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Mon, 05 Jan 2026 20:25:25 -0800 (PST) Received: from tigerii.tok.corp.google.com ([2a00:79e0:2031:6:f5eb:5af6:e88e:8a77]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-34f5f8afba8sm775477a91.13.2026.01.05.20.25.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Jan 2026 20:25:23 -0800 (PST) From: Sergey Senozhatsky To: Andrew Morton , Yosry Ahmed , Nhat Pham Cc: Minchan Kim , Johannes Weiner , Brian Geffon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Sergey Senozhatsky Subject: [PATCH] zsmalloc: use actual object size to detect spans Date: Tue, 6 Jan 2026 13:25:07 +0900 Message-ID: <20260106042507.2579150-1-senozhatsky@chromium.org> X-Mailer: git-send-email 2.52.0.351.gbe84eed79e-goog Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Using class->size to detect spanning objects is not entirely correct, because some size classes can hold a range of object sizes of up to class->size bytes in length, due to size-classes merge. Such classes use padding for cases when actually written objects are smaller than class->size. zs_obj_read_begin() can incorrectly hit the slow path and perform memcpy of such objects, basically copying padding bytes. Instead of class->size zs_obj_read_begin() should use the actual compressed object length (both zram and zswap know it) so that it can correctly handle situations when a written object is small enough to fit into the first physical page. Signed-off-by: Sergey Senozhatsky --- drivers/block/zram/zram_drv.c | 14 ++++++++------ include/linux/zsmalloc.h | 4 ++-- mm/zsmalloc.c | 16 ++++++++++++---- mm/zswap.c | 5 +++-- 4 files changed, 25 insertions(+), 14 deletions(-) diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index a6587bed6a03..76a54eabe889 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -2065,11 +2065,11 @@ static int read_incompressible_page(struct zram *zr= am, struct page *page, void *src, *dst; =20 handle =3D get_slot_handle(zram, index); - src =3D zs_obj_read_begin(zram->mem_pool, handle, NULL); + src =3D zs_obj_read_begin(zram->mem_pool, handle, PAGE_SIZE, NULL); dst =3D kmap_local_page(page); copy_page(dst, src); kunmap_local(dst); - zs_obj_read_end(zram->mem_pool, handle, src); + zs_obj_read_end(zram->mem_pool, handle, PAGE_SIZE, src); =20 return 0; } @@ -2087,11 +2087,12 @@ static int read_compressed_page(struct zram *zram, = struct page *page, u32 index) prio =3D get_slot_comp_priority(zram, index); =20 zstrm =3D zcomp_stream_get(zram->comps[prio]); - src =3D zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy); + src =3D zs_obj_read_begin(zram->mem_pool, handle, size, + zstrm->local_copy); dst =3D kmap_local_page(page); ret =3D zcomp_decompress(zram->comps[prio], zstrm, src, size, dst); kunmap_local(dst); - zs_obj_read_end(zram->mem_pool, handle, src); + zs_obj_read_end(zram->mem_pool, handle, size, src); zcomp_stream_put(zstrm); =20 return ret; @@ -2114,9 +2115,10 @@ static int read_from_zspool_raw(struct zram *zram, s= truct page *page, u32 index) * takes place here, as we read raw compressed data. */ zstrm =3D zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP]); - src =3D zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy); + src =3D zs_obj_read_begin(zram->mem_pool, handle, size, + zstrm->local_copy); memcpy_to_page(page, 0, src, size); - zs_obj_read_end(zram->mem_pool, handle, src); + zs_obj_read_end(zram->mem_pool, handle, size, src); zcomp_stream_put(zstrm); =20 return 0; diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h index f3ccff2d966c..5565c3171007 100644 --- a/include/linux/zsmalloc.h +++ b/include/linux/zsmalloc.h @@ -40,9 +40,9 @@ unsigned int zs_lookup_class_index(struct zs_pool *pool, = unsigned int size); void zs_pool_stats(struct zs_pool *pool, struct zs_pool_stats *stats); =20 void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle, - void *local_copy); + size_t mem_len, void *local_copy); void zs_obj_read_end(struct zs_pool *pool, unsigned long handle, - void *handle_mem); + size_t mem_len, void *handle_mem); void zs_obj_write(struct zs_pool *pool, unsigned long handle, void *handle_mem, size_t mem_len); =20 diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 84da164dcbc5..ab7767d9d87d 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -1065,7 +1065,7 @@ unsigned long zs_get_total_pages(struct zs_pool *pool) EXPORT_SYMBOL_GPL(zs_get_total_pages); =20 void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle, - void *local_copy) + size_t mem_len, void *local_copy) { struct zspage *zspage; struct zpdesc *zpdesc; @@ -1087,7 +1087,11 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsign= ed long handle, class =3D zspage_class(pool, zspage); off =3D offset_in_page(class->size * obj_idx); =20 - if (off + class->size <=3D PAGE_SIZE) { + /* Normal classes have inlined handle */ + if (!ZsHugePage(zspage)) + mem_len +=3D ZS_HANDLE_SIZE; + + if (off + mem_len <=3D PAGE_SIZE) { /* this object is contained entirely within a page */ addr =3D kmap_local_zpdesc(zpdesc); addr +=3D off; @@ -1115,7 +1119,7 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigne= d long handle, EXPORT_SYMBOL_GPL(zs_obj_read_begin); =20 void zs_obj_read_end(struct zs_pool *pool, unsigned long handle, - void *handle_mem) + size_t mem_len, void *handle_mem) { struct zspage *zspage; struct zpdesc *zpdesc; @@ -1129,7 +1133,11 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned = long handle, class =3D zspage_class(pool, zspage); off =3D offset_in_page(class->size * obj_idx); =20 - if (off + class->size <=3D PAGE_SIZE) { + /* Normal classes have inlined handle */ + if (!ZsHugePage(zspage)) + mem_len +=3D ZS_HANDLE_SIZE; + + if (off + mem_len <=3D PAGE_SIZE) { if (!ZsHugePage(zspage)) off +=3D ZS_HANDLE_SIZE; handle_mem -=3D off; diff --git a/mm/zswap.c b/mm/zswap.c index de8858ff1521..a3811b05ab57 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -937,7 +937,8 @@ static bool zswap_decompress(struct zswap_entry *entry,= struct folio *folio) u8 *src, *obj; =20 acomp_ctx =3D acomp_ctx_get_cpu_lock(pool); - obj =3D zs_obj_read_begin(pool->zs_pool, entry->handle, acomp_ctx->buffer= ); + obj =3D zs_obj_read_begin(pool->zs_pool, entry->handle, entry->length, + acomp_ctx->buffer); =20 /* zswap entries of length PAGE_SIZE are not compressed. */ if (entry->length =3D=3D PAGE_SIZE) { @@ -966,7 +967,7 @@ static bool zswap_decompress(struct zswap_entry *entry,= struct folio *folio) dlen =3D acomp_ctx->req->dlen; =20 read_done: - zs_obj_read_end(pool->zs_pool, entry->handle, obj); + zs_obj_read_end(pool->zs_pool, entry->handle, entry->length, obj); acomp_ctx_put_unlock(acomp_ctx); =20 if (!decomp_ret && dlen =3D=3D PAGE_SIZE) --=20 2.52.0.351.gbe84eed79e-goog