linux-kernel - Re: [PATCH v3 3/5] rust: scatterlist: Add abstraction for sg

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Message-ID: <aK3BsV_s-xMQLF4d@google.com>
Date: Tue, 26 Aug 2025 14:16:17 +0000
From: Alice Ryhl <aliceryhl@...gle.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, tmgross@...ch.edu, 
	abdiel.janulgue@...il.com, acourbot@...dia.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 v3 3/5] rust: scatterlist: Add abstraction for sg_table

On Mon, Aug 25, 2025 at 03:24:42PM +0200, 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 generic
> parameter to provide compile-time guarantees about ownership and lifetime.
> 
> The abstraction provides two primary states:
> - `SGTable<Owned<P>>`: Represents a table whose resources are fully
>   managed by Rust. It takes ownership of a page provider `P`, allocates
>   the underlying `struct sg_table`, maps it for DMA, and handles all
>   cleanup automatically upon drop. The DMA mapping's lifetime is tied to
>   the associated device using `Devres`, ensuring it is correctly unmapped
>   before the device is unbound.
> - `SGTable<Borrowed>` (or just `SGTable`): A zero-cost representation of
>   an externally managed `struct sg_table`. It is created from a raw
>   pointer using `SGTable::as_ref()` and provides a lifetime-bound
>   reference (`&'a SGTable`) for operations like iteration.
> 
> The API exposes a safe iterator that yields `&SGEntry` references,
> allowing drivers to easily access the DMA address and length of each
> segment in the list.
> 
> Co-developed-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Overall LGTM. With comments addressed:
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>

> +impl RawSGTable {
> +    fn new(
> +        pages: &mut [*mut bindings::page],

This should probably be unsafe due to the raw pointer. Or could we pass
any pointer here?

Alice