On 7/16/20 6:51 PM, 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
> B. It leads to create a non-root kmem_cache for allocating memory.
> 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>
> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Acked-by: Roman Gushchin <guro@xxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
And it will need to go before the series that starts with
mm-memcg-factor-out-memcg-and-lruvec-level-changes-out-of-__mod_lruvec_state.patch
most likely causing some collisions there to be fixed up...
> ---
>
> changelog in v3:
> 1) Simplify the code suggested by Roman.
>
> changelog in v2:
> 1) Fix a confusing typo in the commit log.
> 2) Remove flush_memcg_workqueue() for !CONFIG_MEMCG_KMEM.
> 3) Introduce a new helper memcg_set_kmem_cache_dying() to fix a race
> condition between flush_memcg_workqueue() and slab_unmergeable().
>
> mm/slab_common.c | 35 ++++++++++++++++++++++++++++-------
> 1 file changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 37d48a56431d..fe8b68482670 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -326,6 +326,14 @@ int slab_unmergeable(struct kmem_cache *s)
> if (s->refcount < 0)
> return 1;
>
> +#ifdef CONFIG_MEMCG_KMEM
> + /*
> + * Skip the dying kmem_cache.
> + */
> + if (s->memcg_params.dying)
> + return 1;
> +#endif
> +
> return 0;
> }
>
> @@ -886,12 +894,15 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
> return 0;
> }
>
> -static void flush_memcg_workqueue(struct kmem_cache *s)
> +static void memcg_set_kmem_cache_dying(struct kmem_cache *s)
> {
> spin_lock_irq(&memcg_kmem_wq_lock);
> s->memcg_params.dying = true;
> spin_unlock_irq(&memcg_kmem_wq_lock);
> +}
>
> +static void flush_memcg_workqueue(struct kmem_cache *s)
> +{
> /*
> * SLAB and SLUB deactivate the kmem_caches through call_rcu. Make
> * sure all registered rcu callbacks have been invoked.
> @@ -923,10 +934,6 @@ static inline int shutdown_memcg_caches(struct kmem_cache *s)
> {
> return 0;
> }
> -
> -static inline void flush_memcg_workqueue(struct kmem_cache *s)
> -{
> -}
> #endif /* CONFIG_MEMCG_KMEM */
>
> void slab_kmem_cache_release(struct kmem_cache *s)
> @@ -944,8 +951,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
> if (unlikely(!s))
> return;
>
> - flush_memcg_workqueue(s);
> -
> get_online_cpus();
> get_online_mems();
>
> @@ -955,6 +960,22 @@ void kmem_cache_destroy(struct kmem_cache *s)
> if (s->refcount)
> goto out_unlock;
>
> +#ifdef CONFIG_MEMCG_KMEM
> + memcg_set_kmem_cache_dying(s);
> +
> + mutex_unlock(&slab_mutex);
> +
> + put_online_mems();
> + put_online_cpus();
> +
> + flush_memcg_workqueue(s);
> +
> + get_online_cpus();
> + get_online_mems();
> +
> + mutex_lock(&slab_mutex);
> +#endif
> +
> err = shutdown_memcg_caches(s);
> if (!err)
> err = shutdown_cache(s);
>
