linux-kernel - Re: [PATCH v6 09/26] rust: alloc: implement kernel `Box`

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Message-ID: <CAH5fLggUzUuiVuKOYmvo1fAcXGpsHUiRKx6DTg2cgkMe2YcF4g@mail.gmail.com>
Date: Tue, 20 Aug 2024 11:47:45 +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 v6 09/26] rust: alloc: implement kernel `Box`

On Fri, Aug 16, 2024 at 2:13 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> `Box` provides the simplest way to allocate memory for a generic type
> with one of the kernel's allocators, e.g. `Kmalloc`, `Vmalloc` or
> `KVmalloc`.
>
> In contrast to Rust's `Box` type, the kernel `Box` type considers the
> kernel's GFP flags for all appropriate functions, always reports
> allocation failures through `Result<_, AllocError>` and remains
> independent from unstable features.
>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Overall looks good to me, but I have a question:

> +impl<T: 'static, A> ForeignOwnable for Box<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Borrowed<'a> = &'a T;
> +    type BorrowedMut<'a> = &'a mut T;
> [..]
> +    unsafe fn borrow_mut<'a>(ptr: *const core::ffi::c_void) -> &'a mut T {
> +        // SAFETY: The safety requirements of this method ensure that the pointer is valid and that
> +        // nothing else will access the value for the duration of 'a.
> +        unsafe { &mut *ptr.cast_mut().cast() }
> +    }

Where does this come from? It looks like you've based the series on
top of [1], but I dropped that patch a long time ago, and I don't see
it in rust-dev anymore.

Alice

[1]: https://lore.kernel.org/all/20230710074642.683831-1-aliceryhl@google.com/