Re: [PATCH 4/6] memcg, slab: check and init memcg_cahes under slab_mutex

[Vladimir Davydov <vdavydov@parallels.com>]

On 12/18/2013 09:41 PM, Michal Hocko wrote:
> On Wed 18-12-13 17:16:55, Vladimir Davydov wrote:
>> The memcg_params::memcg_caches array can be updated concurrently from
>> memcg_update_cache_size() and memcg_create_kmem_cache(). Although both
>> of these functions take the slab_mutex during their operation, the
>> latter checks if memcg's cache has already been allocated w/o taking the
>> mutex. This can result in a race as described below.
>>
>> Asume two threads schedule kmem_cache creation works for the same
>> kmem_cache of the same memcg from __memcg_kmem_get_cache(). One of the
>> works successfully creates it. Another work should fail then, but if it
>> interleaves with memcg_update_cache_size() as follows, it does not:
> I am not sure I understand the race. memcg_update_cache_size is called
> when we start accounting a new memcg or a child is created and it
> inherits accounting from the parent. memcg_create_kmem_cache is called
> when a new cache is first allocated from, right?

memcg_update_cache_size() is called when kmem accounting is activated
for a memcg, no matter how.

memcg_create_kmem_cache() is scheduled from __memcg_kmem_get_cache().
It's OK to have a bunch of such methods trying to create the same memcg
cache concurrently, but only one of them should succeed.

> Why cannot we simply take slab_mutex inside memcg_create_kmem_cache?
> it is running from the workqueue context so it should clash with other
> locks.

Hmm, Glauber's code never takes the slab_mutex inside memcontrol.c. I
have always been wondering why, because it could simplify flow paths
significantly (e.g. update_cache_sizes() -> update_all_caches() ->
update_cache_size() - from memcontrol.c to slab_common.c and back again
just to take the mutex).

I don't see any reason preventing us from taking the mutex in
memcontrol.c. This would allow us to move all memcg-related kmem cache
initialization (addition to the memcg slab list, initialization of the
pointer in memcg_caches) to memcg_kmem_cache_create() and remove a bunch
of public functions. I guess I'll do this in my next iteration.

Thanks.

>
>>   memcg_create_kmem_cache()                     memcg_update_cache_size()
>>   (called w/o mutexes held)                     (called with slab_mutex held)
>>   -------------------------                     -------------------------
>>   mutex_lock(&memcg_cache_mutex)
>>                                                 s->memcg_params=kzalloc(...)
>>   new_cachep=cache_from_memcg_idx(cachep,idx)
>>   // new_cachep==NULL => proceed to creation
>>                                                 s->memcg_params->memcg_caches[i]
>>                                                     =cur_params->memcg_caches[i]
>>   // kmem_cache_dup takes slab_mutex so we will
>>   // hang around here until memcg_update_cache_size()
>>   // finishes, but ...
>>   new_cachep = kmem_cache_dup(memcg, cachep)
>>   // nothing will prevent kmem_cache_dup from
>>   // succeeding so ...
>>   cachep->memcg_params->memcg_caches[idx]=new_cachep
>>   // we've overwritten an existing cache ptr!
>>
>> Let's fix this by moving both the check and the update of
>> memcg_params::memcg_caches from memcg_create_kmem_cache() to
>> kmem_cache_create_memcg() to be called under the slab_mutex.
>>
>> 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/memcontrol.h |    9 ++--
>>  mm/memcontrol.c            |   98 +++++++++++++++-----------------------------
>>  mm/slab_common.c           |    8 +++-
>>  3 files changed, 44 insertions(+), 71 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index b357ae3..fdd3f30 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -500,8 +500,8 @@ int memcg_cache_id(struct mem_cgroup *memcg);
>>  int memcg_init_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
>>  			    struct kmem_cache *root_cache);
>>  void memcg_free_cache_params(struct kmem_cache *s);
>> -void memcg_release_cache(struct kmem_cache *cachep);
>> -void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep);
>> +void memcg_register_cache(struct kmem_cache *s);
>> +void memcg_release_cache(struct kmem_cache *s);
>>  
>>  int memcg_update_cache_size(struct kmem_cache *s, int num_groups);
>>  void memcg_update_array_size(int num_groups);
>> @@ -652,12 +652,11 @@ static inline void memcg_free_cache_params(struct kmem_cache *s);
>>  {
>>  }
>>  
>> -static inline void memcg_release_cache(struct kmem_cache *cachep)
>> +static inline void memcg_register_cache(struct kmem_cache *s)
>>  {
>>  }
>>  
>> -static inline void memcg_cache_list_add(struct mem_cgroup *memcg,
>> -					struct kmem_cache *s)
>> +static inline void memcg_release_cache(struct kmem_cache *s)
>>  {
>>  }
>>  
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index e37fdb5..62b9991 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3059,16 +3059,6 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
>>  		css_put(&memcg->css);
>>  }
>>  
>> -void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
>> -{
>> -	if (!memcg)
>> -		return;
>> -
>> -	mutex_lock(&memcg->slab_caches_mutex);
>> -	list_add(&cachep->memcg_params->list, &memcg->memcg_slab_caches);
>> -	mutex_unlock(&memcg->slab_caches_mutex);
>> -}
>> -
>>  /*
>>   * helper for acessing a memcg's index. It will be used as an index in the
>>   * child cache array in kmem_cache, and also to derive its name. This function
>> @@ -3229,6 +3219,35 @@ void memcg_free_cache_params(struct kmem_cache *s)
>>  	kfree(s->memcg_params);
>>  }
>>  
>> +void memcg_register_cache(struct kmem_cache *s)
>> +{
>> +	struct kmem_cache *root;
>> +	struct mem_cgroup *memcg;
>> +	int id;
>> +
>> +	if (is_root_cache(s))
>> +		return;
>> +
>> +	memcg = s->memcg_params->memcg;
>> +	id = memcg_cache_id(memcg);
>> +	root = s->memcg_params->root_cache;
>> +
>> +	css_get(&memcg->css);
>> +
>> +	/*
>> +	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
>> +	 * barrier here to ensure nobody will see the kmem_cache partially
>> +	 * initialized.
>> +	 */
>> +	smp_wmb();
>> +
>> +	root->memcg_params->memcg_caches[id] = s;
>> +
>> +	mutex_lock(&memcg->slab_caches_mutex);
>> +	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
>> +	mutex_unlock(&memcg->slab_caches_mutex);
>> +}
>> +
>>  void memcg_release_cache(struct kmem_cache *s)
>>  {
>>  	struct kmem_cache *root;
>> @@ -3356,26 +3375,13 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
>>  	schedule_work(&cachep->memcg_params->destroy);
>>  }
>>  
>> -/*
>> - * This lock protects updaters, not readers. We want readers to be as fast as
>> - * they can, and they will either see NULL or a valid cache value. Our model
>> - * allow them to see NULL, in which case the root memcg will be selected.
>> - *
>> - * We need this lock because multiple allocations to the same cache from a non
>> - * will span more than one worker. Only one of them can create the cache.
>> - */
>> -static DEFINE_MUTEX(memcg_cache_mutex);
>> -
>> -/*
>> - * Called with memcg_cache_mutex held
>> - */
>> -static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
>> -					 struct kmem_cache *s)
>> +static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
>> +						  struct kmem_cache *s)
>>  {
>>  	struct kmem_cache *new;
>>  	static char *tmp_name = NULL;
>>  
>> -	lockdep_assert_held(&memcg_cache_mutex);
>> +	BUG_ON(!memcg_can_account_kmem(memcg));
>>  
>>  	/*
>>  	 * kmem_cache_create_memcg duplicates the given name and
>> @@ -3403,45 +3409,6 @@ static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
>>  	return new;
>>  }
>>  
>> -static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
>> -						  struct kmem_cache *cachep)
>> -{
>> -	struct kmem_cache *new_cachep;
>> -	int idx;
>> -
>> -	BUG_ON(!memcg_can_account_kmem(memcg));
>> -
>> -	idx = memcg_cache_id(memcg);
>> -
>> -	mutex_lock(&memcg_cache_mutex);
>> -	new_cachep = cache_from_memcg_idx(cachep, idx);
>> -	if (new_cachep) {
>> -		css_put(&memcg->css);
>> -		goto out;
>> -	}
>> -
>> -	new_cachep = kmem_cache_dup(memcg, cachep);
>> -	if (new_cachep == NULL) {
>> -		new_cachep = cachep;
>> -		css_put(&memcg->css);
>> -		goto out;
>> -	}
>> -
>> -	atomic_set(&new_cachep->memcg_params->nr_pages , 0);
>> -
>> -	/*
>> -	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
>> -	 * barrier here to ensure nobody will see the kmem_cache partially
>> -	 * initialized.
>> -	 */
>> -	smp_wmb();
>> -
>> -	cachep->memcg_params->memcg_caches[idx] = new_cachep;
>> -out:
>> -	mutex_unlock(&memcg_cache_mutex);
>> -	return new_cachep;
>> -}
>> -
>>  void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
>>  {
>>  	struct kmem_cache *c;
>> @@ -3516,6 +3483,7 @@ static void memcg_create_cache_work_func(struct work_struct *w)
>>  
>>  	cw = container_of(w, struct create_work, work);
>>  	memcg_create_kmem_cache(cw->memcg, cw->cachep);
>> +	css_put(&cw->memcg->css);
>>  	kfree(cw);
>>  }
>>  
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 62712fe..51dc106 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -176,6 +176,12 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
>>  	get_online_cpus();
>>  	mutex_lock(&slab_mutex);
>>  
>> +	if (memcg) {
>> +		s = cache_from_memcg_idx(parent_cache, memcg_cache_id(memcg));
>> +		if (s)
>> +			goto out_unlock;
>> +	}
>> +
>>  	err = kmem_cache_sanity_check(memcg, name, size);
>>  	if (err)
>>  		goto out_unlock;
>> @@ -218,7 +224,7 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
>>  
>>  	s->refcount = 1;
>>  	list_add(&s->list, &slab_caches);
>> -	memcg_cache_list_add(memcg, s);
>> +	memcg_register_cache(s);
>>  
>>  out_unlock:
>>  	mutex_unlock(&slab_mutex);
>> -- 
>> 1.7.10.4
>>

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