linux-kernel - Re: [PATCH 2/2] rust: alloc: take the allocator into account for FOREIGN

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Message-Id: <DBCR5IFQFMUU.23UNP95G4NKWA@kernel.org>
Date: Tue, 15 Jul 2025 18:00:14 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>, "Lorenzo Stoakes"
 <lorenzo.stoakes@...cle.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 "Andrew Morton" <akpm@...ux-foundation.org>, "Danilo Krummrich"
 <dakr@...nel.org>, "Matthew Wilcox" <willy@...radead.org>, "Tamir
 Duberstein" <tamird@...il.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Miguel Ojeda" <ojeda@...nel.org>
Cc: "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Trevor Gross"
 <tmgross@...ch.edu>, <linux-mm@...ck.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] rust: alloc: take the allocator into account for
 FOREIGN_ALIGN

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@...gle.com>

Hey this is cool!

Reviewed-by: Benno Lossin <lossin@...nel.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>;