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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53b8201..9715c0c 100644 (file)
--- 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)
 {