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

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Message-ID: <SA1PR11MB8476756D7255F1EA1EBE322AC9DEA@SA1PR11MB8476.namprd11.prod.outlook.com>
Date: Wed, 26 Nov 2025 20:05:40 +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>,
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	"senozhatsky@...omium.org" <senozhatsky@...omium.org>, "kasong@...cent.com"
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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, November 25, 2025 10:35 PM
> 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 Wed, Nov 26, 2025 at 01:46:57PM +0800, Herbert Xu wrote:
> > On Fri, Nov 14, 2025 at 03:37:53PM +0000, Yosry Ahmed wrote:
> > >
> > > Thanks for the clarification. I understand that the error code has
> > > different sources for SW and HW compressors, but I do not like using
> > > errp as an indirection. It makes the code unclear. I would rather we
> > > explicitly check err for SW compressors and dlen for HW compressors.
> > >
> > > That being said, I thought what Herbert suggested was that the same API
> > > is used for both SW and HW compressors. IOW, either way we submit a
> > > batch of pages (8 pages for SW compressors), and then the crypto API
> > > would either give the entire batch to the compressor if it supports
> > > batching, or loop over them internally and hand them page-by-page to
> > > the compressor.
> > >
> > > This would simplify usage as we do not have to handle the differences in
> > > zswap.
> > >
> > > If that is not doable, at the very least the API should be consistent.
> > > Right now the error code and length are propagated differently to the
> > > caller based on whether or not the compressor support batching.
> >
> > Yes we should only have one code path in zswap, regardless of whether
> > batching is used or not.
> >
> > The degenerate case of a batch with a single page should be handled
> > by the Crypto API.
> >
> > So I will change crypto_acomp to take care of this case.
> 
> Nice :)

Thanks Herbert and Yosry!

Herbert, to make sure I understand, will you be implementing all of these
features in crypto_acomp for software compressors? I would appreciate it
if you can clarify:

1) Error & compressed length propagation to the dst sg->length only for
    non-batching compressors.
    a) For batching compressors, this wouldn't apply since errors could occur
        for any page in the batch, and the first page (dst sg->length) could have
        successfully compressed.

2) Will you also be handling the case where zswap can send an SG list batch
     with multiple pages to a non-batching compressor, and the crypto_acomp
     API will internally compress each page sequentially, propagate
     errors/compress lengths before returning?
        
If so, this would really standardize the code in zswap for batching and
non-batching compressors.

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