Re: [PATCH] mm/slab: align kmalloc to cacheline when DMA API debugging is active

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

On Fri, Mar 27, 2026 at 1:43 PM Harry Yoo (Oracle) <harry@kernel.org> wrote:
>
> Probably what I said was misleading...
>
> I didn't mean "the architecture is fine with overlapping cacheline".
>
> I meant "not defining ARCH_DMA_MINALIGN or defining it as smaller than
> L1_CACHE_BYTES is how architectures tell kmalloc subsystem that
> kmalloc objects don't have to be aligned with cacheline size."
>

Hi Harry,

You're right, I was wrong about dma_get_cache_alignment() -- on
x86_64 without ARCH_HAS_DMA_MINALIGN it returns 1, not
L1_CACHE_BYTES.  Sorry for the confusion.

So your suggestion to suppress the warning in dma-debug when
dma_get_cache_alignment() < L1_CACHE_BYTES would indeed work
on x86_64 and other coherent platforms.

I don't have a strong preference either way.  Both approaches
solve the problem:

  - slab side: prevents the overlap from happening
  - dma-debug side: tolerates the overlap when the arch says
    cacheline alignment isn't required for DMA

Would you prefer I send a v2 with the dma-debug approach instead?
Happy to go whichever direction the maintainers prefer.

-- 
Best Regards,
Mike Gavrilov.