Re: [RFC PATCH v2 7/7] mm: zswap: remove zswap_header

Yosry Ahmed
2023-06-07T09:31:11Z

On Tue, Jun 6, 2023 at 7:56 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> Previously, zswap_header served the purpose of storing the swpentry
> within zpool pages. This allowed zpool implementations to pass relevant
> information to the writeback function. However, with the current
> implementation, writeback is directly handled within zswap.
> Consequently, there is no longer a necessity for zswap_header, as the
> swp_entry_t can be stored directly in zswap_entry.
>
> Suggested-by: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>

Thanks for this cleanup. It gives us back some of the memory used by
list_head in zswap entry, and remove an unnecessary zpool map
operation.

> ---
> mm/zswap.c | 52 ++++++++++++++++++++++------------------------------
> 1 file changed, 22 insertions(+), 30 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index ef8604812352..f689444dd5a7 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -193,7 +193,7 @@ struct zswap_pool {
> */
> struct zswap_entry {
> struct rb_node rbnode;
> - pgoff_t offset;
> + swp_entry_t swpentry;
> int refcount;
> unsigned int length;
> struct zswap_pool *pool;
> @@ -205,10 +205,6 @@ struct zswap_entry {
> struct list_head lru;
> };
>
> -struct zswap_header {
> - swp_entry_t swpentry;
> -};
> -
> /*
> * The tree lock in the zswap_tree struct protects a few things:
> * - the rbtree
> @@ -250,7 +246,7 @@ static bool zswap_has_pool;
> pr_debug("%s pool %s/%s\n", msg, (p)->tfm_name, \
> zpool_get_type((p)->zpool))
>
> -static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header *zhdr,
> +static int zswap_writeback_entry(struct zswap_entry *entry,
> struct zswap_tree *tree);
> static int zswap_pool_get(struct zswap_pool *pool);
> static void zswap_pool_put(struct zswap_pool *pool);
> @@ -311,12 +307,14 @@ static struct zswap_entry *zswap_rb_search(struct rb_root *root, pgoff_t offset)
> {
> struct rb_node *node = root->rb_node;
> struct zswap_entry *entry;
> + pgoff_t entry_offset;
>
> while (node) {
> entry = rb_entry(node, struct zswap_entry, rbnode);
> - if (entry->offset > offset)
> + entry_offset = swp_offset(entry->swpentry);
> + if (entry_offset > offset)
> node = node->rb_left;
> - else if (entry->offset < offset)
> + else if (entry_offset < offset)
> node = node->rb_right;
> else
> return entry;
> @@ -333,13 +331,15 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry,
> {
> struct rb_node **link = &root->rb_node, *parent = NULL;
> struct zswap_entry *myentry;
> + pgoff_t myentry_offset, entry_offset = swp_offset(entry->swpentry);
>
> while (*link) {
> parent = *link;
> myentry = rb_entry(parent, struct zswap_entry, rbnode);
> - if (myentry->offset > entry->offset)
> + myentry_offset = swp_offset(myentry->swpentry);
> + if (myentry_offset > entry_offset)
> link = &(*link)->rb_left;
> - else if (myentry->offset < entry->offset)
> + else if (myentry_offset < entry_offset)

This whole myentry thing is very confusing to me. I initially thought
myentry would be the entry passed in as an argument. Can we change the
naming here to make it more consistent with zswap_rb_search() naming?

> link = &(*link)->rb_right;
> else {
> *dupentry = myentry;
> @@ -598,7 +598,6 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor)
> static int zswap_shrink(struct zswap_pool *pool)
> {
> struct zswap_entry *lru_entry, *tree_entry = NULL;
> - struct zswap_header *zhdr;
> struct zswap_tree *tree;
> int swpoffset;
> int ret;
> @@ -611,15 +610,13 @@ static int zswap_shrink(struct zswap_pool *pool)
> }
> lru_entry = list_last_entry(&pool->lru, struct zswap_entry, lru);
> list_del_init(&lru_entry->lru);
> - zhdr = zpool_map_handle(pool->zpool, lru_entry->handle, ZPOOL_MM_RO);
> - tree = zswap_trees[swp_type(zhdr->swpentry)];
> - zpool_unmap_handle(pool->zpool, lru_entry->handle);
> /*
> * Once the pool lock is dropped, the lru_entry might get freed. The
> * swpoffset is copied to the stack, and lru_entry isn't deref'd again
> * until the entry is verified to still be alive in the tree.
> */
> - swpoffset = swp_offset(zhdr->swpentry);
> + swpoffset = swp_offset(lru_entry->swpentry);
> + tree = zswap_trees[swp_type(lru_entry->swpentry)];
> spin_unlock(&pool->lru_lock);
>
> /* hold a reference from tree so it won't be freed during writeback */
> @@ -633,7 +630,7 @@ static int zswap_shrink(struct zswap_pool *pool)
> }
> spin_unlock(&tree->lock);
>
> - ret = zswap_writeback_entry(lru_entry, zhdr, tree);
> + ret = zswap_writeback_entry(lru_entry, tree);
>
> spin_lock(&tree->lock);
> if (ret) {
> @@ -1046,10 +1043,10 @@ static int zswap_get_swap_cache_page(swp_entry_t entry,
> * the swap cache, the compressed version stored by zswap can be
> * freed.
> */
> -static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header *zhdr,
> +static int zswap_writeback_entry(struct zswap_entry *entry,
> struct zswap_tree *tree)
> {
> - swp_entry_t swpentry = zhdr->swpentry;
> + swp_entry_t swpentry = entry->swpentry;
> struct page *page;
> struct scatterlist input, output;
> struct crypto_acomp_ctx *acomp_ctx;
> @@ -1089,7 +1086,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header
> * writing.
> */
> spin_lock(&tree->lock);
> - if (zswap_rb_search(&tree->rbroot, entry->offset) != entry) {
> + if (zswap_rb_search(&tree->rbroot, swp_offset(entry->swpentry)) != entry) {
> spin_unlock(&tree->lock);
> delete_from_swap_cache(page_folio(page));
> ret = -ENOMEM;
> @@ -1101,8 +1098,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry, struct zswap_header
> acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
> dlen = PAGE_SIZE;
>
> - zhdr = zpool_map_handle(pool, entry->handle, ZPOOL_MM_RO);
> - src = (u8 *)zhdr + sizeof(struct zswap_header);
> + src = zpool_map_handle(pool, entry->handle, ZPOOL_MM_RO);
> if (!zpool_can_sleep_mapped(pool)) {
> memcpy(tmp, src, entry->length);
> src = tmp;
> @@ -1196,11 +1192,10 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> struct obj_cgroup *objcg = NULL;
> struct zswap_pool *pool;
> int ret;
> - unsigned int hlen, dlen = PAGE_SIZE;
> + unsigned int dlen = PAGE_SIZE;
> unsigned long handle, value;
> char *buf;
> u8 *src, *dst;
> - struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
> gfp_t gfp;
>
> /* THP isn't supported */
> @@ -1245,7 +1240,7 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> src = kmap_atomic(page);
> if (zswap_is_page_same_filled(src, &value)) {
> kunmap_atomic(src);
> - entry->offset = offset;
> + entry->swpentry = swp_entry(type, offset);
> entry->length = 0;
> entry->value = value;
> atomic_inc(&zswap_same_filled_pages);
> @@ -1299,11 +1294,10 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> }
>
> /* store */
> - hlen = sizeof(zhdr);
> gfp = __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM;
> if (zpool_malloc_support_movable(entry->pool->zpool))
> gfp |= __GFP_HIGHMEM | __GFP_MOVABLE;
> - ret = zpool_malloc(entry->pool->zpool, hlen + dlen, gfp, &handle);
> + ret = zpool_malloc(entry->pool->zpool, dlen, gfp, &handle);
> if (ret == -ENOSPC) {
> zswap_reject_compress_poor++;
> goto put_dstmem;
> @@ -1313,13 +1307,12 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
> goto put_dstmem;
> }
> buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_WO);
> - memcpy(buf, &zhdr, hlen);
> - memcpy(buf + hlen, dst, dlen);
> + memcpy(buf, dst, dlen);
> zpool_unmap_handle(entry->pool->zpool, handle);
> mutex_unlock(acomp_ctx->mutex);
>
> /* populate entry */
> - entry->offset = offset;
> + entry->swpentry = swp_entry(type, offset);
> entry->handle = handle;
> entry->length = dlen;
>
> @@ -1418,7 +1411,6 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
> /* decompress */
> dlen = PAGE_SIZE;
> src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
> - src += sizeof(struct zswap_header);
>
> if (!zpool_can_sleep_mapped(entry->pool->zpool)) {
> memcpy(tmp, src, entry->length);
> --
> 2.34.1
>