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

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Message-ID: <CAH5fLggbXYYSTKMUR3jRXwWn7WPi8GtZpHqhLPWW9XHieRPQRA@mail.gmail.com>
Date: Wed, 14 Aug 2024 14:36:44 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
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 2:29 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On Wed, Aug 14, 2024 at 10:42:28AM +0200, Alice Ryhl wrote:
> > On Mon, Aug 12, 2024 at 8:25 PM Danilo Krummrich <dakr@...nel.org> 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. You can do the same by
> > utilizing new_uninit. A sketch:
> >
> > match KBox::<[_; _]>::new_uninit(GFP_KERNEL) {
> >     Ok(b) => Ok(KVec::from(KBox::write(b, [$($x),+]))),
> >     Err(e) => Err(e),
> > }
>
> Generally, I'm fine changing that, but what's the reason for the suggestion? It
> shouldn't make a difference, does it?

The compiler is much more likely to not put the array on the stack
before it is copied to the heap.

In the case where $x constructs new values, it also avoids
create-then-destroy on allocator failure.

Alice