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

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Message-ID: <CAKEwX=MgJPFJNFuj7vQ+RjuJQiC+dOeTTEBLv6=Dna=MNFubJw@mail.gmail.com>
Date: Tue, 2 Dec 2025 10:18:58 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Chris Li <chrisl@...nel.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, Nov 25, 2025 at 1:31 PM Johannes Weiner <hannes@...xchg.org> wrote:
>
> On Tue, Nov 25, 2025 at 11:27:04PM +0400, Chris Li wrote:
> > On Mon, Nov 24, 2025 at 11:33 PM Johannes Weiner <hannes@...xchg.org> wrote:
> > > > > Do you have a link to that proposal?
> > > >
> > > > My 2024 LSF swap pony talk already has a mechanism to redirect page
> > > > cache swap entries to different physical locations.
> > > > That can also work for redirecting swap entries in different swapfiles.
> > > >
> > > > https://lore.kernel.org/linux-mm/CANeU7QnPsTouKxdK2QO8Opho6dh1qMGTox2e5kFOV8jKoEJwig@mail.gmail.com/
> > >
> > > I looked through your slides and the LWN article, but it's very hard
> > > for me to find answers to my questions in there.
> >
> > Naturally, the slide is only intended to cover what is in the current
> > swap table may be phase VII.
> > But it does have the physical location pointer consideration.
> >
> > > In your proposal, let's say you have a swp_entry_t in the page
> > > table. What does it describe, and what are the data structures to get
> > > from this key to user data in the following scenarios:
> >
> > Please keep in mind that I don't have every detail design laid out. I
> > follow the first principles that redirect a swap entry page should
> > only take an additional 4 byte per swap entry. VS blow up the swap
> > entry size by something like 24 bytes?
>
> Nhat can lay this out in more detail, but there isn't much new stuff
> in the virtual swap descriptor. It's mostly just a consolidation of
> state we currently track elsewhere - swap count, swapcache pointer,
> cgroup ownership etc.
>
> The actual indirection is just a word for the backend type,offset.
>
> 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.

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
space dynamicization, operational overhead reduction) of swap
virtualization.