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@xxxxxxxxxxxxx>
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);
>
