Re: [v3 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 06.01.2021 01:58, Yang Shi wrote:
> Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem
> exclusively, the read side can be protected by holding read lock, so it sounds
> superfluous to have a dedicated mutex.  This should not exacerbate the contention
> to shrinker_rwsem since just one read side critical section is added.
> 
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
>  mm/vmscan.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9db7b4d6d0ae..ddb9f972f856 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem);
>  #ifdef CONFIG_MEMCG
>  
>  static int memcg_shrinker_map_size;
> -static DEFINE_MUTEX(memcg_shrinker_map_mutex);
>  
>  static void memcg_free_shrinker_map_rcu(struct rcu_head *head)
>  {
> @@ -200,8 +199,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg,
>  	struct memcg_shrinker_map *new, *old;
>  	int nid;
>  
> -	lockdep_assert_held(&memcg_shrinker_map_mutex);
> -
>  	for_each_node(nid) {
>  		old = rcu_dereference_protected(
>  			mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true);
> @@ -250,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
>  	if (mem_cgroup_is_root(memcg))
>  		return 0;
>  
> -	mutex_lock(&memcg_shrinker_map_mutex);
> +	down_read(&shrinker_rwsem);
>  	size = memcg_shrinker_map_size;
>  	for_each_node(nid) {
>  		map = kvzalloc(sizeof(*map) + size, GFP_KERNEL);
> @@ -261,7 +258,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
>  		}
>  		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);

Here we do STORE operation, and since we want the assignment is visible
for shrink_slab_memcg() under down_read(), we have to use down_write()
in memcg_alloc_shrinker_maps().

>  	}
> -	mutex_unlock(&memcg_shrinker_map_mutex);
> +	up_read(&shrinker_rwsem);
>  
>  	return ret;
>  }
> @@ -276,9 +273,8 @@ static int memcg_expand_shrinker_maps(int new_id)
>  	if (size <= old_size)
>  		return 0;
>  
> -	mutex_lock(&memcg_shrinker_map_mutex);
>  	if (!root_mem_cgroup)
> -		goto unlock;
> +		goto out;
>  
>  	memcg = mem_cgroup_iter(NULL, NULL, NULL);
>  	do {
> @@ -287,13 +283,13 @@ static int memcg_expand_shrinker_maps(int new_id)
>  		ret = memcg_expand_one_shrinker_map(memcg, size, old_size);
>  		if (ret) {
>  			mem_cgroup_iter_break(NULL, memcg);
> -			goto unlock;
> +			goto out;
>  		}
>  	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> -unlock:
> +out:
>  	if (!ret)
>  		memcg_shrinker_map_size = size;
> -	mutex_unlock(&memcg_shrinker_map_mutex);
> +
>  	return ret;
>  }
>  
>