Re: [PATCH 2/2] rust: alloc: take the allocator into account for FOREIGN_ALIGN

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

On Tue Jul 15, 2025 at 3:46 PM CEST, Alice Ryhl wrote:
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index bffe72f44cb33a265018e67d92d9f0abe82f8e22..fd3f1e0b9c3b3437fb50d8f1b28c92bc7cefd565 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -400,12 +400,19 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
>  }
>  
>  // SAFETY: The pointer returned by `into_foreign` comes from a well aligned
> -// pointer to `T`.
> +// pointer to `T` allocated by `A`.
>  unsafe impl<T: 'static, A> ForeignOwnable for Box<T, A>
>  where
>      A: Allocator,
>  {
> -    const FOREIGN_ALIGN: usize = core::mem::align_of::<T>();
> +    const FOREIGN_ALIGN: usize = {
> +        let mut align = core::mem::align_of::<T>();
> +        if align < A::MIN_ALIGN {
> +            align = A::MIN_ALIGN;
> +        }
> +        align
> +    };

Pretty unfortunate that core::cmp::max() can't be used from const context. :(

What do you think about

	const FOREIGN_ALIGN: usize =
	    if core::mem::align_of::<T>() < A::MIN_ALIGN {
	        A::MIN_ALIGN
	    } else {
	        core::mem::align_of::<T>()
	    };

instead? I think that reads a bit better.