Re: [PATCH 6/6] memcg, slab: RCU protect memcg_params for root caches

[Michal Hocko <mhocko@suse.cz>]

On Wed 18-12-13 17:16:57, Vladimir Davydov wrote:
> We update root cache's memcg_params whenever we need to grow the
> memcg_caches array to accommodate all kmem-active memory cgroups.
> Currently we free the old version immediately then, which can lead to
> use-after-free, because the memcg_caches array is accessed lock-free.
> This patch fixes this by making memcg_params RCU-protected.

yes, I was thinking about something like this when talking about RCU
usage.

> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Glauber Costa <glommer@gmail.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
>  include/linux/slab.h |    5 ++++-
>  mm/memcontrol.c      |   15 ++++++++-------
>  mm/slab.h            |    8 +++++++-
>  3 files changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 1e2f4fe..f7e5649 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -528,7 +528,10 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
>  struct memcg_cache_params {
>  	bool is_root_cache;
>  	union {
> -		struct kmem_cache *memcg_caches[0];
> +		struct {
> +			struct rcu_head rcu_head;
> +			struct kmem_cache *memcg_caches[0];
> +		};
>  		struct {
>  			struct mem_cgroup *memcg;
>  			struct list_head list;
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ad8de6a..379fc5f 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3142,18 +3142,17 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
>  
>  	if (num_groups > memcg_limited_groups_array_size) {
>  		int i;
> +		struct memcg_cache_params *new_params;
>  		ssize_t size = memcg_caches_array_size(num_groups);
>  
>  		size *= sizeof(void *);
>  		size += offsetof(struct memcg_cache_params, memcg_caches);
>  
> -		s->memcg_params = kzalloc(size, GFP_KERNEL);
> -		if (!s->memcg_params) {
> -			s->memcg_params = cur_params;
> +		new_params = kzalloc(size, GFP_KERNEL);
> +		if (!new_params)
>  			return -ENOMEM;
> -		}
>  
> -		s->memcg_params->is_root_cache = true;
> +		new_params->is_root_cache = true;
>  
>  		/*
>  		 * There is the chance it will be bigger than
> @@ -3167,7 +3166,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
>  		for (i = 0; i < memcg_limited_groups_array_size; i++) {
>  			if (!cur_params->memcg_caches[i])
>  				continue;
> -			s->memcg_params->memcg_caches[i] =
> +			new_params->memcg_caches[i] =
>  						cur_params->memcg_caches[i];
>  		}
>  
> @@ -3180,7 +3179,9 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
>  		 * bigger than the others. And all updates will reset this
>  		 * anyway.
>  		 */
> -		kfree(cur_params);
> +		rcu_assign_pointer(s->memcg_params, new_params);
> +		if (cur_params)
> +			kfree_rcu(cur_params, rcu_head);
>  	}
>  	return 0;
>  }
> diff --git a/mm/slab.h b/mm/slab.h
> index 1d8b53f..53b81a9 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -164,10 +164,16 @@ static inline struct kmem_cache *
>  cache_from_memcg_idx(struct kmem_cache *s, int idx)
>  {
>  	struct kmem_cache *cachep;
> +	struct memcg_cache_params *params;
>  
>  	if (!s->memcg_params)
>  		return NULL;
> -	cachep = s->memcg_params->memcg_caches[idx];
> +
> +	rcu_read_lock();
> +	params = rcu_dereference(s->memcg_params);
> +	cachep = params->memcg_caches[idx];
> +	rcu_read_unlock();
> +

Consumer has to be covered by the same rcu section otherwise
memcg_params might be freed right after rcu unlock here.

>  	smp_read_barrier_depends();	/* see memcg_register_cache() */
>  	return cachep;
>  }
> -- 
> 1.7.10.4
> 

-- 
Michal Hocko
SUSE Labs

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