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From: Yosry Ahmed To: Sergey Senozhatsky Cc: Andrew Morton , Nhat Pham , Minchan Kim , Johannes Weiner , Brian Geffon , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH 2/2] zsmalloc: chain-length configuration should consider other metrics Message-ID: References: <20260101013814.2312147-1-senozhatsky@chromium.org> <20260101013814.2312147-3-senozhatsky@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Stat-Signature: 9iwgcnt16jtm197ro6pd7e79h658qg49 X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: BA1C9C0014 X-HE-Tag: 1767628952-706009 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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