Re: [RFC PATCH 6/9] mm: zswap: drop support for non-zero same-filled pages handling

Chengming Zhou
2024-03-27T11:25:26Z

On 2024/3/26 07:50, Yosry Ahmed wrote:
> The current same-filled pages handling supports pages filled with any
> repeated word-sized pattern. However, in practice, most of these should
> be zero pages anyway. Other patterns should be nearly as common.
>
> Drop the support for non-zero same-filled pages, but keep the names of
> knobs exposed to userspace as "same_filled", which isn't entirely
> inaccurate.
>
> This yields some nice code simplification and enables a following patch
> that eliminates the need to allocate struct zswap_entry for those pages
> completely.
>
> There is also a very small performance improvement observed over 50 runs
> of kernel build test (kernbench) comparing the mean build time on a
> skylake machine when building the kernel in a cgroup v1 container with a
> 3G limit:
>
> base patched % diff
> real 70.167 69.915 -0.359%
> user 2953.068 2956.147 +0.104%
> sys 2612.811 2594.718 -0.692%
>
> This probably comes from more optimized operations like memchr_inv() and
> clear_highpage(). Note that the percentage of zero-filled pages during
> this test was only around 1.5% on average, and was not affected by this
> patch. Practical workloads could have a larger proportion of such pages
> (e.g. Johannes observed around 10% [1]), so the performance improvement
> should be larger.
>
> [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@cmpxchg.org/
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>

The code looks good!

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

Thanks.

> ---
> mm/zswap.c | 76 ++++++++++++++----------------------------------------
> 1 file changed, 20 insertions(+), 56 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 0fc27ae950c74..413d9242cf500 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -44,8 +44,8 @@
> **********************************/
> /* 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 number of zero-filled pages currently stored in zswap */
> +static atomic_t zswap_zero_filled_pages = ATOMIC_INIT(0);
>
> /*
> * The statistics below are not protected from concurrent access for
> @@ -123,9 +123,9 @@ static unsigned int zswap_accept_thr_percent = 90; /* of max pool size */
> module_param_named(accept_threshold_percent, zswap_accept_thr_percent,
> uint, 0644);
>
> -/* Enable/disable handling non-same-value filled pages (enabled by default) */
> -static bool zswap_non_same_filled_pages_enabled = true;
> -module_param_named(non_same_filled_pages_enabled, zswap_non_same_filled_pages_enabled,
> +/* Enable/disable handling non-zero-filled pages (enabled by default) */
> +static bool zswap_non_zero_filled_pages_enabled = true;
> +module_param_named(non_same_filled_pages_enabled, zswap_non_zero_filled_pages_enabled,
> bool, 0644);
>
> /* Number of zpools in zswap_pool (empirically determined for scalability) */
> @@ -187,11 +187,10 @@ 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
> + * decompression. For a zero-filled page length is 0, and both
> * pool and lru are invalid and must be ignored.
> * 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.
> */
> @@ -199,10 +198,7 @@ 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;
> };
> @@ -805,7 +801,7 @@ 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);
> + atomic_dec(&zswap_zero_filled_pages);
> else {
> zswap_lru_del(&zswap_list_lru, entry);
> zpool_free(zswap_find_zpool(entry), entry->handle);
> @@ -1377,43 +1373,17 @@ static void shrink_worker(struct work_struct *w)
> } while (zswap_total_pages() > thr);
> }
>
> -static bool zswap_is_folio_same_filled(struct folio *folio, unsigned long *value)
> +static bool zswap_is_folio_zero_filled(struct folio *folio)
> {
> - unsigned long *page;
> - unsigned long val;
> - unsigned int pos, last_pos = PAGE_SIZE / sizeof(*page) - 1;
> + unsigned long *kaddr;
> bool ret;
>
> - page = kmap_local_folio(folio, 0);
> - val = page[0];
> -
> - if (val != page[last_pos]) {
> - ret = false;
> - goto out;
> - }
> -
> - for (pos = 1; pos < last_pos; pos++) {
> - if (val != page[pos]) {
> - ret = false;
> - goto out;
> - }
> - }
> -
> - *value = val;
> - ret = true;
> -out:
> - kunmap_local(page);
> + kaddr = kmap_local_folio(folio, 0);
> + ret = !memchr_inv(kaddr, 0, PAGE_SIZE);
> + kunmap_local(kaddr);
> return ret;
> }
>
> -static void zswap_fill_page(void *ptr, unsigned long value)
> -{
> - unsigned long *page;
> -
> - page = (unsigned long *)ptr;
> - memset_l(page, value, PAGE_SIZE / sizeof(unsigned long));
> -}
> -
> static bool zswap_check_limit(void)
> {
> unsigned long cur_pages = zswap_total_pages();
> @@ -1437,7 +1407,6 @@ bool zswap_store(struct folio *folio)
> struct obj_cgroup *objcg = NULL;
> struct mem_cgroup *memcg = NULL;
> struct zswap_entry *entry;
> - unsigned long value;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
> VM_WARN_ON_ONCE(!folio_test_swapcache(folio));
> @@ -1470,14 +1439,13 @@ bool zswap_store(struct folio *folio)
> goto reject;
> }
>
> - if (zswap_is_folio_same_filled(folio, &value)) {
> + if (zswap_is_folio_zero_filled(folio)) {
> entry->length = 0;
> - entry->value = value;
> - atomic_inc(&zswap_same_filled_pages);
> + atomic_inc(&zswap_zero_filled_pages);
> goto insert_entry;
> }
>
> - if (!zswap_non_same_filled_pages_enabled)
> + if (!zswap_non_zero_filled_pages_enabled)
> goto freepage;
>
> /* if entry is successfully added, it keeps the reference */
> @@ -1532,7 +1500,7 @@ bool zswap_store(struct folio *folio)
>
> store_failed:
> if (!entry->length)
> - atomic_dec(&zswap_same_filled_pages);
> + atomic_dec(&zswap_zero_filled_pages);
> else {
> zpool_free(zswap_find_zpool(entry), entry->handle);
> put_pool:
> @@ -1563,7 +1531,6 @@ bool zswap_load(struct folio *folio)
> struct page *page = &folio->page;
> struct xarray *tree = swap_zswap_tree(swp);
> struct zswap_entry *entry;
> - u8 *dst;
>
> VM_WARN_ON_ONCE(!folio_test_locked(folio));
>
> @@ -1573,11 +1540,8 @@ bool zswap_load(struct folio *folio)
>
> if (entry->length)
> zswap_decompress(entry, page);
> - else {
> - dst = kmap_local_page(page);
> - zswap_fill_page(dst, entry->value);
> - kunmap_local(dst);
> - }
> + else
> + clear_highpage(page);
>
> count_vm_event(ZSWPIN);
> if (entry->objcg)
> @@ -1679,7 +1643,7 @@ static int zswap_debugfs_init(void)
> 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);
> + zswap_debugfs_root, &zswap_zero_filled_pages);
>
> return 0;
> }