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

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Message-ID: <CAH5fLgjYNy3R79nZzPOttk1fx81T+_Q+h8Ug40aVz44TNaCVgg@mail.gmail.com>
Date: Wed, 13 Aug 2025 12:43:02 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
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 12:19 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> 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;
> >> >
> >> > 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.

That sounds right. So yeah, please go ahead and pick it up with the bindgen fix.

> So, I assume you were on arm64? :)

I did in fact build for arm64, yes.

Alice