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

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Message-Id: <DC9UYN56HBRZ.XRDXQHO2QQQ1@kernel.org>
Date: Sat, 23 Aug 2025 15:57:18 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alexandre Courbot" <acourbot@...dia.com>
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 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.

> 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().

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. :)