linux-kernel - Re: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr

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Date:   Thu, 10 Dec 2020 16:33:56 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     guro@...com, ktkhai@...tuozzo.com, shakeelb@...gle.com,
        david@...morbit.com, mhocko@...e.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] mm: memcontrol: add per memcg shrinker nr_deferred

On Wed, Dec 02, 2020 at 10:27:21AM -0800, Yang Shi wrote:
> @@ -504,6 +577,34 @@ int memcg_expand_shrinker_maps(int new_id)
>  	return ret;
>  }
>  
> +int memcg_expand_shrinker_deferred(int new_id)
> +{
> +	int size, old_size, ret = 0;
> +	struct mem_cgroup *memcg;
> +
> +	size = (new_id + 1) * sizeof(atomic_long_t);
> +	old_size = memcg_shrinker_deferred_size;
> +	if (size <= old_size)
> +		return 0;
> +
> +	mutex_lock(&memcg_shrinker_mutex);

The locking is somewhat confusing. I was wondering why we first read
memcg_shrinker_deferred_size "locklessly", then change it while
holding the &memcg_shrinker_mutex.

memcg_shrinker_deferred_size only changes under shrinker_rwsem(write),
correct? This should be documented in a comment, IMO.

memcg_shrinker_mutex looks superfluous then. The memcg allocation path
is the read-side of memcg_shrinker_deferred_size, and so simply needs
to take shrinker_rwsem(read) to lock out shrinker (de)registration.

Also, isn't memcg_shrinker_deferred_size just shrinker_nr_max? And
memcg_expand_shrinker_deferred() is only called when size >= old_size
in the first place (because id >= shrinker_nr_max)?