linux-kernel - Re: [v4 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map

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Date:   Mon, 25 Jan 2021 11:35:58 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Yang Shi <shy828301@...il.com>, guro@...com, shakeelb@...gle.com,
        david@...morbit.com, hannes@...xchg.org, mhocko@...e.com,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map_size

On 22.01.2021 02:06, Yang Shi wrote:
> Both memcg_shrinker_map_size and shrinker_nr_max is maintained, but actually the
> map size can be calculated via shrinker_nr_max, so it seems unnecessary to keep both.
> Remove memcg_shrinker_map_size since shrinker_nr_max is also used by iterating the
> bit map.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>
> ---
>  mm/vmscan.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d3f3701dfcd2..40e7751ef961 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -185,8 +185,7 @@ static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
>  #ifdef CONFIG_MEMCG
> -
> -static int memcg_shrinker_map_size;
> +static int shrinker_nr_max;
>  
>  static void free_shrinker_map_rcu(struct rcu_head *head)
>  {
> @@ -248,7 +247,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
>  		return 0;
>  
>  	down_write(&shrinker_rwsem);
> -	size = memcg_shrinker_map_size;
> +	size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>  	for_each_node(nid) {
>  		map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
>  		if (!map) {
> @@ -266,10 +265,11 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
>  static int expand_shrinker_maps(int new_id)
>  {
>  	int size, old_size, ret = 0;
> +	int new_nr_max = new_id + 1;
>  	struct mem_cgroup *memcg;
>  
> -	size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> -	old_size = memcg_shrinker_map_size;
> +	size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
> +	old_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>
>  	if (size <= old_size)
>  		return 0;

This looks a BUG:

expand_shrinker_maps(id == 1)
{
	old_size = 64;
	size = 64;
	
	===>return 0 and shrinker_nr_max remains 0.
}

Then shrink_slab_memcg() misses this shrinker since shrinker_nr_max == 0.

>  
> @@ -286,9 +286,10 @@ static int expand_shrinker_maps(int new_id)
>  			goto out;
>  		}
>  	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
> +
>  out:
>  	if (!ret)
> -		memcg_shrinker_map_size = size;
> +		shrinker_nr_max = new_nr_max;
>  
>  	return ret;
>  }
> @@ -321,7 +322,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>  #define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
>  
>  static DEFINE_IDR(shrinker_idr);
> -static int shrinker_nr_max;
>  
>  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  {
> @@ -338,8 +338,6 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  			idr_remove(&shrinker_idr, id);
>  			goto unlock;
>  		}
> -
> -		shrinker_nr_max = id + 1;
>  	}
>  	shrinker->id = id;
>  	ret = 0;
>