Re: [PATCH v5 2/2] mm: remove code to handle same filled pages

[Chengming Zhou]

On 2024/6/14 18:07, Usama Arif wrote:
> With an earlier commit to handle zero-filled pages in swap directly,
> and with only 1% of the same-filled pages being non-zero, zswap no
> longer needs to handle same-filled pages and can just work on compressed
> pages.
> 
> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>

Looks good to me, thanks.

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

> ---
>  mm/zswap.c | 86 +++++-------------------------------------------------
>  1 file changed, 8 insertions(+), 78 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index a546c01602aa..e25a6808c2ed 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -44,8 +44,6 @@
>  **********************************/
>  /* The number of compressed pages currently stored in zswap */
>  atomic_t zswap_stored_pages = ATOMIC_INIT(0);
> -/* The number of same-value filled pages currently stored in zswap */
> -static atomic_t zswap_same_filled_pages = ATOMIC_INIT(0);
>  
>  /*
>   * The statistics below are not protected from concurrent access for
> @@ -188,11 +186,9 @@ static struct shrinker *zswap_shrinker;
>   *
>   * swpentry - associated swap entry, the offset indexes into the red-black tree
>   * length - the length in bytes of the compressed page data.  Needed during
> - *          decompression. For a same value filled page length is 0, and both
> - *          pool and lru are invalid and must be ignored.
> + *          decompression.
>   * pool - the zswap_pool the entry's data is in
>   * handle - zpool allocation handle that stores the compressed page data
> - * value - value of the same-value filled pages which have same content
>   * objcg - the obj_cgroup that the compressed memory is charged to
>   * lru - handle to the pool's lru used to evict pages.
>   */
> @@ -200,10 +196,7 @@ struct zswap_entry {
>  	swp_entry_t swpentry;
>  	unsigned int length;
>  	struct zswap_pool *pool;
> -	union {
> -		unsigned long handle;
> -		unsigned long value;
> -	};
> +	unsigned long handle;
>  	struct obj_cgroup *objcg;
>  	struct list_head lru;
>  };
> @@ -820,13 +813,9 @@ static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
>   */
>  static void zswap_entry_free(struct zswap_entry *entry)
>  {
> -	if (!entry->length)
> -		atomic_dec(&zswap_same_filled_pages);
> -	else {
> -		zswap_lru_del(&zswap_list_lru, entry);
> -		zpool_free(zswap_find_zpool(entry), entry->handle);
> -		zswap_pool_put(entry->pool);
> -	}
> +	zswap_lru_del(&zswap_list_lru, entry);
> +	zpool_free(zswap_find_zpool(entry), entry->handle);
> +	zswap_pool_put(entry->pool);
>  	if (entry->objcg) {
>  		obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
>  		obj_cgroup_put(entry->objcg);
> @@ -1268,11 +1257,6 @@ 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);
>  }
> @@ -1376,42 +1360,6 @@ static void shrink_worker(struct work_struct *w)
>  	} while (zswap_total_pages() > thr);
>  }
>  
> -/*********************************
> -* same-filled functions
> -**********************************/
> -static bool zswap_is_folio_same_filled(struct folio *folio, unsigned long *value)
> -{
> -	unsigned long *data;
> -	unsigned long val;
> -	unsigned int pos, last_pos = PAGE_SIZE / sizeof(*data) - 1;
> -	bool ret = false;
> -
> -	data = kmap_local_folio(folio, 0);
> -	val = data[0];
> -
> -	if (val != data[last_pos])
> -		goto out;
> -
> -	for (pos = 1; pos < last_pos; pos++) {
> -		if (val != data[pos])
> -			goto out;
> -	}
> -
> -	*value = val;
> -	ret = true;
> -out:
> -	kunmap_local(data);
> -	return ret;
> -}
> -
> -static void zswap_fill_folio(struct folio *folio, unsigned long value)
> -{
> -	unsigned long *data = kmap_local_folio(folio, 0);
> -
> -	memset_l(data, value, PAGE_SIZE / sizeof(unsigned long));
> -	kunmap_local(data);
> -}
> -
>  /*********************************
>  * main API
>  **********************************/
> @@ -1423,7 +1371,6 @@ bool zswap_store(struct folio *folio)
>  	struct zswap_entry *entry, *old;
>  	struct obj_cgroup *objcg = NULL;
>  	struct mem_cgroup *memcg = NULL;
> -	unsigned long value;
>  
>  	VM_WARN_ON_ONCE(!folio_test_locked(folio));
>  	VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> @@ -1456,13 +1403,6 @@ bool zswap_store(struct folio *folio)
>  		goto reject;
>  	}
>  
> -	if (zswap_is_folio_same_filled(folio, &value)) {
> -		entry->length = 0;
> -		entry->value = value;
> -		atomic_inc(&zswap_same_filled_pages);
> -		goto store_entry;
> -	}
> -
>  	/* if entry is successfully added, it keeps the reference */
>  	entry->pool = zswap_pool_current_get();
>  	if (!entry->pool)
> @@ -1480,7 +1420,6 @@ bool zswap_store(struct folio *folio)
>  	if (!zswap_compress(folio, entry))
>  		goto put_pool;
>  
> -store_entry:
>  	entry->swpentry = swp;
>  	entry->objcg = objcg;
>  
> @@ -1528,13 +1467,9 @@ bool zswap_store(struct folio *folio)
>  	return true;
>  
>  store_failed:
> -	if (!entry->length)
> -		atomic_dec(&zswap_same_filled_pages);
> -	else {
> -		zpool_free(zswap_find_zpool(entry), entry->handle);
> +	zpool_free(zswap_find_zpool(entry), entry->handle);
>  put_pool:
> -		zswap_pool_put(entry->pool);
> -	}
> +	zswap_pool_put(entry->pool);
>  freepage:
>  	zswap_entry_cache_free(entry);
>  reject:
> @@ -1597,10 +1532,7 @@ bool zswap_load(struct folio *folio)
>  	if (!entry)
>  		return false;
>  
> -	if (entry->length)
> -		zswap_decompress(entry, folio);
> -	else
> -		zswap_fill_folio(folio, entry->value);
> +	zswap_decompress(entry, folio);
>  
>  	count_vm_event(ZSWPIN);
>  	if (entry->objcg)
> @@ -1703,8 +1635,6 @@ static int zswap_debugfs_init(void)
>  			    zswap_debugfs_root, NULL, &total_size_fops);
>  	debugfs_create_atomic_t("stored_pages", 0444,
>  				zswap_debugfs_root, &zswap_stored_pages);
> -	debugfs_create_atomic_t("same_filled_pages", 0444,
> -				zswap_debugfs_root, &zswap_same_filled_pages);
>  
>  	return 0;
>  }