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

Johannes Weiner
2013-03-27T14:58:25Z

On Wed, Mar 27, 2013 at 09:36:39AM +0100, Michal Hocko wrote:
> 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>
> ---
> mm/memcontrol.c | 64 ++++++++++++++++++++++++++++---------------------------
> 1 file changed, 33 insertions(+), 31 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f608546..b30547b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3364,52 +3364,54 @@ 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(PAGE_SIZE, GFP_KERNEL);
> + WARN_ON_ONCE(!tmp_name);

Just use the page allocator directly and get a free allocation failure
warning. Then again, order-0 pages are considered cheap enough that
they never even fail in our current implementation.

Which brings me to my other point: why not just a simple single-page
allocation? This just seems a little overelaborate. I think this
path would be taken predominantly after cgroup creation and fork where
we do a bunch of allocations anyway. And it happens asynchroneously
from userspace, so it's not even really performance critical.

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