linux-kernel - Re: [PATCH RFC] mm: ghost swapfile support for zswap

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Message-ID: <CACePvbVmf2Yyfe9+uEXZUB2yoBQFNc_s1HwkJ-RPLoTDqGgj3g@mail.gmail.com>
Date: Wed, 3 Dec 2025 01:07:34 +0400
From: Chris Li <chrisl@...nel.org>
To: Nhat Pham <nphamcs@...il.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kairui Song <kasong@...cent.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, Yosry Ahmed <yosry.ahmed@...ux.dev>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	pratmal@...gle.com, sweettea@...gle.com, gthelen@...gle.com, 
	weixugc@...gle.com
Subject: Re: [PATCH RFC] mm: ghost swapfile support for zswap

On Tue, Dec 2, 2025 at 10:19 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> > That indirection is the tradeoff for swapped pages. In turn you're
> > getting back all that other stuff for swap slots that *aren't*
> > currently used. This is a win for the vast majority of users.
>
> I will also note though, that we will merge the zswap tree with the
> virtual swap descriptors as well.

What is the merged per swap slot entry size? If your descritor is over
48 bytes plus some zswap pool handles and compressed buffer size etc I
am not sure that is an overall win. Provide actual number helps.

> So for zswap entries there are actually no extra overhead induced by
> the backend indirection pointer :)
>
> IOW, overhead for zswap-only users (such as Google) will be much
> smaller than what Johannes is describing here - pretty much
> non-existent :) While you will still gain all the other benefits (swap

The per swap slot memory usage size, zswap+ swap core, is it smaller
than the ghost swap file patch I posted here?
Do you have a number in bytes?

Chris