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Message-ID: <yr7cekjrssjiwqlkrmreugl6fhywssutjzg3ll45mcvdjklnzy@5vkgju4wwtrg>
Date: Mon, 5 Jan 2026 16:01:54 +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: [RFC PATCH 2/2] zsmalloc: chain-length configuration should
consider other metrics
On Mon, Jan 05, 2026 at 04:23:39PM +0900, Sergey Senozhatsky wrote:
> On (26/01/05 10:42), Sergey Senozhatsky wrote:
> > On (26/01/02 18:29), Yosry Ahmed wrote:
> > > On Thu, Jan 01, 2026 at 10:38:14AM +0900, Sergey Senozhatsky wrote:
> > [..]
> > >
> > > I worry that the heuristics are too hand-wavy
> >
> > I don't disagree. Am not super excited about the heuristics either.
> >
> > > and I wonder if the memcpy savings actually show up as perf improvements
> > > in any real life workload. Do we have data about this?
> >
> > I don't have real life 16K PAGE_SIZE devices. However, on 16K PAGE_SIZE
> > systems we have "normal" size-classes up to a very large size, and normal
> > class means chaining of 0-order physical pages, and chaining means spanning.
> > So on 16K memcpy overhead is expected to be somewhat noticeable.
>
> By the way, while looking at it, I think we need to "fix" obj_read_begin().
> Currently, it uses "off + class->size" to detect spanning objects, which is
> incorrect: size classes get merged, so a typical size class can hold a range
> of sizes, using padding for smaller objects. So instead of class->size we
> need to use the actual compressed objects size, just in case if actual written
> size was small enough to fit into the first physical page (we do that in
> obj_write()). I'll cook a patch.
We also need to handle zs_obj_read_end() to do the kunmap() call
correctly.
>
> Something like this:
>
> ---
>
> drivers/block/zram/zram_drv.c | 8 +++++---
> include/linux/zsmalloc.h | 2 +-
> mm/zsmalloc.c | 4 ++--
> mm/zswap.c | 3 ++-
> 4 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index a6587bed6a03..b371ba6bfec2 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -2065,7 +2065,7 @@ 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);
> @@ -2087,7 +2087,8 @@ 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);
> @@ -2114,7 +2115,8 @@ 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);
> zcomp_stream_put(zstrm);
> diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
> index f3ccff2d966c..64f65c1f14d6 100644
> --- a/include/linux/zsmalloc.h
> +++ b/include/linux/zsmalloc.h
> @@ -40,7 +40,7 @@ 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);
> void zs_obj_write(struct zs_pool *pool, unsigned long handle,
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index be385609ef8a..2da60c23cd18 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1070,7 +1070,7 @@ 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;
> @@ -1092,7 +1092,7 @@ 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 (off + mem_len <= PAGE_SIZE) {
> /* this object is contained entirely within a page */
> addr = kmap_local_zpdesc(zpdesc);
> addr += off;
> diff --git a/mm/zswap.c b/mm/zswap.c
> index de8858ff1521..291352629616 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) {
> --
> 2.52.0.351.gbe84eed79e-goog
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