linux-kernel - RE: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with compress batching of large folios.

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Message-ID: <SJ2PR11MB8472CE03A67C1161469CDE9EC9A3A@SJ2PR11MB8472.namprd11.prod.outlook.com>
Date: Tue, 9 Dec 2025 17:21:06 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>, Herbert Xu
	<herbert@...dor.apana.org.au>
CC: SeongJae Park <sj@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hannes@...xchg.org" <hannes@...xchg.org>, "nphamcs@...il.com"
	<nphamcs@...il.com>, "chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "ryan.roberts@....com"
	<ryan.roberts@....com>, "21cnbao@...il.com" <21cnbao@...il.com>,
	"ying.huang@...ux.alibaba.com" <ying.huang@...ux.alibaba.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"senozhatsky@...omium.org" <senozhatsky@...omium.org>, "kasong@...cent.com"
	<kasong@...cent.com>, "linux-crypto@...r.kernel.org"
	<linux-crypto@...r.kernel.org>, "davem@...emloft.net" <davem@...emloft.net>,
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	<ardb@...nel.org>, "ebiggers@...gle.com" <ebiggers@...gle.com>,
	"surenb@...gle.com" <surenb@...gle.com>, "Accardi, Kristen C"
	<kristen.c.accardi@...el.com>, "Gomes, Vinicius" <vinicius.gomes@...el.com>,
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh"
	<vinodh.gopal@...el.com>, "Sridhar, Kanchana P"
	<kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with
 compress batching of large folios.


> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@...ux.dev>
> Sent: Tuesday, December 9, 2025 8:55 AM
> To: Herbert Xu <herbert@...dor.apana.org.au>
> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>; SeongJae Park
> <sj@...nel.org>; linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; nphamcs@...il.com; chengming.zhou@...ux.dev;
> usamaarif642@...il.com; ryan.roberts@....com; 21cnbao@...il.com;
> ying.huang@...ux.alibaba.com; akpm@...ux-foundation.org;
> senozhatsky@...omium.org; kasong@...cent.com; linux-
> crypto@...r.kernel.org; davem@...emloft.net; clabbe@...libre.com;
> ardb@...nel.org; ebiggers@...gle.com; surenb@...gle.com; Accardi,
> Kristen C <kristen.c.accardi@...el.com>; Gomes, Vinicius
> <vinicius.gomes@...el.com>; Feghali, Wajdi K <wajdi.k.feghali@...el.com>;
> Gopal, Vinodh <vinodh.gopal@...el.com>
> Subject: Re: [PATCH v13 22/22] mm: zswap: Batched zswap_compress() with
> compress batching of large folios.
> 
> On Tue, Dec 09, 2025 at 10:32:20AM +0800, Herbert Xu wrote:
> > On Tue, Dec 09, 2025 at 01:15:02AM +0000, Yosry Ahmed wrote:
> > >
> > > Just to clarify, does this mean that zswap can pass a batch of (eight)
> > > pages to the acomp API, and get the results for the batch uniformly
> > > whether or not the underlying compressor supports batching?
> >
> > Correct.  In fact I'd like to remove the batch size exposure to zswap
> > altogether.  zswap should just pass along whatever maximum number of
> > pages that is convenient to itself.
> 
> I think exposing the batch size is still useful as a hint for zswap. In
> the current series, zswap allocates as many per-CPU buffers as the
> compressor's batch size, so no extra buffers for non-batching
> compressors (including SW compressors).
> 
> If we use the same batch size regardless, we'll have to always allocate
> 8 (or N) per-CPU buffers, for little to no benefit on non-batching
> compressors.
> 
> So we still want the batch size on the zswap side, but we want the
> crypto API to be uniform whether or not the compressor supports
> batching.

Thanks Yosry, you bring up a good point. I currently have the outer for
loop in zswap_compress() due to the above constraint. For non-batching
compressors, we allocate only one per-CPU buffer. Hence, we need to
call crypto_acomp_compress() and write the compressed data to the
zs_poll for each page in the batch. Wouldn't we need to allocate
8 per-CPU buffers for non-batching compressors if we want zswap to
send a batch of 8 pages uniformly to the crypto API, so that
zswap_compress() can store the 8 pages in zs_pool after the crypto
API returns?

Thanks,
Kanchana

> 
> >
> > Cheers,
> > --
> > Email: Herbert Xu <herbert@...dor.apana.org.au>
> > Home Page: http://gondor.apana.org.au/~herbert/
> > PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt