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Date:   Wed, 21 Mar 2018 07:56:25 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
        vdavydov.dev@gmail.com, akpm@linux-foundation.org,
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        linux-mm@kvack.org
Subject: Re: [PATCH 03/10] mm: Assign memcg-aware shrinkers bitmap to memcg
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On Wed, Mar 21, 2018 at 04:21:40PM +0300, Kirill Tkhai wrote:
> +++ b/include/linux/memcontrol.h
> @@ -151,6 +151,11 @@ struct mem_cgroup_thresholds {
>  	struct mem_cgroup_threshold_ary *spare;
>  };
>  
> +struct shrinkers_map {
> +	struct rcu_head rcu;
> +	unsigned long *map[0];
> +};
> +
>  enum memcg_kmem_state {
>  	KMEM_NONE,
>  	KMEM_ALLOCATED,
> @@ -182,6 +187,9 @@ struct mem_cgroup {
>  	unsigned long low;
>  	unsigned long high;
>  
> +	/* Bitmap of shrinker ids suitable to call for this memcg */
> +	struct shrinkers_map __rcu *shrinkers_map;
> +
>  	/* Range enforcement for interrupt charges */
>  	struct work_struct high_work;
>  

Why use your own bitmap here?  Why not use an IDA which can grow and
shrink automatically without you needing to play fun games with RCU?