linux-kernel - Re: [PATCH v4 6/8] mm: vmscan: hold write lock to reparent shrinker nr

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Message-ID: <3056542f-5598-9c60-30fe-4949400dd39e@suse.cz>
Date:   Thu, 9 Mar 2023 10:36:58 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Qi Zheng <zhengqi.arch@...edance.com>, akpm@...ux-foundation.org,
        tkhai@...ru, hannes@...xchg.org, shakeelb@...gle.com,
        mhocko@...nel.org, roman.gushchin@...ux.dev, muchun.song@...ux.dev,
        david@...hat.com, shy828301@...il.com, rppt@...nel.org
Cc:     sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/8] mm: vmscan: hold write lock to reparent shrinker
 nr_deferred

On 3/7/23 07:56, Qi Zheng wrote:
> For now, reparent_shrinker_deferred() is the only holder
> of read lock of shrinker_rwsem. And it already holds the
> global cgroup_mutex, so it will not be called in parallel.
> 
> Therefore, in order to convert shrinker_rwsem to shrinker_mutex
> later, here we change to hold the write lock of shrinker_rwsem
> to reparent.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>

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

> ---
>  mm/vmscan.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9a5a3da5c8b5..7aaf6f94ac1b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -451,7 +451,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>  		parent = root_mem_cgroup;
>  
>  	/* Prevent from concurrent shrinker_info expand */
> -	down_read(&shrinker_rwsem);
> +	down_write(&shrinker_rwsem);
>  	for_each_node(nid) {
>  		child_info = shrinker_info_protected(memcg, nid);
>  		parent_info = shrinker_info_protected(parent, nid);
> @@ -460,7 +460,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>  			atomic_long_add(nr, &parent_info->nr_deferred[i]);
>  		}
>  	}
> -	up_read(&shrinker_rwsem);
> +	up_write(&shrinker_rwsem);
>  }
>  
>  static bool cgroup_reclaim(struct scan_control *sc)