Re: [PATCH] memcg: fix memcg_cache_name() to use cgroup_name()

Michal Hocko
2013-03-28T07:48:14Z

On Wed 27-03-13 18:32:23, Michal Hocko wrote:
[...]
> Removed WARN_ON_ONCE as suggested by Johannes and kept kmalloc with
> PATH_MAX used instead of PAGE_SIZE. I've kept Glauber's acked-by but I
> can remove it.

And hopefully the last version. I forgot to s/PAGE_SIZE/MAX_PATH/ in
snprintf.
---
From 551d7b5960904503da8f050faa533278a1d1bc6c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
Date: Thu, 28 Mar 2013 08:46:49 +0100
Subject: [PATCH] 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-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
Signed-off-by: Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>
Acked-by: Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>
---
mm/memcontrol.c | 63 ++++++++++++++++++++++++++++---------------------------
1 file changed, 32 insertions(+), 31 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53b8201..9715c0c 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)
{
--
1.7.10.4

--
Michal Hocko
SUSE Labs