Subject: Re: [PATCH v2 28/47] bcache: dynamically allocate the md-bcache shrinker

From: Muchun Song <[email-masked]>
Date: Wed, 26 Jul 2023 15:32:10 +0800

On 2023/7/24 17:43, Qi Zheng wrote:
> In preparation for implementing lockless slab shrink, use new APIs to
> dynamically allocate the md-bcache shrinker, so that it can be freed
> asynchronously using kfree_rcu(). Then it doesn't need to wait for RCU
> read-side critical section when releasing the struct cache_set.
>
> Signed-off-by: Qi Zheng <[email-masked]>
> ---
>   drivers/md/bcache/bcache.h |  2 +-
>   drivers/md/bcache/btree.c  | 27 ++++++++++++++++-----------
>   drivers/md/bcache/sysfs.c  |  3 ++-
>   3 files changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
> index 5a79bb3c272f..c622bc50f81b 100644
> --- a/drivers/md/bcache/bcache.h
> +++ b/drivers/md/bcache/bcache.h
> @@ -541,7 +541,7 @@ struct cache_set {
>   	struct bio_set		bio_split;
>   
>   	/* For the btree cache */
> -	struct shrinker		shrink;
> +	struct shrinker		*shrink;
>   
>   	/* For the btree cache and anything allocation related */
>   	struct mutex		bucket_lock;
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index fd121a61f17c..c176c7fc77d9 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -667,7 +667,7 @@ static int mca_reap(struct btree *b, unsigned int min_order, bool flush)
>   static unsigned long bch_mca_scan(struct shrinker *shrink,
>   				  struct shrink_control *sc)
>   {
> -	struct cache_set *c = container_of(shrink, struct cache_set, shrink);
> +	struct cache_set *c = shrink->private_data;
>   	struct btree *b, *t;
>   	unsigned long i, nr = sc->nr_to_scan;
>   	unsigned long freed = 0;
> @@ -734,7 +734,7 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
>   static unsigned long bch_mca_count(struct shrinker *shrink,
>   				   struct shrink_control *sc)
>   {
> -	struct cache_set *c = container_of(shrink, struct cache_set, shrink);
> +	struct cache_set *c = shrink->private_data;
>   
>   	if (c->shrinker_disabled)
>   		return 0;
> @@ -752,8 +752,8 @@ void bch_btree_cache_free(struct cache_set *c)
>   
>   	closure_init_stack(&cl);
>   
> -	if (c->shrink.list.next)
> -		unregister_shrinker(&c->shrink);
> +	if (c->shrink)
> +		shrinker_unregister(c->shrink);
>   
>   	mutex_lock(&c->bucket_lock);
>   
> @@ -828,14 +828,19 @@ int bch_btree_cache_alloc(struct cache_set *c)
>   		c->verify_data = NULL;
>   #endif
>   
> -	c->shrink.count_objects = bch_mca_count;
> -	c->shrink.scan_objects = bch_mca_scan;
> -	c->shrink.seeks = 4;
> -	c->shrink.batch = c->btree_pages * 2;
> +	c->shrink = shrinker_alloc(0, "md-bcache:%pU", c->set_uuid);
> +	if (!c->shrink) {
> +		pr_warn("bcache: %s: could not allocate shrinker\n", __func__);
> +		return -ENOMEM;

Seems you have cheanged the semantic of this. In the past,
it is better to have a shrinker, but now it becomes a mandatory.
Right? I don't know if it is acceptable. From my point of view,
just do the cleanup, don't change any behaviour.

> +	}
> +
> +	c->shrink->count_objects = bch_mca_count;
> +	c->shrink->scan_objects = bch_mca_scan;
> +	c->shrink->seeks = 4;
> +	c->shrink->batch = c->btree_pages * 2;
> +	c->shrink->private_data = c;
>   
> -	if (register_shrinker(&c->shrink, "md-bcache:%pU", c->set_uuid))
> -		pr_warn("bcache: %s: could not register shrinker\n",
> -				__func__);
> +	shrinker_register(c->shrink);
>   
>   	return 0;
>   }
> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
> index 0e2c1880f60b..45d8af755de6 100644
> --- a/drivers/md/bcache/sysfs.c
> +++ b/drivers/md/bcache/sysfs.c
> @@ -866,7 +866,8 @@ STORE(__bch_cache_set)
>   
>   		sc.gfp_mask = GFP_KERNEL;
>   		sc.nr_to_scan = strtoul_or_return(buf);
> -		c->shrink.scan_objects(&c->shrink, &sc);
> +		if (c->shrink)
> +			c->shrink->scan_objects(c->shrink, &sc);
>   	}
>   
>   	sysfs_strtoul_clamp(congested_read_threshold_us,