linux-kernel - RE: [PATCH v12 22/23] mm: zswap: zswap_store() will process a large folio in batches.

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Message-ID: <PH7PR11MB8121779612BDD6C3A3D54959C9F8A@PH7PR11MB8121.namprd11.prod.outlook.com>
Date: Fri, 31 Oct 2025 22:15:11 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Nhat Pham <nphamcs@...il.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "hannes@...xchg.org"
	<hannes@...xchg.org>, "chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>,
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	<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 v12 22/23] mm: zswap: zswap_store() will process a large
 folio in batches.


> -----Original Message-----
> From: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Sent: Wednesday, October 15, 2025 5:57 PM
> To: Nhat Pham <nphamcs@...il.com>; Yosry Ahmed
> <yosry.ahmed@...ux.dev>
> Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; 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; sj@...nel.org; kasong@...cent.com; 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; 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 v12 22/23] mm: zswap: zswap_store() will process a
> large folio in batches.
> 
> 
> > -----Original Message-----
> > From: Nhat Pham <nphamcs@...il.com>
> > Sent: Wednesday, October 15, 2025 3:36 PM
> > To: Yosry Ahmed <yosry.ahmed@...ux.dev>
> > Cc: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>; linux-
> > kernel@...r.kernel.org; linux-mm@...ck.org; hannes@...xchg.org;
> > 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; sj@...nel.org; kasong@...cent.com; 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; 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 v12 22/23] mm: zswap: zswap_store() will process a
> > large folio in batches.
> >
> > On Wed, Oct 15, 2025 at 3:24 PM Yosry Ahmed <yosry.ahmed@...ux.dev>
> > wrote:
> > >
> > > On Wed, Oct 15, 2025 at 10:15:12PM +0000, Sridhar, Kanchana P wrote:
> > >
> > > I am against increasing the size of struct zswap_entry. On x86_64, there
> > > is a 3 byte hole after 'referenced'. We can technically use that,
> > > although the node id is usually an int, which is 4 bytes on x86_64.
> > >
> > > In practice, I think 2 bytes (i.e. short) should be enough, but it will
> > > be ugly to cast the node id to a short. We should at least WARN on
> > > overflow.
> >
> > Can we pack length and referenced using bit fields? I assume
> > compressed length cannot exceed 2^31 - 1? :)
> >
> > So, something along the line of:
> >
> > struct {
> >     unsigned int length:31;
> >     bool referenced:1;
> > }
> >
> > That way, we save up another 4 bytes hole, which can be repurposed for
> > the node's id.
> >
> > Does that work? I'm not very experienced with this magickery - please
> > fact check me :)
> >
> >
> > >
> > > Or we can take the simple route and drop the bulk allocation.
> >
> > Or this :)
> 
> Thanks Nhat and Yosry, for sharing these constraints and ideas.
> Sure, I will see if I can use the bit fields approach and will make sure
> to not increase the size of struct zswap_entry. If this doesn't work
> for some reason, we have the simple route.

Hi Nhat and Yosry,

I have been able to get the bit fields approach working, as you suggested.
The size of "struct zswap_entry" is unchanged at 56 bytes with this change
and the addition of the "int nid" field.

zswap_lru_add()/del() use the entry->nid.

I am able to continue using kmem_cache_alloc_bulk() to benefit batching
performance.

I will include this in v13 that I expect to post within a couple of days.

Thanks!
Kanchana


> 
> Best regards,
> Kanchana