Re: [PATCH v5 1/2] rust: allocator: add KUnit tests for alignment guarantees

["Danilo Krummrich" <dakr@kernel.org>]

On Fri Jul 25, 2025 at 9:02 AM CEST, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@kylinos.cn>
>
> Add comprehensive tests to verify correct alignment handling in Rust
> allocator wrappers. The tests validate:
>
> That kmalloc respects both standard (128-byte) and page-size
> (8192-byte) alignments when allocating structs with explicit alignment
> attributes.
>
> That vmalloc correctly handles standard alignments but intentionally
> rejects allocations requiring alignments larger than its capabilities.
>
> That kvmalloc mirrors vmalloc's constraints, accepting standard
> alignments but rejecting excessive alignment requirements.
>
> The test infrastructure uses specialized aligned structs (Blob and
> LargeAlignBlob) and a test harness (TestAlign) to validate pointer
> alignment through different allocation paths. This ensures our Rust
> allocators correctly propagate kernel allocation constraints.
>
> Co-developed-by: Geliang Tang <geliang@kernel.org>
> Signed-off-by: Geliang Tang <geliang@kernel.org>
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>

Thanks, this looks good. I think it would be good to rebase onto [1], since it
will likely land in the same cycle. Additionally, two nits below.

As a follow-up we could also test alignment in the context of
Allocator::realloc(), i.e. when growing and shrinking buffers or requesting a
different NUMA node.

[1] https://lore.kernel.org/lkml/20250715135645.2230065-1-vitaly.wool@konsulko.se/

> ---
>  rust/kernel/alloc/allocator.rs | 58 ++++++++++++++++++++++++++++++++++
>  1 file changed, 58 insertions(+)
>
> diff --git a/rust/kernel/alloc/allocator.rs b/rust/kernel/alloc/allocator.rs
> index aa2dfa9dca4c..bcc916240f11 100644
> --- a/rust/kernel/alloc/allocator.rs
> +++ b/rust/kernel/alloc/allocator.rs
> @@ -187,3 +187,61 @@ unsafe fn realloc(
>          unsafe { ReallocFunc::KVREALLOC.call(ptr, layout, old_layout, flags) }
>      }
>  }
> +
> +#[macros::kunit_tests(rust_allocator_kunit)]
> +mod tests {
> +    use super::*;
> +    use core::mem::MaybeUninit;
> +    use kernel::prelude::*;
> +

--8<--

> +    const TEST_SIZE: usize = 1024;
> +    const TEST_LARGE_ALIGN_SIZE: usize = kernel::page::PAGE_SIZE * 4;
> +
> +    // These two structs are used to test allocating aligned memory.
> +    // they don't need to be accessed, so they're marked as dead_code.
> +    #[allow(dead_code)]

This should be #[expect(dead_code)].

> +    #[repr(align(128))]
> +    struct Blob([u8; TEST_SIZE]);
> +    #[allow(dead_code)]
> +    #[repr(align(8192))]
> +    struct LargeAlignBlob([u8; TEST_LARGE_ALIGN_SIZE]);
> +
> +    struct TestAlign<T, A: Allocator>(Box<MaybeUninit<T>, A>);
> +    impl<T, A: Allocator> TestAlign<T, A> {
> +        fn new() -> Result<Self> {
> +            Ok(Self(Box::<_, A>::new_uninit(GFP_KERNEL)?))
> +        }
> +
> +        fn alignment_valid(&self, align: usize) -> bool {
> +            assert!(align.is_power_of_two());
> +
> +            let addr = self.0.as_ptr() as usize;
> +            if addr & (align - 1) != 0 {
> +                false
> +            } else {
> +                true
> +            }

This can just be

	addr & (align - 1) == 0

instead of the conditional clause.

> +        }
> +    }

We could move all the above into test_alignment() given that it's likely only
needed from there.

> +
> +    #[test]
> +    fn test_alignment() -> Result<()> {
> +        let ta = TestAlign::<Blob, Kmalloc>::new()?;
> +        assert!(ta.alignment_valid(128));
> +
> +        let ta = TestAlign::<LargeAlignBlob, Kmalloc>::new()?;
> +        assert!(ta.alignment_valid(8192));
> +
> +        let ta = TestAlign::<Blob, Vmalloc>::new()?;
> +        assert!(ta.alignment_valid(128));
> +
> +        assert!(TestAlign::<LargeAlignBlob, Vmalloc>::new().is_err());
> +
> +        let ta = TestAlign::<Blob, KVmalloc>::new()?;
> +        assert!(ta.alignment_valid(128));
> +
> +        assert!(TestAlign::<LargeAlignBlob, KVmalloc>::new().is_err());
> +
> +        Ok(())
> +    }
> +}
> -- 
> 2.43.0