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Fri, 04 Sep 2026 06:25:17 -0700 (PDT) From: Jianyue Wu To: Johannes Weiner , Yosry Ahmed , Nhat Pham , Chengming Zhou , Andrew Morton Cc: Chris Li , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v5 3/3] mm/zswap: reference the pool by id to shrink struct zswap_entry Date: Fri, 4 Sep 2026 21:24:56 +0800 Message-ID: <5c4f72f1ffbd5f15d0b57fb1a87f73b4249ab424.1788528216.git.wujianyue000@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: References: <20260830114731.8322-1-wujianyue000@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit struct zswap_entry is one allocation per stored page, so its size is pure overhead. It currently embeds an 8-byte pool pointer, even though the live pools now sit in an allocating xarray keyed by a small integer id that fits in a u8. Replace the per-entry pool pointer with that u8 id and resolve it through the xarray with xa_load(). A live entry holds a reference to its pool, so the id cannot be reused under it; xa_load() is lockless and only needs an rcu_read_lock() section, no zswap_pools_lock. A live entry never uses the reserved id 0, so looking up that id resolves to NULL and trips a WARN rather than aliasing a live pool. The u8 fits in the padding after the bool referenced field, shrinking the entry from 56 to 48 bytes on 64-bit. This raises objs_per_slab from 73 to 85 and saves about 2MiB of metadata per 1GiB of data held in zswap. Suggested-by: Chris Li Signed-off-by: Jianyue Wu --- mm/zswap.c | 45 ++++++++++++++++++++++++++++++++++++++------- 1 file changed, 38 insertions(+), 7 deletions(-) diff --git a/mm/zswap.c b/mm/zswap.c index 74876acfa9dc..31cf0ef43d23 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -195,7 +195,7 @@ static struct shrinker *zswap_shrinker; * writeback logic. The entry is only reclaimed by the writeback * logic if referenced is unset. See comments in the shrinker * section for context. - * pool - the zswap_pool the entry's data is in + * pool_idx - id of the zswap_pool that the entry's data is in. * handle - zsmalloc allocation handle that stores the compressed page data * objcg - the obj_cgroup that the compressed memory is charged to * lru - handle to the pool's lru used to evict pages. @@ -204,12 +204,27 @@ struct zswap_entry { swp_entry_t swpentry; unsigned int length; bool referenced; - struct zswap_pool *pool; + u8 pool_idx; unsigned long handle; struct obj_cgroup *objcg; struct list_head lru; }; +/* + * The pool stays alive after this returns because a stored entry holds a + * reference to its pool (taken in zswap_store_page()). + */ +static struct zswap_pool *zswap_entry_pool(struct zswap_entry *entry) +{ + struct zswap_pool *pool; + + rcu_read_lock(); + pool = xa_load(&zswap_pools, entry->pool_idx); + rcu_read_unlock(); + + return pool; +} + static struct xarray *zswap_trees[MAX_SWAPFILES]; static unsigned int nr_zswap_trees[MAX_SWAPFILES]; @@ -770,9 +785,13 @@ static void zswap_entry_cache_free(struct zswap_entry *entry) */ static void zswap_entry_free(struct zswap_entry *entry) { + struct zswap_pool *pool = zswap_entry_pool(entry); + zswap_lru_del(entry); - zs_free(entry->pool->zs_pool, entry->handle); - zswap_pool_put(entry->pool); + if (!WARN_ON_ONCE(!pool)) { + zs_free(pool->zs_pool, entry->handle); + zswap_pool_put(pool); + } if (entry->objcg) { obj_cgroup_uncharge_zswap(entry->objcg, entry->length); obj_cgroup_put(entry->objcg); @@ -929,12 +948,15 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry, static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio) { - struct zswap_pool *pool = entry->pool; + struct zswap_pool *pool = zswap_entry_pool(entry); struct scatterlist input[2]; /* zsmalloc returns an SG list 1-2 entries */ struct scatterlist output; struct crypto_acomp_ctx *acomp_ctx; int ret = 0, dlen; + if (WARN_ON_ONCE(!pool)) + return false; + acomp_ctx = raw_cpu_ptr(pool->acomp_ctx); mutex_lock(&acomp_ctx->mutex); zs_obj_read_sg_begin(pool->zs_pool, entry->handle, input, entry->length); @@ -970,7 +992,7 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio) pr_alert_ratelimited("Decompression error from zswap (%d:%lu %s %u->%d)\n", swp_type(entry->swpentry), swp_offset(entry->swpentry), - entry->pool->tfm_name, + pool->tfm_name, entry->length, dlen); return false; } @@ -1428,6 +1450,16 @@ static bool zswap_store_page(struct page *page, if (!zswap_compress(page, entry, pool)) goto compress_failed; + /* + * Set pool_idx before publishing the entry: compression has + * succeeded and the pool is already pinned by this store, so the id is + * final. Doing it here (rather than after xa_store()) means the entry + * is never briefly visible with a stale pool_idx left over from slab + * reuse, which zswap_entry_pool() would otherwise resolve to an + * unrelated live pool. + */ + entry->pool_idx = pool->idx; + old = xa_store(swap_zswap_tree(page_swpentry), swp_offset(page_swpentry), entry, GFP_KERNEL); @@ -1473,7 +1505,6 @@ static bool zswap_store_page(struct page *page, * The publishing order matters to prevent writeback from seeing * an incoherent entry. */ - entry->pool = pool; entry->swpentry = page_swpentry; entry->objcg = objcg; entry->referenced = true; -- 2.43.0