Re: [PATCH v5.4.y, v4.19.y] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy

[Shakeel Butt <shakeelb@google.com>]

Sorry I missed this email.


On Mon, Jul 6, 2020 at 11:28 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> If the kmem_cache refcount is greater than one, we should not
> mark the root kmem_cache as dying. If we mark the root kmem_cache
> dying incorrectly, the non-root kmem_cache can never be destroyed.
> It resulted in memory leak when memcg was destroyed. We can use the
> following steps to reproduce.
>
>   1) Use kmem_cache_create() to create a new kmem_cache named A.
>   2) Coincidentally, the kmem_cache A is an alias for kmem_cache B,
>      so the refcount of B is just increased.

I definitely missed the alias kmem cache case.

>   3) Use kmem_cache_destroy() to destroy the kmem_cache A, just
>      decrease the B's refcount but mark the B as dying.
>   4) Create a new memory cgroup and alloc memory from the kmem_cache
>      A. It leads to create a non-root kmem_cache for allocating.

I think in (4) you meant alloc memory from kmem_cache B instead of A.
There should not be any allocation from A after kmem_cache_destroy()
in (3).

>   5) When destroy the memory cgroup created in the step 4), the
>      non-root kmem_cache can never be destroyed.
>
> If we repeat steps 4) and 5), this will cause a lot of memory leak.
> So only when refcount reach zero, we mark the root kmem_cache as dying.
>
> Fixes: 92ee383f6daa ("mm: fix race between kmem_cache destroy, create and deactivate")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

The patch looks fine.

Reviewed-by: Shakeel Butt <shakeelb@google.com>

> ---
>  mm/slab_common.c | 43 +++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 41 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 8c1ffbf7de45..83ee6211aec7 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -258,6 +258,11 @@ static void memcg_unlink_cache(struct kmem_cache *s)
>                 list_del(&s->memcg_params.kmem_caches_node);
>         }
>  }
> +
> +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> +{
> +       return is_root_cache(s) && s->memcg_params.dying;
> +}
>  #else
>  static inline int init_memcg_params(struct kmem_cache *s,
>                                     struct kmem_cache *root_cache)
> @@ -272,6 +277,11 @@ static inline void destroy_memcg_params(struct kmem_cache *s)
>  static inline void memcg_unlink_cache(struct kmem_cache *s)
>  {
>  }
> +
> +static inline bool memcg_kmem_cache_dying(struct kmem_cache *s)
> +{
> +       return false;
> +}
>  #endif /* CONFIG_MEMCG_KMEM */
>
>  /*
> @@ -326,6 +336,13 @@ int slab_unmergeable(struct kmem_cache *s)
>         if (s->refcount < 0)
>                 return 1;
>
> +       /*
> +        * If the kmem_cache is dying. We should also skip this
> +        * kmem_cache.
> +        */
> +       if (memcg_kmem_cache_dying(s))
> +               return 1;
> +
>         return 0;
>  }
>
> @@ -944,8 +961,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
>         if (unlikely(!s))
>                 return;
>
> -       flush_memcg_workqueue(s);
> -
>         get_online_cpus();
>         get_online_mems();
>
> @@ -955,6 +970,30 @@ void kmem_cache_destroy(struct kmem_cache *s)
>         if (s->refcount)
>                 goto out_unlock;
>
> +#ifdef CONFIG_MEMCG_KMEM
> +       mutex_unlock(&slab_mutex);
> +
> +       put_online_mems();
> +       put_online_cpus();
> +
> +       flush_memcg_workqueue(s);
> +
> +       get_online_cpus();
> +       get_online_mems();
> +
> +       mutex_lock(&slab_mutex);
> +
> +       if (WARN(s->refcount,
> +                "kmem_cache_destroy %s: Slab cache is still referenced\n",
> +                s->name)) {
> +               /*
> +                * Reset the dying flag setted by flush_memcg_workqueue().
> +                */
> +               s->memcg_params.dying = false;
> +               goto out_unlock;
> +       }
> +#endif
> +
>         err = shutdown_memcg_caches(s);
>         if (!err)
>                 err = shutdown_cache(s);
> --
> 2.11.0
>