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

[Vladimir Davydov <vdavydov@parallels.com>]

On 12/19/2013 01:28 PM, Michal Hocko wrote:
> 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.

Not exactly (if you mean your replies to this series). We do not protect
kmem_caches, but we do protect the memcg_caches array, which can grow.

>
>> 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.

No. We protect only accesses to kmem_cache::memcg_params, which can
potentially be relocated for root caches. But as soon as we get the
pointer to a kmem_cache from this array, we can freely dereference it,
because the cache cannot be freed when we use it. This is, because we
access a kmem_cache either under the slab_mutex or
memcg->slab_caches_mutex, or when we allocate/free from it. While doing
the latter, the cache can't go away, it would be a bug. IMO.

Thanks.

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