm/slab_common.c |    3 ++-
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0e11c72047d0..b07e08f97460 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3205,6 +3205,7 @@ 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;
 
@@ -3213,6 +3214,10 @@ 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);
 }
 
@@ -3235,9 +3240,6 @@ 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
@@ -3286,10 +3288,8 @@ 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);
-- 
1.7.10.4

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