Re: [PATCH 1/2] rust: alloc: specify the minimum alignment of each allocator

["Danilo Krummrich" <dakr@kernel.org>]

On Tue Jul 15, 2025 at 3:46 PM CEST, Alice Ryhl wrote:
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index a2c49e5494d334bfde67328464dafcdb31052947..c12753a5fb1c7423a4063553674b537a775c860e 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -137,6 +137,14 @@ pub mod flags {
>  /// - Implementers must ensure that all trait functions abide by the guarantees documented in the
>  ///   `# Guarantees` sections.
>  pub unsafe trait Allocator {
> +    /// The minimum alignment satisfied by all allocations from this allocator.
> +    ///
> +    /// # Guarantees
> +    ///
> +    /// Any pointer allocated by this allocator must be aligned to `MIN_ALIGN` even if the
> +    /// requested layout has a smaller alignment.

I'd say "is guaranteed to be aligned to" instead, "must be" reads like a
requirement.

Speaking of which, I think this also needs to be expressed as a safety
requirement of the Allocator trait itself, which the specific allocator
implementations need to justify.

> +    const MIN_ALIGN: usize;
> +
>      /// Allocate memory based on `layout` and `flags`.
>      ///
>      /// On success, returns a buffer represented as `NonNull<[u8]>` that satisfies the layout
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index aa2dfa9dca4c309e5a9eafc7da6a8a9bd7b54b11..25fc9f9ae3b4e471a08d77130b374bd1397f7384 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -17,6 +17,8 @@
>  use crate::bindings;
>  use crate::pr_warn;
>  
> +const ARCH_KMALLOC_MINALIGN: usize = bindings::ARCH_KMALLOC_MINALIGN as usize;
> +
>  /// The contiguous kernel allocator.
>  ///
>  /// `Kmalloc` is typically used for physically contiguous allocations up to page size, but also
> @@ -128,6 +130,8 @@ unsafe fn call(
>  // - passing a pointer to a valid memory allocation is OK,
>  // - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
>  unsafe impl Allocator for Kmalloc {
> +    const MIN_ALIGN: usize = ARCH_KMALLOC_MINALIGN;
> +
>      #[inline]
>      unsafe fn realloc(
>          ptr: Option<NonNull<u8>>,
> @@ -145,6 +149,8 @@ unsafe fn realloc(
>  // - passing a pointer to a valid memory allocation is OK,
>  // - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
>  unsafe impl Allocator for Vmalloc {
> +    const MIN_ALIGN: usize = kernel::page::PAGE_SIZE;
> +
>      #[inline]
>      unsafe fn realloc(
>          ptr: Option<NonNull<u8>>,
> @@ -169,6 +175,8 @@ unsafe fn realloc(
>  // - passing a pointer to a valid memory allocation is OK,
>  // - `realloc` satisfies the guarantees, since `ReallocFunc::call` has the same.
>  unsafe impl Allocator for KVmalloc {
> +    const MIN_ALIGN: usize = ARCH_KMALLOC_MINALIGN;
> +
>      #[inline]
>      unsafe fn realloc(
>          ptr: Option<NonNull<u8>>,