Re: [PATCHv4] mm: slab: shutdown caches only after releasing sysfs file

[Vladimir Davydov <vdavydov@virtuozzo.com>]

On Thu, Feb 04, 2016 at 07:39:48PM +0300, Dmitry Safonov wrote:
...
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index b7e57927..a6bf41a 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -103,9 +103,10 @@ struct kmem_cache {
>  
>  #ifdef CONFIG_SYSFS
>  #define SLAB_SUPPORTS_SYSFS
> -void sysfs_slab_remove(struct kmem_cache *);
> +int sysfs_slab_remove(struct kmem_cache *);
> +void sysfs_slab_remove_cancel(struct kmem_cache *s);
>  #else
> -static inline void sysfs_slab_remove(struct kmem_cache *s)
> +void sysfs_slab_remove_cancel(struct kmem_cache *s)
>  {
>  }
>  #endif
> diff --git a/mm/slab.h b/mm/slab.h
> index 834ad24..ec87600 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -141,7 +141,7 @@ static inline unsigned long kmem_cache_flags(unsigned long object_size,
>  
>  int __kmem_cache_shutdown(struct kmem_cache *);
>  int __kmem_cache_shrink(struct kmem_cache *, bool);
> -void slab_kmem_cache_release(struct kmem_cache *);
> +int slab_kmem_cache_release(struct kmem_cache *);
>  
>  struct seq_file;
>  struct file;
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index b50aef0..3ad3d22 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -448,33 +448,58 @@ out_unlock:
>  }
>  EXPORT_SYMBOL(kmem_cache_create);
>  
> -static int shutdown_cache(struct kmem_cache *s,
> +static void prepare_caches_release(struct kmem_cache *s,
>  		struct list_head *release, bool *need_rcu_barrier)
>  {
> -	if (__kmem_cache_shutdown(s) != 0)
> -		return -EBUSY;
> -
>  	if (s->flags & SLAB_DESTROY_BY_RCU)
>  		*need_rcu_barrier = true;
>  
>  	list_move(&s->list, release);
> -	return 0;
>  }
>  
> -static void release_caches(struct list_head *release, bool need_rcu_barrier)
> +#ifdef SLAB_SUPPORTS_SYSFS
> +#define release_one_cache sysfs_slab_remove
> +#else
> +#define release_one_cache slab_kmem_cache_release
> +#endif
> +
> +static int release_caches_type(struct list_head *release, bool children)
>  {
>  	struct kmem_cache *s, *s2;
> +	int ret = 0;
>  
> +	list_for_each_entry_safe(s, s2, release, list) {
> +		if (is_root_cache(s) == children)
> +			continue;
> +
> +		ret += release_one_cache(s);
> +	}
> +	return ret;
> +}
> +
> +static void release_caches(struct list_head *release, bool need_rcu_barrier)
> +{
>  	if (need_rcu_barrier)
>  		rcu_barrier();

You must issue an rcu barrier before freeing kmem_cache structure, not
before issuing __kmem_cache_shutdown. Otherwise a slab freed by
__kmem_cache_shutdown might hit use-after-free bug.

>  
> -	list_for_each_entry_safe(s, s2, release, list) {
> -#ifdef SLAB_SUPPORTS_SYSFS
> -		sysfs_slab_remove(s);
> -#else
> -		slab_kmem_cache_release(s);
> -#endif
> -	}
> +	/* remove children in the first place, remove root on success */
> +	if (!release_caches_type(release, true))
> +		release_caches_type(release, false);
> +}
> +
> +static void release_cache_cancel(struct kmem_cache *s)
> +{
> +	sysfs_slab_remove_cancel(s);
> +
> +	get_online_cpus();
> +	get_online_mems();

What's point taking these locks when you just want to add a cache to the
slab_list?

Besides, if cpu/mem hotplug happens *between* prepare_cache_release and
release_cache_cancel we'll get a cache on the list with not all per
node/cpu structures allocated.

> +	mutex_lock(&slab_mutex);
> +
> +	list_move(&s->list, &slab_caches);
> +
> +	mutex_unlock(&slab_mutex);
> +	put_online_mems();
> +	put_online_cpus();
>  }
>  
>  #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
> @@ -589,16 +614,14 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
>  	put_online_cpus();
>  }
>  
> -static int __shutdown_memcg_cache(struct kmem_cache *s,
> +static void prepare_memcg_empty_caches(struct kmem_cache *s,
>  		struct list_head *release, bool *need_rcu_barrier)
>  {
>  	BUG_ON(is_root_cache(s));
>  
> -	if (shutdown_cache(s, release, need_rcu_barrier))
> -		return -EBUSY;
> +	prepare_caches_release(s, release, need_rcu_barrier);
>  
> -	list_del(&s->memcg_params.list);
> -	return 0;
> +	list_del_init(&s->memcg_params.list);

AFAICS if kmem_cache_destroy fails, you don't add per-memcg cache back
to the list. Not good.

>  }
>  
>  void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
> @@ -614,11 +637,12 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
>  	list_for_each_entry_safe(s, s2, &slab_caches, list) {
>  		if (is_root_cache(s) || s->memcg_params.memcg != memcg)
>  			continue;
> +
>  		/*
>  		 * The cgroup is about to be freed and therefore has no charges
>  		 * left. Hence, all its caches must be empty by now.
>  		 */
> -		BUG_ON(__shutdown_memcg_cache(s, &release, &need_rcu_barrier));

It was a nice little check if everything works fine on memcg side. And
you wiped it out. Sad.

> +		prepare_memcg_empty_caches(s, &release, &need_rcu_barrier);
>  	}
>  	mutex_unlock(&slab_mutex);
>  
> @@ -628,81 +652,53 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)
>  	release_caches(&release, need_rcu_barrier);
>  }
>  
> -static int shutdown_memcg_caches(struct kmem_cache *s,
> +static void prepare_memcg_filled_caches(struct kmem_cache *s,
>  		struct list_head *release, bool *need_rcu_barrier)
>  {
>  	struct memcg_cache_array *arr;
>  	struct kmem_cache *c, *c2;
> -	LIST_HEAD(busy);
> -	int i;
>  
>  	BUG_ON(!is_root_cache(s));
>  
> -	/*
> -	 * First, shutdown active caches, i.e. caches that belong to online
> -	 * memory cgroups.
> -	 */

>  	arr = rcu_dereference_protected(s->memcg_params.memcg_caches,
>  					lockdep_is_held(&slab_mutex));

And now what's that for?

> -	for_each_memcg_cache_index(i) {
> -		c = arr->entries[i];
> -		if (!c)
> -			continue;
> -		if (__shutdown_memcg_cache(c, release, need_rcu_barrier))
> -			/*
> -			 * The cache still has objects. Move it to a temporary
> -			 * list so as not to try to destroy it for a second
> -			 * time while iterating over inactive caches below.
> -			 */
> -			list_move(&c->memcg_params.list, &busy);
> -		else
> -			/*
> -			 * The cache is empty and will be destroyed soon. Clear
> -			 * the pointer to it in the memcg_caches array so that
> -			 * it will never be accessed even if the root cache
> -			 * stays alive.
> -			 */
> -			arr->entries[i] = NULL;

So you don't clean arr->entries on global cache destruction. If we fail
to destroy a cache, this will result in use-after-free when the global
cache gets reused.

> -	}
>  
> -	/*
> -	 * Second, shutdown all caches left from memory cgroups that are now
> -	 * offline.
> -	 */
> +	/* move children caches to release list */
>  	list_for_each_entry_safe(c, c2, &s->memcg_params.list,
>  				 memcg_params.list)
> -		__shutdown_memcg_cache(c, release, need_rcu_barrier);
> -
> -	list_splice(&busy, &s->memcg_params.list);
> +		prepare_caches_release(c, release, need_rcu_barrier);
>  
> -	/*
> -	 * A cache being destroyed must be empty. In particular, this means
> -	 * that all per memcg caches attached to it must be empty too.
> -	 */
> -	if (!list_empty(&s->memcg_params.list))
> -		return -EBUSY;
> -	return 0;
> +	/* root cache to the same place */
> +	prepare_caches_release(s, release, need_rcu_barrier);
>  }
> +
>  #else
> -static inline int shutdown_memcg_caches(struct kmem_cache *s,
> -		struct list_head *release, bool *need_rcu_barrier)
> -{
> -	return 0;
> -}
> +#define prepare_memcg_filled_caches prepare_caches_release
>  #endif /* CONFIG_MEMCG && !CONFIG_SLOB */
>  
> -void slab_kmem_cache_release(struct kmem_cache *s)
> +int slab_kmem_cache_release(struct kmem_cache *s)
>  {
> +	if (__kmem_cache_shutdown(s)) {
> +		WARN(1, "release slub cache %s failed: it still has objects\n",
> +			s->name);
> +		release_cache_cancel(s);
> +		return 1;
> +	}
> +
> +#ifdef CONFIG_MEMCG
> +	list_del(&s->memcg_params.list);
> +#endif
> +
>  	destroy_memcg_params(s);
>  	kfree_const(s->name);
>  	kmem_cache_free(kmem_cache, s);
> +	return 0;
>  }
>  
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
>  	LIST_HEAD(release);
>  	bool need_rcu_barrier = false;
> -	int err;
>  
>  	if (unlikely(!s))
>  		return;
> @@ -716,15 +712,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	if (s->refcount)
>  		goto out_unlock;
>  
> -	err = shutdown_memcg_caches(s, &release, &need_rcu_barrier);
> -	if (!err)
> -		err = shutdown_cache(s, &release, &need_rcu_barrier);
> +	prepare_memcg_filled_caches(s, &release, &need_rcu_barrier);
>  
> -	if (err) {
> -		pr_err("kmem_cache_destroy %s: "
> -		       "Slab cache still has objects\n", s->name);
> -		dump_stack();
> -	}
>  out_unlock:
>  	mutex_unlock(&slab_mutex);
>  
> diff --git a/mm/slub.c b/mm/slub.c
> index 2e1355a..373aa6d 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5429,14 +5429,14 @@ out_del_kobj:
>  	goto out;
>  }
>  
> -void sysfs_slab_remove(struct kmem_cache *s)
> +int sysfs_slab_remove(struct kmem_cache *s)
>  {
>  	if (slab_state < FULL)
>  		/*
>  		 * Sysfs has not been setup yet so no need to remove the
>  		 * cache from sysfs.
>  		 */
> -		return;
> +		return 0;
>  
>  #ifdef CONFIG_MEMCG
>  	kset_unregister(s->memcg_kset);
> @@ -5444,6 +5444,24 @@ void sysfs_slab_remove(struct kmem_cache *s)
>  	kobject_uevent(&s->kobj, KOBJ_REMOVE);
>  	kobject_del(&s->kobj);
>  	kobject_put(&s->kobj);
> +	return 0;
> +}
> +
> +void sysfs_slab_remove_cancel(struct kmem_cache *s)
> +{
> +	int ret;
> +
> +	if (slab_state < FULL)
> +		return;
> +
> +	/* tricky */

What purpose is this comment supposed to serve for?

> +	kobject_get(&s->kobj);
> +	ret = kobject_add(&s->kobj, NULL, "%s", s->name);

ret is not used.

> +	kobject_uevent(&s->kobj, KOBJ_ADD);
> +
> +#ifdef CONFIG_MEMCG
> +	s->memcg_kset = kset_create_and_add("cgroup", NULL, &s->kobj);
> +#endif

For per-memcg cache we must not create memcg_kset.

And what if any of these functions fail? I don't think that silently
ignoring failures is good.

Anyway, I really don't like how this patch approaches the problem. AFAIU
we only want to postpone kmem_cache_node free until sysfs file is
destroyed. Can't we just move the code freeing kmem_cache_node to a
separate function which will be called from sysfs release instead of
messing things up?

Thanks,
Vladimir

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