Re: [PATCH v2] mm: zswap: remove nr_zswap_stored atomic

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2024/3/22 08:10, Yosry Ahmed wrote:
> nr_stored was introduced by commit b5ba474f3f51 ("zswap: shrink zswap
> pool based on memory pressure") as a per zswap_pool counter of the
> number of stored pages that are not same-filled pages. It is used in
> zswap_shrinker_count() to scale the number of freeable compressed pages
> by the compression ratio.  That is, to reduce the amount of writeback
> from zswap with higher compression ratios as the ROI from IO diminishes.
> 
> Later on, commit bf9b7df23cb3 ("mm/zswap: global lru and shrinker shared
> by all zswap_pools") made the shrinker global (not per zswap_pool), and
> replaced nr_stored with nr_zswap_stored (initially introduced as
> zswap.nr_stored), which is now a global counter.
> 
> The counter is now awfully close to zswap_stored_pages. The only
> difference is that the latter also includes same-filled pages. Also,
> when memcgs are enabled, we use memcg_page_state(memcg, MEMCG_ZSWAPPED),
> which includes same-filled pages anyway (i.e.  equivalent to
> zswap_stored_pages).
> 
> Use zswap_stored_pages instead in zswap_shrinker_count() to keep things
> consistent whether memcgs are enabled or not, and add a comment about
> the number of freeable pages possibly being scaled down more than it
> should if we have lots of same-filled pages (i.e. inflated compression
> ratio).
> 
> Remove nr_zswap_stored and one atomic operation in the store and free
> paths.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> Reviewed-by: Nhat Pham <nphamcs@gmail.com>

Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks.

> ---
>  mm/zswap.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index b31c977f53e9c..1a79f99606cef 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -181,8 +181,6 @@ struct zswap_pool {
>  
>  /* Global LRU lists shared by all zswap pools. */
>  static struct list_lru zswap_list_lru;
> -/* counter of pages stored in all zswap pools. */
> -static atomic_t zswap_nr_stored = ATOMIC_INIT(0);
>  
>  /* The lock protects zswap_next_shrink updates. */
>  static DEFINE_SPINLOCK(zswap_shrink_lock);
> @@ -885,7 +883,6 @@ static void zswap_entry_free(struct zswap_entry *entry)
>  	else {
>  		zswap_lru_del(&zswap_list_lru, entry);
>  		zpool_free(zswap_find_zpool(entry), entry->handle);
> -		atomic_dec(&zswap_nr_stored);
>  		zswap_pool_put(entry->pool);
>  	}
>  	if (entry->objcg) {
> @@ -1310,7 +1307,7 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  #else
>  	/* use pool stats instead of memcg stats */
>  	nr_backing = zswap_total_pages();
> -	nr_stored = atomic_read(&zswap_nr_stored);
> +	nr_stored = atomic_read(&zswap_stored_pages);
>  #endif
>  
>  	if (!nr_stored)
> @@ -1330,6 +1327,11 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker,
>  	 * This ensures that the better zswap compresses memory, the fewer
>  	 * pages we will evict to swap (as it will otherwise incur IO for
>  	 * relatively small memory saving).
> +	 *
> +	 * The memory saving factor calculated here takes same-filled pages into
> +	 * account, but those are not freeable since they almost occupy no
> +	 * space. Hence, we may scale nr_freeable down a little bit more than we
> +	 * should if we have a lot of same-filled pages.
>  	 */
>  	return mult_frac(nr_freeable, nr_backing, nr_stored);
>  }
> @@ -1575,7 +1577,6 @@ bool zswap_store(struct folio *folio)
>  	if (entry->length) {
>  		INIT_LIST_HEAD(&entry->lru);
>  		zswap_lru_add(&zswap_list_lru, entry);
> -		atomic_inc(&zswap_nr_stored);
>  	}
>  	spin_unlock(&tree->lock);
>