memcg: fix memcg_cache_name() to use cgroup_name()

As cgroup supports rename, it's unsafe to dereference dentry->d_name
without proper vfs locks. Fix this by using cgroup_name() rather than
dentry directly.

Also open code memcg_cache_name because it is called only from
kmem_cache_dup which frees the returned name right after
kmem_cache_create_memcg makes a copy of it. Such a short-lived
allocation doesn't make too much sense. So replace it by a static
buffer as kmem_cache_dup is called with memcg_cache_mutex.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Glauber Costa <glommer@parallels.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 32 insertions, 31 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53b8201b31eb..9715c0c491b0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3214,52 +3214,53 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
         schedule_work(&cachep->memcg_params->destroy);
 }
 
-static char *memcg_cache_name(struct mem_cgroup *memcg, struct kmem_cache *s)
-{
-        char *name;
-        struct dentry *dentry;
-
-        rcu_read_lock();
-        dentry = rcu_dereference(memcg->css.cgroup->dentry);
-        rcu_read_unlock();
-
-        BUG_ON(dentry == NULL);
-
-        name = kasprintf(GFP_KERNEL, "%s(%d:%s)", s->name,
-                         memcg_cache_id(memcg), dentry->d_name.name);
-
-        return name;
-}
+/*
+ * This lock protects updaters, not readers. We want readers to be as fast as
+ * they can, and they will either see NULL or a valid cache value. Our model
+ * allow them to see NULL, in which case the root memcg will be selected.
+ *
+ * We need this lock because multiple allocations to the same cache from a non
+ * will span more than one worker. Only one of them can create the cache.
+ */
+static DEFINE_MUTEX(memcg_cache_mutex);
 
+/*
+ * Called with memcg_cache_mutex held
+ */
 static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
                                          struct kmem_cache *s)
 {
-        char *name;
         struct kmem_cache *new;
+        static char *tmp_name = NULL;
 
-        name = memcg_cache_name(memcg, s);
-        if (!name)
-                return NULL;
+        lockdep_assert_held(&memcg_cache_mutex);
+
+        /*
+         * kmem_cache_create_memcg duplicates the given name and
+         * cgroup_name for this name requires RCU context.
+         * This static temporary buffer is used to prevent from
+         * pointless shortliving allocation.
+         */
+        if (!tmp_name) {
+                tmp_name = kmalloc(PATH_MAX, GFP_KERNEL);
+                if (!tmp_name)
+                        return NULL;
+        }
+
+        rcu_read_lock();
+        snprintf(tmp_name, PATH_MAX, "%s(%d:%s)", s->name,
+                         memcg_cache_id(memcg), cgroup_name(memcg->css.cgroup));
+        rcu_read_unlock();
 
-        new = kmem_cache_create_memcg(memcg, name, s->object_size, s->align,
+        new = kmem_cache_create_memcg(memcg, tmp_name, s->object_size, s->align,
                                       (s->flags & ~SLAB_PANIC), s->ctor, s);
 
         if (new)
                 new->allocflags |= __GFP_KMEMCG;
 
-        kfree(name);
         return new;
 }
 
-/*
- * This lock protects updaters, not readers. We want readers to be as fast as
- * they can, and they will either see NULL or a valid cache value. Our model
- * allow them to see NULL, in which case the root memcg will be selected.
- *
- * We need this lock because multiple allocations to the same cache from a non
- * will span more than one worker. Only one of them can create the cache.
- */
-static DEFINE_MUTEX(memcg_cache_mutex);
 static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
                                                   struct kmem_cache *cachep)
 {