linux-kernel - Re: [v5 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect shrinker

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Message-ID: <73c724ba-a1ef-a862-4c31-153d92826f8d@suse.cz>
Date:   Thu, 28 Jan 2021 17:19:08 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <shy828301@...il.com>, guro@...com, ktkhai@...tuozzo.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: [v5 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect
 shrinker_maps allocation

On 1/28/21 12:33 AM, Yang Shi wrote:
> Since memcg_shrinker_map_size just can be changed under holding shrinker_rwsem
> exclusively, the read side can be protected by holding read lock, so it sounds
> superfluous to have a dedicated mutex.
> 
> Kirill Tkhai suggested use write lock since:
> 
>   * We want the assignment to shrinker_maps is visible for shrink_slab_memcg().
>   * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but
>     in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing
>     is not actually protected.
>   * READ lock makes alloc_shrinker_info() racy against memory allocation fail.
>     alloc_shrinker_info()->free_shrinker_info() may free memory right after
>     shrink_slab_memcg() dereferenced it. You may say
>     shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure,
>     but this is not the thing we want to remember in the future, since this
>     spreads modularity.
> 
> And a test with heavy paging workload didn't show write lock makes things worse.
> 
> Signed-off-by: Yang Shi <shy828301@...il.com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>