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

[Vlastimil Babka <vbabka@suse.cz>]

On 7/7/20 8:27 AM, Muchun Song 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.
>   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.
>   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>

CC Roman, who worked in this area recently.

Also why is this marked "[PATCH v5.4.y, v4.19.y]"? Has it been fixed otherwise
in 5.5+ ?

> ---
>  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);
>