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

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

(Cc: Andrew)

On Fri Jul 25, 2025 at 11:50 AM CEST, Danilo Krummrich wrote:
> 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.

Please also Cc: Andrew for subsequent submissions, since this will, due to the
interaction with [1] likely go through his tree.

> 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