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

Michal Hocko
2013-03-21T09:08:49Z

On Thu 21-03-13 09:22:21, Li Zefan wrote:
> As cgroup supports rename, it's unsafe to dereference dentry->d_name
> without proper vfs locks. Fix this by using cgroup_name().
>
> Signed-off-by: Li Zefan <lizefan@huawei.com>
> ---
>
> This patch depends on "cgroup: fix cgroup_path() vs rename() race",
> which has been queued for 3.10.
>
> ---
> mm/memcontrol.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 53b8201..72be5c9 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3217,17 +3217,16 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
> static char *memcg_cache_name(struct mem_cgroup *memcg, struct kmem_cache *s)
> {
> char *name;
> - struct dentry *dentry;
> +
> + name = (char *)__get_free_page(GFP_TEMPORARY);

Ouch. Can we use a static temporary buffer instead? This is called from
workqueue context so we do not have to be afraid of the deep call chain.
It is also not a hot path AFAICS.

Even GFP_ATOMIC for kasprintf would be an improvement IMO.

> + if (!name)
> + return NULL;
>
> rcu_read_lock();
> - dentry = rcu_dereference(memcg->css.cgroup->dentry);
> + snprintf(name, PAGE_SIZE, "%s(%d:%s)", s->name, memcg_cache_id(memcg),
> + cgroup_name(memcg->css.cgroup));
> 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;
> }
>
> @@ -3247,7 +3246,7 @@ static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
> if (new)
> new->allocflags |= __GFP_KMEMCG;
>
> - kfree(name);
> + free_page((unsigned long)name);
> return new;
> }
>
> --
> 1.8.0.2
> --
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--
Michal Hocko
SUSE Labs

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