linux-kernel - Re: [PATCH v5 13/26] rust: alloc: implement kernel `Vec` type

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Message-ID: <Zr0z0fBM_acHFezv@cassiopeiae>
Date: Thu, 15 Aug 2024 00:46:41 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: ojeda@...nel.org, alex.gaynor@...il.com, wedsonaf@...il.com,
	boqun.feng@...il.com, gary@...yguo.net, bjorn3_gh@...tonmail.com,
	benno.lossin@...ton.me, a.hindborg@...sung.com,
	akpm@...ux-foundation.org, daniel.almeida@...labora.com,
	faith.ekstrand@...labora.com, boris.brezillon@...labora.com,
	lina@...hilina.net, mcanal@...lia.com, zhiw@...dia.com,
	cjia@...dia.com, jhubbard@...dia.com, airlied@...hat.com,
	ajanulgu@...hat.com, lyude@...hat.com, linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 13/26] rust: alloc: implement kernel `Vec` type

On Wed, Aug 14, 2024 at 10:42:28AM +0200, Alice Ryhl wrote:
> > +#[macro_export]
> > +macro_rules! kvec {
> > +    () => (
> > +        {
> > +            $crate::alloc::KVec::new()
> > +        }
> > +    );
> > +    ($elem:expr; $n:expr) => (
> > +        {
> > +            $crate::alloc::KVec::from_elem($elem, $n, GFP_KERNEL)
> > +        }
> > +    );
> > +    ($($x:expr),+ $(,)?) => (
> > +        {
> > +            match $crate::alloc::KBox::new([$($x),+], GFP_KERNEL) {
> > +                Ok(b) => Ok($crate::alloc::KBox::into_vec(b)),
> > +                Err(e) => Err(e),
> 
> Hmm. This currently generates code that:
> 
> 1. Creates the array.
> 2. Allocates the memory.
> 3. Moves the array into the box.
> 
> Whereas the stdlib macro swaps step 1 and 2.

Isn't stdlib [1] doing the same thing I do?

[1] https://doc.rust-lang.org/1.80.1/src/alloc/macros.rs.html#49

> You can do the same by utilizing new_uninit. A sketch:
> 
> match KBox::<[_; _]>::new_uninit(GFP_KERNEL) {

How do we get the size here? `#![feature(generic_arg_infer)]` seems to be
unstable.

>     Ok(b) => Ok(KVec::from(KBox::write(b, [$($x),+]))),
>     Err(e) => Err(e),
> }