linux-kernel - Re: [PATCH v4 5/8] mm: shrinkers: make count and scan in shrinker debugfs lockless

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Date:   Thu, 9 Mar 2023 10:36:34 +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 5/8] mm: shrinkers: make count and scan in shrinker
 debugfs lockless

On 3/7/23 07:56, Qi Zheng wrote:
> Like global and memcg slab shrink, also use SRCU to
> make count and scan operations in memory shrinker
> debugfs lockless.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>

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

> ---
>  mm/shrinker_debug.c | 24 +++++++-----------------
>  1 file changed, 7 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
> index 39c3491e28a3..6aa7a7ec69da 100644
> --- a/mm/shrinker_debug.c
> +++ b/mm/shrinker_debug.c
> @@ -9,6 +9,7 @@
>  /* defined in vmscan.c */
>  extern struct rw_semaphore shrinker_rwsem;
>  extern struct list_head shrinker_list;
> +extern struct srcu_struct shrinker_srcu;
>  
>  static DEFINE_IDA(shrinker_debugfs_ida);
>  static struct dentry *shrinker_debugfs_root;
> @@ -49,18 +50,13 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>  	struct mem_cgroup *memcg;
>  	unsigned long total;
>  	bool memcg_aware;
> -	int ret, nid;
> +	int ret = 0, nid, srcu_idx;
>  
>  	count_per_node = kcalloc(nr_node_ids, sizeof(unsigned long), GFP_KERNEL);
>  	if (!count_per_node)
>  		return -ENOMEM;
>  
> -	ret = down_read_killable(&shrinker_rwsem);
> -	if (ret) {
> -		kfree(count_per_node);
> -		return ret;
> -	}
> -	rcu_read_lock();
> +	srcu_idx = srcu_read_lock(&shrinker_srcu);
>  
>  	memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
>  
> @@ -91,8 +87,7 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
>  		}
>  	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
>  
> -	rcu_read_unlock();
> -	up_read(&shrinker_rwsem);
> +	srcu_read_unlock(&shrinker_srcu, srcu_idx);
>  
>  	kfree(count_per_node);
>  	return ret;
> @@ -115,9 +110,8 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
>  		.gfp_mask = GFP_KERNEL,
>  	};
>  	struct mem_cgroup *memcg = NULL;
> -	int nid;
> +	int nid, srcu_idx;
>  	char kbuf[72];
> -	ssize_t ret;
>  
>  	read_len = size < (sizeof(kbuf) - 1) ? size : (sizeof(kbuf) - 1);
>  	if (copy_from_user(kbuf, buf, read_len))
> @@ -146,11 +140,7 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
>  		return -EINVAL;
>  	}
>  
> -	ret = down_read_killable(&shrinker_rwsem);
> -	if (ret) {
> -		mem_cgroup_put(memcg);
> -		return ret;
> -	}
> +	srcu_idx = srcu_read_lock(&shrinker_srcu);
>  
>  	sc.nid = nid;
>  	sc.memcg = memcg;
> @@ -159,7 +149,7 @@ static ssize_t shrinker_debugfs_scan_write(struct file *file,
>  
>  	shrinker->scan_objects(shrinker, &sc);
>  
> -	up_read(&shrinker_rwsem);
> +	srcu_read_unlock(&shrinker_srcu, srcu_idx);
>  	mem_cgroup_put(memcg);
>  
>  	return size;