linux-kernel - Re: [PATCH v2 3/5] rust: scatterlist: Add type-state abstraction for sg

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Message-Id: <DC9VD4LXAK9R.1RIJYWF1MFCMS@nvidia.com>
Date: Sat, 23 Aug 2025 23:16:14 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: <akpm@...ux-foundation.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <boqun.feng@...il.com>, <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>, <a.hindborg@...nel.org>,
 <aliceryhl@...gle.com>, <tmgross@...ch.edu>, <abdiel.janulgue@...il.com>,
 <jgg@...pe.ca>, <lyude@...hat.com>, <robin.murphy@....com>,
 <daniel.almeida@...labora.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/5] rust: scatterlist: Add type-state abstraction
 for sg_table

On Sat Aug 23, 2025 at 10:57 PM JST, Danilo Krummrich wrote:
> On Sat Aug 23, 2025 at 3:47 PM CEST, Alexandre Courbot wrote:
>> Oops, forgot to mention a couple more things:
>>
>> On Thu Aug 21, 2025 at 1:52 AM JST, Danilo Krummrich wrote:
>>> Add a safe Rust abstraction for the kernel's scatter-gather list
>>> facilities (`struct scatterlist` and `struct sg_table`).
>>>
>>> This commit introduces `SGTable<T>`, a wrapper that uses a type-state
>>> pattern to provide compile-time guarantees about ownership and lifetime.
>>
>> Is this actually a typestate? From my understanding, the typestate
>> pattern implies transitions from one state to the other (such as
>> Unmapped -> Mapped), but in this version there are no such transitions
>> (the previous ones had, though). We are just using a generic parameter,
>> so mentioning typestate sounds a bit misleading to me.
>
> I'd argue that it's still kind of a typestate. You can derive &SGTable (i.e.
> &SGTable<Borrowed>) from SGTabe<Owned>. So, technically there is an
> uni-directional transition I guess.

That's technically correct, but is also not the intent of the design, at
least compared to something like Unmapped <-> Mapped. Not a big problem
if you prefer to keep the current naming though.

>
>> Another random thought, in the owned case, do we want to provide an
>> accessor to the provider of the backing pages? Or do we expect the
>> caller to take dispositions to keep such a reference if they need to
>> access the backing buffer post-mapping?
>
> That's not going to work that easily. Once the backing pages are DMA mapped, the
> backing buffer can be accessed safely an more.
>
> See also the safety requirements of dma::CoherentAllocation::as_slice() and
> dma::CoherentAllocation::as_slice_mut().

Yup. So couldn't similar accessors (marked unsafe of course) be
convenient?

>
> If we want to support that, we have to provide a new type for this and maybe
> want to define a common trait for DMA mapped memory accessors, etc.
>
> Not the scope for this series, I believe. :)

I've had a few thoughts in that direction as well, but completely agree
we should debate about this *after* this series is merged. :)