linux-kernel - Re: [PATCH v11 23/24] mm: zswap: zswap_store() will process a large folio in batches.

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Message-ID: <CAGsJ_4zFXzXKVKu9M2_R1Nqqt6FBEktT1YBGAFPRgjTxv=+1_Q@mail.gmail.com>
Date: Fri, 29 Aug 2025 11:59:43 +1200
From: Barry Song <21cnbao@...il.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	yosry.ahmed@...ux.dev, nphamcs@...il.com, chengming.zhou@...ux.dev, 
	usamaarif642@...il.com, ryan.roberts@....com, ying.huang@...ux.alibaba.com, 
	akpm@...ux-foundation.org, senozhatsky@...omium.org, 
	linux-crypto@...r.kernel.org, herbert@...dor.apana.org.au, 
	davem@...emloft.net, clabbe@...libre.com, ardb@...nel.org, 
	ebiggers@...gle.com, surenb@...gle.com, kristen.c.accardi@...el.com, 
	vinicius.gomes@...el.com, wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v11 23/24] mm: zswap: zswap_store() will process a large
 folio in batches.

On Fri, Aug 1, 2025 at 4:36 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> This patch modifies zswap_store() to store a batch of pages in large
> folios at a time, instead of storing one page at a time. It does this by
> calling a new procedure zswap_store_pages() with a range of
> "pool->batch_size" indices in the folio.
>
> zswap_store_pages() implements all the computes done earlier in
> zswap_store_page() for a single-page, for multiple pages in a folio,
> namely the "batch":
>
> 1) It starts by allocating all zswap entries required to store the
>    batch. New procedures, zswap_entries_cache_alloc_batch() and
>    zswap_entries_cache_free_batch() call kmem_cache_[free]alloc_bulk()
>    to optimize the performance of this step.
>
> 2) Next, the entries fields are written, computes that need to be happen
>    anyway, without modifying the zswap xarray/LRU publishing order. This
>    improves latency by avoiding having the bring the entries into the
>    cache for writing in different code blocks within this procedure.
>
> 3) Next, it calls zswap_compress() to sequentially compress each page in
>    the batch.
>
> 4) Finally, it adds the batch's zswap entries to the xarray and LRU,
>    charges zswap memory and increments zswap stats.
>
> 5) The error handling and cleanup required for all failure scenarios
>    that can occur while storing a batch in zswap are consolidated to a
>    single "store_pages_failed" label in zswap_store_pages(). Here again,
>    we optimize performance by calling kmem_cache_free_bulk().
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  mm/zswap.c | 218 ++++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 149 insertions(+), 69 deletions(-)

This seems fine overall. However, could we pull some data from the
cover letter. For example, even with hardware batching, we are still
improving performance. Since your cover letter is very long, readers
might fail to connect this data with the patches.

Thanks
Barry