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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKEwX=NT_3QJSQGRgogGYmx_M262O+F4Hs9BtbCUqK3qbh+t8g@mail.gmail.com>
Date: Wed, 15 Oct 2025 15:36:07 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>, 
	"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>, 
	"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>, "sj@...nel.org" <sj@...nel.org>, 
	"kasong@...cent.com" <kasong@...cent.com>, 
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>, 
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>, "davem@...emloft.net" <davem@...emloft.net>, 
	"clabbe@...libre.com" <clabbe@...libre.com>, "ardb@...nel.org" <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>
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 :)