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Message-ID: <dvdaf3cuwvk332nvds46uqqvpoo46bf34zgqorpq4ldwxjo7sk@uh5v5bn5nrab>
Date: Wed, 7 Jan 2026 18:57:08 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Nhat Pham <nphamcs@...il.com>, Minchan Kim <minchan@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Brian Geffon <bgeffon@...gle.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCHv2 1/2] zsmalloc: use actual object size to detect spans
On Wed, Jan 07, 2026 at 02:21:44PM +0900, Sergey Senozhatsky wrote:
> 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 <senozhatsky@...omium.org>
For zsmalloc and zswap bits:
Reviewed-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
> ---
>
> v1->v2:
> - use mem_len for second memcpy size calculation (Yosry)
> - simplified read_begin/end logic (Yosry)
>
> drivers/block/zram/zram_drv.c | 14 ++++++++------
> include/linux/zsmalloc.h | 4 ++--
> mm/zsmalloc.c | 16 +++++++++++-----
> mm/zswap.c | 5 +++--
> 4 files changed, 24 insertions(+), 15 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 *zram, struct page *page,
> void *src, *dst;
>
> handle = get_slot_handle(zram, index);
> - src = zs_obj_read_begin(zram->mem_pool, handle, NULL);
> + src = zs_obj_read_begin(zram->mem_pool, handle, PAGE_SIZE, NULL);
> dst = 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);
>
> return 0;
> }
> @@ -2087,11 +2087,12 @@ static int read_compressed_page(struct zram *zram, struct page *page, u32 index)
> prio = get_slot_comp_priority(zram, index);
>
> zstrm = zcomp_stream_get(zram->comps[prio]);
> - src = zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy);
> + src = zs_obj_read_begin(zram->mem_pool, handle, size,
> + zstrm->local_copy);
> dst = kmap_local_page(page);
> ret = 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);
>
> return ret;
> @@ -2114,9 +2115,10 @@ static int read_from_zspool_raw(struct zram *zram, struct page *page, u32 index)
> * takes place here, as we read raw compressed data.
> */
> zstrm = zcomp_stream_get(zram->comps[ZRAM_PRIMARY_COMP]);
> - src = zs_obj_read_begin(zram->mem_pool, handle, zstrm->local_copy);
> + src = 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);
>
> 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);
>
> 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);
>
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 84da164dcbc5..119c196a287a 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);
>
> 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 +1087,10 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
> class = zspage_class(pool, zspage);
> off = offset_in_page(class->size * obj_idx);
>
> - if (off + class->size <= PAGE_SIZE) {
> + if (!ZsHugePage(zspage))
> + mem_len += ZS_HANDLE_SIZE;
> +
> + if (off + mem_len <= PAGE_SIZE) {
> /* this object is contained entirely within a page */
> addr = kmap_local_zpdesc(zpdesc);
> addr += off;
> @@ -1096,7 +1099,7 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
>
> /* this object spans two pages */
> sizes[0] = PAGE_SIZE - off;
> - sizes[1] = class->size - sizes[0];
> + sizes[1] = mem_len - sizes[0];
> addr = local_copy;
>
> memcpy_from_page(addr, zpdesc_page(zpdesc),
> @@ -1115,7 +1118,7 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
> EXPORT_SYMBOL_GPL(zs_obj_read_begin);
>
> 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 +1132,10 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
> class = zspage_class(pool, zspage);
> off = offset_in_page(class->size * obj_idx);
>
> - if (off + class->size <= PAGE_SIZE) {
> + if (!ZsHugePage(zspage))
> + mem_len += ZS_HANDLE_SIZE;
> +
> + if (off + mem_len <= PAGE_SIZE) {
> if (!ZsHugePage(zspage))
> off += ZS_HANDLE_SIZE;
> handle_mem -= 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;
>
> acomp_ctx = acomp_ctx_get_cpu_lock(pool);
> - obj = zs_obj_read_begin(pool->zs_pool, entry->handle, acomp_ctx->buffer);
> + obj = zs_obj_read_begin(pool->zs_pool, entry->handle, entry->length,
> + acomp_ctx->buffer);
>
> /* zswap entries of length PAGE_SIZE are not compressed. */
> if (entry->length == PAGE_SIZE) {
> @@ -966,7 +967,7 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)
> dlen = acomp_ctx->req->dlen;
>
> 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);
>
> if (!decomp_ret && dlen == PAGE_SIZE)
> --
> 2.52.0.351.gbe84eed79e-goog
>
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