linux-kernel - Re: [PATCH 1/6] rust: types: Add Ownable/Owned types

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Message-ID: <CAH5fLggscATtCgQwCYSms77oSFOMkjTscRDqAOZsSgoHsOoPQw@mail.gmail.com>
Date: Mon, 3 Feb 2025 10:13:56 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Asahi Lina <lina@...hilina.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Jann Horn <jannh@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	Danilo Krummrich <dakr@...nel.org>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Valentin Obst <kernel@...entinobst.de>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	airlied@...hat.com, Abdiel Janulgue <abdiel.janulgue@...il.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	asahi@...ts.linux.dev
Subject: Re: [PATCH 1/6] rust: types: Add Ownable/Owned types

On Sun, Feb 2, 2025 at 2:06 PM Asahi Lina <lina@...hilina.net> wrote:
>
> By analogy to AlwaysRefCounted and ARef, an Ownable type is a (typically
> C FFI) type that *may* be owned by Rust, but need not be. Unlike
> AlwaysRefCounted, this mechanism expects the reference to be unique
> within Rust, and does not allow cloning.
>
> Conceptually, this is similar to a KBox<T>, except that it delegates
> resource management to the T instead of using a generic allocator.
>
> Signed-off-by: Asahi Lina <lina@...hilina.net>

Overall LGTM.

> +/// A subtrait of Ownable that asserts that an `Owned<T>` Rust reference is not only unique
> +/// within Rust and keeps the `T` alive, but also guarantees that the C code follows the
> +/// usual mutable reference requirements. That is, the kernel will never mutate the
> +/// `T` (excluding internal mutability that follows the usual rules) while Rust owns it.
> +///
> +/// When this type is implemented for an [`Ownable`] type, it allows `Owned<T>` to be
> +/// dereferenced into a &mut T.
> +///
> +/// # Safety
> +///
> +/// Implementers must ensure that the kernel never mutates the underlying type while
> +/// Rust owns it.
> +pub unsafe trait OwnableMut: Ownable {}

Giving out mutable references allows users to call core::mem::swap on
the object. We must require that this is allowed.

> +impl<T: Ownable> Owned<T> {
> +    /// Creates a new instance of [`Owned`].
> +    ///
> +    /// It takes over ownership of the underlying object.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that the underlying object is acquired and can be considered owned by
> +    /// Rust.
> +    pub(crate) unsafe fn from_raw(ptr: NonNull<T>) -> Self {
> +        // INVARIANT: The safety requirements guarantee that the new instance now owns the
> +        // reference.
> +        Self {
> +            ptr,
> +            _p: PhantomData,
> +        }
> +    }
> +
> +    /// Consumes the `Owned`, returning a raw pointer.
> +    ///
> +    /// This function does not actually relinquish ownership of the object.
> +    /// After calling this function, the caller is responsible for ownership previously managed
> +    /// by the `Owned`.
> +    #[allow(dead_code)]
> +    pub(crate) fn into_raw(me: Self) -> NonNull<T> {

I would just make these methods public, like the ARef ones. Then you
can drop the #[allow(dead_code)] annotation.

Alice