linux-kernel - Re: [PATCH v3] rust: allocator_test: use `posix

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Message-ID: <Z6UwCtaO-qVYEN2Z@cassiopeiae>
Date: Thu, 6 Feb 2025 22:56:26 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Tamir Duberstein <tamird@...il.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rust: allocator_test: use `posix_memalign`

On Thu, Feb 06, 2025 at 03:49:00PM -0500, Tamir Duberstein wrote:
> The implementation added in commit dd09538fb409 ("rust: alloc: implement
> `Cmalloc` in module allocator_test") used `aligned_malloc` which has
> implementation-defined requirements of its `alignment` parameter.
> 
> The macOS implementation of `aligned_alloc` appears to be based on
> `posix_memalign` and inherits the stricter requirements of that
> function, causing test failures on macOS.
> 
> Replace `aligned_alloc` with `posix_memalign` and comply with its
> requirements. This ensures uniform behavior across systems.
> 
> Fixes: dd09538fb409 ("rust: alloc: implement `Cmalloc` in module allocator_test")
> 
> Signed-off-by: Tamir Duberstein <tamird@...il.com>

Let's wait for clarification before moving on. :)

> ---
> I've intentionally not picked up Danilo's Acked-by from v2 because the
> approach has changed quite a bit.
> ---
> Changes in v3:
> - Replace `aligned_malloc` with `posix_memalign` for portability.
> - Link to v2: https://lore.kernel.org/r/20250202-aligned-alloc-v2-1-5af0b5fdd46f@gmail.com
> 
> Changes in v2:
> - Shorten some variable names. (Danilo Krummrich)
> - Replace shadowing alignment variable with a second call to
>   Layout::align. (Danilo Krummrich)
> - Link to v1: https://lore.kernel.org/r/20250201-aligned-alloc-v1-1-c99a73f3cbd4@gmail.com
> ---
>  rust/kernel/alloc/allocator_test.rs | 27 +++++++++++++++++++++++----
>  1 file changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/rust/kernel/alloc/allocator_test.rs b/rust/kernel/alloc/allocator_test.rs
> index e3240d16040b..0aa68d955b39 100644
> --- a/rust/kernel/alloc/allocator_test.rs
> +++ b/rust/kernel/alloc/allocator_test.rs
> @@ -23,8 +23,19 @@
>  pub type KVmalloc = Kmalloc;
>  
>  extern "C" {
> -    #[link_name = "aligned_alloc"]
> -    fn libc_aligned_alloc(align: usize, size: usize) -> *mut crate::ffi::c_void;
> +    // NB: `posix_memalign` is intentionally used instead of `aligned_malloc`.
> +    //
> +    // ISO C (ISO/IEC 9899:2011) defines `aligned_malloc`:
> +    //
> +    // > The value of alignment shall be a valid alignment supported by the implementation [...].
> +    //
> +    // POSIX.1-2001 (IEEE 1003.1-2001) defines `posix_memalign`:
> +    //
> +    // > The value of alignment shall be a power of two multiple of sizeof (void *).
> +    //
> +    // `posix_memalign` is more portable than (but otherwise identical to) `aligned_malloc`.
> +    #[link_name = "posix_memalign"]
> +    fn libc_posix_memalign(align: usize, size: usize) -> *mut crate::ffi::c_void;

I don't think this can work. posix_memalign() is defined as follows.

int posix_memalign(void **memptr, size_t alignment, size_t size)

Besides that, I don't think switching to posix_memalign() is desirable, it only
seems to make the code more complicated.