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

[Kairui Song <ryncsn@gmail.com>]

On Tue, Dec 2, 2025 at 12:47 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Sun, Nov 30, 2025 at 12:38:38AM +0400, Chris Li wrote:
> > On Sat, Nov 29, 2025 at 12:46 AM Nhat Pham <nphamcs@gmail.com> wrote:
> > >
> > > On Thu, Nov 27, 2025 at 11:10 AM Chris Li <chrisl@kernel.org> wrote:
> > > >
> > > > On Thu, Nov 27, 2025 at 6:28 AM Rik van Riel <riel@surriel.com> wrote:
> > > > >
> > > > > Sorry, I am talking about upstream.
> > > >
> > > > So far I have not had a pleasant upstream experience when submitting
> > > > this particular patch to upstream.
> > > >
> > > > > I really appreciate anybody participating in Linux
> > > > > kernel development. Linux is good because different
> > > > > people bring different perspectives to the table.
> > > >
> > > > Of course everybody is welcome. However, NACK without technical
> > > > justification is very bad for upstream development. I can't imagine
> > > > what a new hacker would think after going through what I have gone
> > > > through for this patch. He/she will likely quit contributing upstream.
> > > > This is not the kind of welcome we want.
> > > >
> > > > Nhat needs to be able to technically justify his NACK as a maintainer.
> > > > Sorry there is no other way to sugar coat it.
> > >
> > > I am NOT the only zswap maintainer who expresses concerns. Other
> > > people also have their misgivings, so I have let them speak and not
> > > put words in their mouths.
> >
> > You did not mention the fact that both two NACK from zswap maintainers
> > are from the same company. I assume you have some kind of team sync.
> > There is a term for that, called "person acting in concert".
>
> For the benefit of anybody following this from the sidelines, the
> third zswap maintainer also expressed concerns about Chris's proposal
> upthread. He works for the same company as Chris.
>
> The reality is that Chris is failing to convince others of his design
> direction, and is now obviously resorting to manipulation and hominem
> attacks.
>
> During the course of this thread, Chris has asked for "a little faith"
> that his idea will work for all stated requirements, without deeming
> it necessary to explain how.
>
> When probed on technical details, he stated that he doesn't like to
> plan that far ahead, and prefers having somebody else iron out the
> implementation details. He also referred to high-level slides from his
> LSFMM '24 session - which was received thusly[1]:
>
>   Matthew Wilcox agreed, warning Li that he was setting himself up for "a world of pain".
>
>   Jan Kara said that existing filesystem designs are not suited to this task
>
>   Hildenbrand said that this plan was introducing too much complexity
>
> His first response to criticism was to invoke his <4 week status of
> swap maintainer.
>
> Meanwhile, the design direction that Chris is construing as a single
> company conspiracy is anything but. The collaborative origins of these
> patches are well documented. Chris was CC'd on those RFCs. He notably
> did not engage in them. He is now lying about the narrative and
> choosing to attack these patches in bad faith and out of context.
>
> This pattern of behavior gives me low confidence that Chris is able to
> collaborate and compromise on a design that works for all users.
>
> And while Chris has been quite vocal and opinionated in mailing list
> discussions, his actual code contributions to the kernel do not
> instill confidence that he can solve this problem by himself, either.

Hi all,

I’d really prefer we all let things cool off a bit before the thread
gets too dramatic. :)

Sorry to see that the discussion went quite off topic, still I believe
this is some kind of misunderstanding on Chris' intention to improve
the kernel in a more generic way.

From my perspective, Chris did co-developed, suggested, reviewed or
authored many of the implementation details around the swap-table
idea, and he implemented the swap cluster allocator in 6.11, which
unlocked a bunch of follow-on optimizations.

I’ve been working on swap for a while as well and have rewritten and
refactored large parts of the swap, swap allocator and swap cache
(mm/swapfile.c, mm/swap_state.c, swap.h, swap_table.h). Maybe, yeah,
I’m not a kernel vet with decades of patches yet, but I do think I'm
familiar enough with swap. I think Chris' work, words or code, has
been looking good in the end results.

It's hard to put a penthouse on a sandcastle, and maybe that's the
reason makes it hard to describe or layout the further implementations
of swap.

We all struggled with swap subsystem a lot, the code base served us
well, but it had accumulated a lot of historical complexity and
awkward workarounds overtime (we had so many people in the community
complaining about it for so many years). I think we all agree that
pursuing incremental cleanups and improvement (eg. swap slot cache
cleanup, swap lock cleanup, swap_has_cache cleanup,
direct-swap workarounds removal, etc.) is more suitable upstream.
Chris also help a lot on this (eg. the LPC talk last year) and we
finally got rid of many long time burdens, quite some of these
works are directly enabled by his swap allocator rework first.

And I do have a more completed branch that I posted several times
showing the end results of swap tables have better memory consumption
& performance, and the code is much simpler than what we had in
upstream. It's getting merged step by step, and each step is a gain. I
believe that is the right way to improve things upstream, everyone and
every workload benefits, and progressively. And based on that, we will
be able to implement things much easier.

I believe things will look much better and cleaner as we process (eg.
resizing might be doable for generic swap too), and make it easier for
all of us, and make the swap subsystem better in a collaborative way.

Cheers.