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

Michal Hocko
2013-03-22T09:31:41Z

On Fri 22-03-13 12:22:23, Glauber Costa wrote:
> On 03/22/2013 12:17 PM, Li Zefan wrote:
> >> GFP_TEMPORARY groups short lived allocations but the mem cache is not
> >> > an ideal candidate of this type of allocations..
> >> >
> > I'm not sure I'm following you...
> >
> > char *memcg_cache_name()
> > {
> > char *name = alloc();
> > return name;
> > }
> >
> > kmem_cache_dup()
> > {
> > name = memcg_cache_name();
> > kmem_cache_create_memcg(name);
> > free(name);
> > }
> >
> > Isn't this a short lived allocation?
> >
>
> Hi,
>
> Thanks for identifying and fixing this.
>
> Li is right. The cache name will live long, but this is because the
> slab/slub caches will strdup it internally. So the actual memcg
> allocation is short lived.

OK, I have totally missed that. Sorry about the confusion. Then all the
churn around the allocation is pointless, no?
What about:
---
From 7ed7f53bb597e8cb40d9ac91ce16142fb60f1e93 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.cz>
Date: Fri, 22 Mar 2013 10:22:54 +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@huawei.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
---
mm/memcontrol.c | 33 +++++++++++----------------------
1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 53b8201..9452b56 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3214,40 +3214,29 @@ 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;
-}
-
+/*
+ * 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[PAGE_SIZE];

- name = memcg_cache_name(memcg, s);
- if (!name)
- return NULL;
+ lockdep_assert_held(&memcg_cache_mutex);
+
+ rcu_read_lock();
+ tmp_name = snprintf(tmp_name, sizeof(tmp_name), "%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;
}

--
1.7.10.4

--
Michal Hocko
SUSE Labs

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