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

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Message-ID: <CACePvbUCeEu3dkq-M5fL-k-bca2c_h8A4HTCmLvRaV1aFW5qnw@mail.gmail.com>
Date: Thu, 27 Nov 2025 06:07:33 +0400
From: Chris Li <chrisl@...nel.org>
To: Rik van Riel <riel@...riel.com>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <kasong@...cent.com>, 
	Kemeng Shi <shikemeng@...weicloud.com>, Nhat Pham <nphamcs@...il.com>, 
	Baoquan He <bhe@...hat.com>, Barry Song <baohua@...nel.org>, 
	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 Thu, Nov 27, 2025 at 1:59 AM Rik van Riel <riel@...riel.com> wrote:
>
> On Tue, 2025-11-25 at 22:50 +0400, Chris Li wrote:
> >
> > > - We still cannot do swapoff efficiently as we need to walk the
> > > page
> > >   tables (and some swap tables) to find and swapin all entries in a
> > >   swapfile. Not as important as other things, but worth mentioning.
> >
> > That need rmap for swap entries. It It is an independent issue.
> >
>
> Wouldn't rmap for swap entries be more expensive than
> simply always having indirection for swap entries that
> are in use?

It might be, to be frank. I consider this pretty far and late in the
stage of the game to evaluate the rmap and its alternatives. Do you
agree?

I might or might not try the rmap for swap entry. Right now I don't
have many data points nor insights.

> With indirection, swapoff can just move pages from
> the being-swapoffed device into the swap cache, and
> if needed the memory can then be moved to another
> swap device, without ever needing to find the page
> tables.

Ack. I don't think we have any disagreement here.

> This sounds like an uncommon scenario, but it is
> functionally identical to what is done to pages
> during zswap writeback, where the page table entries
> stay unchanged, and the swap page is simply moved
> to another backend location.
>
> Why implement two things, when we can have one
> thing that does both, with no extra complexity
> over what zswap writeback needs?

Let me ask you a clarifying question, then.

1) What exactly are you trying to propose here in what project? VS or
swap the pony?
2) What stage of the code change do you have in mind should this
change apply to?

I can't speak for VS,  I am open to embrace what you suggest in order
to swap the pony project, that is after I understand it first.

Chris