linux-kernel - Re: [PATCH v4 0/2] rust: zpool: add abstraction for zpool drivers

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Message-Id: <47BCEE04-1759-4242-BF5F-727E2C2E0772@konsulko.se>
Date: Tue, 2 Sep 2025 17:16:04 +0200
From: Vitaly Wool <vitaly.wool@...sulko.se>
To: Johannes Weiner <hannes@...xchg.org>
Cc: rust-for-linux <rust-for-linux@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 Uladzislau Rezki <urezki@...il.com>,
 Danilo Krummrich <dakr@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Vlastimil Babka <vbabka@...e.cz>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>,
 Bjorn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 Trevor Gross <tmgross@...ch.edu>,
 Yosry Ahmed <yosry.ahmed@...ux.dev>,
 Nhat Pham <nphamcs@...il.com>,
 linux-mm@...ck.org
Subject: Re: [PATCH v4 0/2] rust: zpool: add abstraction for zpool drivers

> On Aug 27, 2025, at 3:07 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> 
> On Tue, Aug 26, 2025 at 04:56:46PM +0200, Vitaly Wool wrote:
>> 
>> 
>>> On Aug 26, 2025, at 2:44 PM, Johannes Weiner <hannes@...xchg.org> wrote:
>>> 
>>> On Sat, Aug 23, 2025 at 03:04:19PM +0200, Vitaly Wool wrote:
>>>> Zpool is a common frontend for memory storage pool implementations.
>>>> These pools are typically used to store compressed memory objects,
>>>> e. g. for Zswap, the lightweight compressed cache for swap pages.
>>>> 
>>>> This patch provides the interface to use Zpool in Rust kernel code,
>>>> thus enabling Rust implementations of Zpool allocators for Zswap.
>>> 
>>> The zpool indirection is on its way out.
>>> 
>>> When you submitted an alternate allocator backend recently, the
>>> resounding feedback from the zswap maintainers was that improvements
>>> should happen to zsmalloc incrementally. It is a lot of code and has a
>>> lot of features that go beyond allocation strategy. We do not want to
>>> fork it and fragment this space again with niche, incomplete backends.
>>> 
>>> It's frustrating that you not only ignored this, but then went ahead
>>> and made other people invest their time and effort into this as well.
>>> 
>> 
>> I don’t think we have a consensus on that.
>> 
>> And zblock is, after some additional improvements, just better than
>> zsmalloc in all meaningful aspects, let alone the simplicity. It is
>> fas easier to implement in Rust than zsmalloc, too. Besides, zram is
>> a good candidate to be rewritten in Rust as well and after that is
>> done, zblock will be even safer and faster. So while not being
>> “incomplete", it’s zsmalloc that is becoming a niche backend moving
>> forward, and I would argue that it could make more sense to
>> eventually obsolete *it* rather than the zpool API.
> 
> That's your opinion, and I disagree with all of these claims. I would
> also be surprised if you found much alignment on this with the other
> folks who develop and use these features on a daily basis.

By features you mean zsmalloc? I certainly respect the effort put in it but if not for it being rather sluggish and error prone in non-4K page environments, we’d not come up with zblock.

> That being said, by all means, you can propose alternate
> allocators. But you don't need the zpool API for that. Just provide
> alternate implementations of the "zs_*" API and make it compile-time
> selectable.
> 
zpool API is neutral and well-defined, I don’t see *any* good reason for it to be phased out.

> As it stands, it's hard to justify the almost 700 lines of code to
> support *runtime-switching* of zswap backends when there is only one
> backend in-tree (and even you suggest there should only be one, albeit
> a different one).

No, I don’t. What I said was that zsmalloc was arguably becoming a niche allocator. And the point here is, the toy allocator written in Rust (just as an example of how zpool API could be used) shows in some tests similar results to zsmalloc both performance and compression density wise.