linux-kernel - Re: [PATCH 1/2] rust: add initial scatterlist bindings

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Message-ID: <aEHDS0YPmkB4Uto_@Mac.home>
Date: Thu, 5 Jun 2025 09:18:19 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Alexandre Courbot <acourbot@...dia.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>, dakr@...nel.org,
	lyude@...hat.com, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Valentin Obst <kernel@...entinobst.de>,
	open list <linux-kernel@...r.kernel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, airlied@...hat.com,
	rust-for-linux@...r.kernel.org,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
	Petr Tesarik <petr@...arici.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Sui Jingfeng <sui.jingfeng@...ux.dev>,
	Randy Dunlap <rdunlap@...radead.org>,
	Michael Kelley <mhklinux@...look.com>
Subject: Re: [PATCH 1/2] rust: add initial scatterlist bindings

On Thu, Jun 05, 2025 at 01:02:04PM -0300, Jason Gunthorpe wrote:
> On Thu, Jun 05, 2025 at 08:35:59AM -0700, Boqun Feng wrote:
> > Late to the party, but seems to me the main problem here is that we
> > cannot pass a reference to .set_page(), note that there is some work
> > that would change the Rust struct Page from a `*mut page` to a
> > `page`[0], and then we can impl Ownable[1] and AlwaysRefCounted for
> > `Page`, if that's done, then I believe the correct parameter for
> > set_page() would be an ARef<Page>.
> 
> There are alot of things that want to go into scatterlists that don't
> have struct pages that are refcountable (eg frozen pages used by
> kmalloc).
> 

I see.

> So I don't think you want to go in the direction of forcing
> struct page refcounting in scatter table. That is not how the C API
> works for good reason.
> 

Ok, that's fair.

> I also don't think it is a good idea to push more struct page stuff
> into Rust as we are trying to eliminate struct page from the

Well, that's why I suggested Owned<Page> and ARef<Folio>, but let's not
get too deep into that direction unless necessary ;-)

> kernel. It is better for rust to stick to KVAs and convert to struct
> page in core code only where absolutely necessary for someon reason.
> 

Sounds reasonable to me for now.

Regards,
Boqun

> Which is another part of why I suggested set_page should not be part
> of the driver facing rust API for scatterlist.
> 
> Jason