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

["Benno Lossin" <lossin@kernel.org>]

On Tue Jul 15, 2025 at 3:46 PM CEST, Alice Ryhl wrote:
> When converting a Box<T> into a void pointer, the allocator might
> guarantee a higher alignment than the type itself does, and in that case
> it is guaranteed that the void pointer has that higher alignment.
>
> This is quite useful when combined with the XArray, which you can only
> create using a ForeignOwnable whose FOREIGN_ALIGN is at least 4. This
> means that you can now always use a Box<T> with the XArray no matter the
> alignment of T.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Hey this is cool!

Reviewed-by: Benno Lossin <lossin@kernel.org>

One question below.

> ---
>  rust/kernel/alloc/kbox.rs | 15 +++++++++++----
>  rust/kernel/sync/arc.rs   |  6 +++---
>  2 files changed, 14 insertions(+), 7 deletions(-)

> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 63a66761d0c7d752e09ce7372bc230661b2f7c6d..74121cf935f364c16799b5c31cc88714dfd6b702 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -373,10 +373,10 @@ pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
>      }
>  }
>  
> -// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
> -// pointer to `ArcInner<T>`.
> +// SAFETY: The pointer returned by `into_foreign` was originally allocated as an
> +// `KBox<ArcInner<T>>`, so that type is what determines the alignment.
>  unsafe impl<T: 'static> ForeignOwnable for Arc<T> {
> -    const FOREIGN_ALIGN: usize = core::mem::align_of::<ArcInner<T>>();
> +    const FOREIGN_ALIGN: usize = <KBox<ArcInner<T>> as ForeignOwnable>::FOREIGN_ALIGN;

Do we at some point also want to give people the option to use vmalloc
for `Arc`?

---
Cheers,
Benno

>  
>      type Borrowed<'a> = ArcBorrow<'a, T>;
>      type BorrowedMut<'a> = Self::Borrowed<'a>;