Date: Tue, 13 Feb 2024 12:57:42 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/zswap: global lru and shrinker shared by all zswap_pools

On Sun, Feb 11, 2024 at 01:57:04PM +0000, Chengming Zhou wrote:
> Dynamic zswap_pool creation may create/reuse to have multiple
> zswap_pools in a list, only the first will be current used.
> 
> Each zswap_pool has its own lru and shrinker, which is not
> necessary and has its problem:
> 
> 1. When memory has pressure, all shrinker of zswap_pools will
>    try to shrink its own lru, there is no order between them.
> 
> 2. When zswap limit hit, only the last zswap_pool's shrink_work
>    will try to shrink its lru, which is inefficient.
> 
> Anyway, having a global lru and shrinker shared by all zswap_pools
> is better and efficient.

It is also a great simplification.

> 
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> ---
>  mm/zswap.c | 153 ++++++++++++++++++++++---------------------------------------
>  1 file changed, 55 insertions(+), 98 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 62fe307521c9..7668db8c10e3 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -176,14 +176,17 @@ struct zswap_pool {
>  	struct kref kref;
>  	struct list_head list;
>  	struct work_struct release_work;
> -	struct work_struct shrink_work;
>  	struct hlist_node node;
>  	char tfm_name[CRYPTO_MAX_ALG_NAME];
> +};
> +
> +struct {

static?

>  	struct list_lru list_lru;
> -	struct mem_cgroup *next_shrink;
> -	struct shrinker *shrinker;

Just curious, any reason to change the relative ordering of members
here? It produces a couple more lines of diff :)

>  	atomic_t nr_stored;
> -};
> +	struct shrinker *shrinker;
> +	struct work_struct shrink_work;
> +	struct mem_cgroup *next_shrink;
> +} zswap;
>  
>  /*
>   * struct zswap_entry
> @@ -301,9 +304,6 @@ static void zswap_update_total_size(void)
>  * pool functions
>  **********************************/
>  
> -static void zswap_alloc_shrinker(struct zswap_pool *pool);
> -static void shrink_worker(struct work_struct *w);
> -
>  static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  {
>  	int i;
> @@ -353,30 +353,16 @@ static struct zswap_pool *zswap_pool_create(char *type, char *compressor)
>  	if (ret)
>  		goto error;
>  
> -	zswap_alloc_shrinker(pool);
> -	if (!pool->shrinker)
> -		goto error;
> -
> -	pr_debug("using %s compressor\n", pool->tfm_name);
> -

Why are we removing this debug print?

>  	/* being the current pool takes 1 ref; this func expects the
>  	 * caller to always add the new pool as the current pool
>  	 */
>  	kref_init(&pool->kref);
>  	INIT_LIST_HEAD(&pool->list);
> -	if (list_lru_init_memcg(&pool->list_lru, pool->shrinker))
> -		goto lru_fail;
> -	shrinker_register(pool->shrinker);
> -	INIT_WORK(&pool->shrink_work, shrink_worker);
> -	atomic_set(&pool->nr_stored, 0);
>  
>  	zswap_pool_debug("created", pool);
>  
>  	return pool;
>  
> -lru_fail:
> -	list_lru_destroy(&pool->list_lru);
> -	shrinker_free(pool->shrinker);
>  error:
>  	if (pool->acomp_ctx)
>  		free_percpu(pool->acomp_ctx);
[..]
> @@ -816,14 +777,10 @@ void zswap_folio_swapin(struct folio *folio)
>  
>  void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg)
>  {
> -	struct zswap_pool *pool;
> -
> -	/* lock out zswap pools list modification */
> +	/* lock out zswap shrinker walking memcg tree */
>  	spin_lock(&zswap_pools_lock);
> -	list_for_each_entry(pool, &zswap_pools, list) {
> -		if (pool->next_shrink == memcg)
> -			pool->next_shrink = mem_cgroup_iter(NULL, pool->next_shrink, NULL);
> -	}
> +	if (zswap.next_shrink == memcg)
> +		zswap.next_shrink = mem_cgroup_iter(NULL, zswap.next_shrink, NULL);

Now that next_shrink has nothing to do with zswap pools, it feels weird
that we are using zswap_pools_lock for its synchronization. Does it make
sense to have a separate lock for it just for semantic purposes?

>  	spin_unlock(&zswap_pools_lock);
>  }
>  
[..]
> @@ -1328,7 +1284,6 @@ static unsigned long zswap_shrinker_scan(struct shrinker *shrinker,
>  static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  		struct shrink_control *sc)
>  {
> -	struct zswap_pool *pool = shrinker->private_data;
>  	struct mem_cgroup *memcg = sc->memcg;
>  	struct lruvec *lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(sc->nid));
>  	unsigned long nr_backing, nr_stored, nr_freeable, nr_protected;
> @@ -1343,7 +1298,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  #else
>  	/* use pool stats instead of memcg stats */
>  	nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT;

"pool" is still being used here.

> -	nr_stored = atomic_read(&pool->nr_stored);
> +	nr_stored = atomic_read(&zswap.nr_stored);
>  #endif
>  
>  	if (!nr_stored)
[..]  
> @@ -1804,6 +1749,21 @@ static int zswap_setup(void)
>  	if (ret)
>  		goto hp_fail;
>  
> +	shrink_wq = alloc_workqueue("zswap-shrink",
> +			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> +	if (!shrink_wq)
> +		goto hp_fail;

I think we need a new label here to call cpuhp_remove_multi_state(), but
apparently this is missing from the current code for some reason.

> +
> +	zswap.shrinker = zswap_alloc_shrinker();
> +	if (!zswap.shrinker)
> +		goto shrinker_fail;
> +	if (list_lru_init_memcg(&zswap.list_lru, zswap.shrinker))
> +		goto lru_fail;
> +	shrinker_register(zswap.shrinker);
> +
> +	INIT_WORK(&zswap.shrink_work, shrink_worker);
> +	atomic_set(&zswap.nr_stored, 0);
> +
>  	pool = __zswap_pool_create_fallback();
>  	if (pool) {
>  		pr_info("loaded using pool %s/%s\n", pool->tfm_name,
> @@ -1815,19 +1775,16 @@ static int zswap_setup(void)
>  		zswap_enabled = false;
>  	}
>  
> -	shrink_wq = alloc_workqueue("zswap-shrink",
> -			WQ_UNBOUND|WQ_MEM_RECLAIM, 1);
> -	if (!shrink_wq)
> -		goto fallback_fail;
> -
>  	if (zswap_debugfs_init())
>  		pr_warn("debugfs initialization failed\n");
>  	zswap_init_state = ZSWAP_INIT_SUCCEED;
>  	return 0;
>  
> -fallback_fail:
> -	if (pool)
> -		zswap_pool_destroy(pool);
> +lru_fail:
> +	list_lru_destroy(&zswap.list_lru);

Do we need to call list_lru_destroy() here? I know it is currently being
called if list_lru_init_memcg() fails, but I fail to understand why. It
seems like list_lru_destroy() will do nothing anyway.

> +	shrinker_free(zswap.shrinker);
> +shrinker_fail:
> +	destroy_workqueue(shrink_wq);
>  hp_fail:
>  	kmem_cache_destroy(zswap_entry_cache);
>  cache_fail:
> 
> -- 
> b4 0.10.1

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