linux-kernel - Re: [PATCH v2 1/2] rust: alloc: specify the minimum alignment of each allocator

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Message-Id: <DC182DB9HNS9.21ZVBYT6DXHDE@kernel.org>
Date: Wed, 13 Aug 2025 12:19:25 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Lorenzo Stoakes" <lorenzo.stoakes@...cle.com>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, "Andrew Morton" <akpm@...ux-foundation.org>,
 "Matthew Wilcox" <willy@...radead.org>, "Tamir Duberstein"
 <tamird@...il.com>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Trevor
 Gross" <tmgross@...ch.edu>, <linux-mm@...ck.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] rust: alloc: specify the minimum alignment of
 each allocator

On Wed Aug 13, 2025 at 11:32 AM CEST, Alice Ryhl wrote:
> On Wed, Aug 13, 2025 at 11:14 AM Danilo Krummrich <dakr@...nel.org> wrote:
>>
>> On Wed Aug 13, 2025 at 9:52 AM CEST, Alice Ryhl wrote:
>> > On Tue, Aug 12, 2025 at 07:52:35PM +0200, Danilo Krummrich wrote:
>> >> On Mon Aug 11, 2025 at 2:31 PM CEST, Alice Ryhl wrote:
>> >> > 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;
>> >>
>> >> I think this needs the following diff:
>> >>
>> >> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
>> >> index 84d60635e8a9..4ad9add117ea 100644
>> >> --- a/rust/bindings/bindings_helper.h
>> >> +++ b/rust/bindings/bindings_helper.h
>> >> @@ -84,6 +84,7 @@
>> >>
>> >>  /* `bindgen` gets confused at certain things. */
>> >>  const size_t RUST_CONST_HELPER_ARCH_SLAB_MINALIGN = ARCH_SLAB_MINALIGN;
>> >> +const size_t RUST_CONST_HELPER_ARCH_KMALLOC_MINALIGN = ARCH_KMALLOC_MINALIGN;
>> >>  const size_t RUST_CONST_HELPER_PAGE_SIZE = PAGE_SIZE;
>> >>  const gfp_t RUST_CONST_HELPER_GFP_ATOMIC = GFP_ATOMIC;
>> >>  const gfp_t RUST_CONST_HELPER_GFP_KERNEL = GFP_KERNEL;
>> >> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
>> >> index 25fc9f9ae3b4..5003907f0240 100644
>> >> --- a/rust/kernel/alloc/allocator.rs
>> >> +++ b/rust/kernel/alloc/allocator.rs
>> >> @@ -17,7 +17,7 @@
>> >>  use crate::bindings;
>> >>  use crate::pr_warn;
>> >>
>> >> -const ARCH_KMALLOC_MINALIGN: usize = bindings::ARCH_KMALLOC_MINALIGN as usize;
>> >> +const ARCH_KMALLOC_MINALIGN: usize = bindings::ARCH_KMALLOC_MINALIGN;
>> >>
>> >>  /// The contiguous kernel allocator.
>> >>  ///
>> >>
>> >>
>> >> No need to resend I can fix it up when applying the patch.
>> >
>> > Hmm. Maybe that depends on the configuration? The constant was generated
>> > for me. Either way, happy with the suggested change.
>>
>> That is a bit odd, I'd like to understand this before merging.
>>
>> All of the definitions in the kernel are defines that shouldn't be picked up by
>> bindgen.
>
> It is possible for bindgen to pick up a #define in some cases. The
> main case where bindgen fails is when the macro is defined in terms of
> a function-like macro. This is why we see so many failures with _IO*
> macros.

I think I see it now, ARCH_KMALLOC_MINALIGN seems to be either a literal or
__alignof__(unsigned long long), either directly or indirecty through
ARCH_DMA_MINALIGN. bindgen doesn't like the __alignof__() extension.

So, I assume you were on arm64? :)