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

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Aug 12, 2024 at 8:25 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> `Vec` provides a contiguous growable array type (such as `Vec`) with
> contents allocated with the kernel's allocators (e.g. `Kmalloc`,
> `Vmalloc` or `KVmalloc`).
>
> In contrast to Rust's `Vec` type, the kernel `Vec` 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@kernel.org>
> [...]
> +impl<T, A, const N: usize> Box<[T; N], A>
> +where
> +    A: Allocator,
> +{
> +    /// Convert a `Box<[T, N], A>` to a `Vec<T, A>`.
> +    pub fn into_vec(b: Self) -> Vec<T, A> {

Nit: I would probably make this a From impl.

> +#[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),
}

> +// SAFETY: `Vec` is `Send` if `T` is `Send` because the data referenced by `self.ptr` is unaliased.
> +unsafe impl<T, A> Send for Vec<T, A>
> +where
> +    T: Send,
> +    A: Allocator,
> +{
> +}
> +
> +// SAFETY: `Vec` is `Sync` if `T` is `Sync` because the data referenced by `self.ptr` is unaliased.
> +unsafe impl<T, A> Sync for Vec<T, A>
> +where
> +    T: Send,
> +    A: Allocator,

Same comment as Box. Ditto about "unaliased". Needs `T: Sync` instead.

> +impl<T: Clone, A: Allocator> Vec<T, A> {
> +    /// Extend the vector by `n` clones of value.
> +    pub fn extend_with(&mut self, n: usize, value: T, flags: Flags) -> Result<(), AllocError> {
> +        self.reserve(n, flags)?;
> +
> +        let spare = self.spare_capacity_mut();
> +
> +        for item in spare.iter_mut().take(n - 1) {

You need to handle `n == 0` here.

Alice