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

["Harry Yoo (Oracle)" <harry@kernel.org>]

On Fri, Mar 27, 2026 at 01:07:21PM +0500, Mikhail Gavrilov wrote:
> On Fri, Mar 27, 2026 at 1:00 PM Harry Yoo (Oracle) <harry@kernel.org> wrote:
> >
> > But dma_get_cache_alignment() < L1_CACHE_BYTES means the architecture
> > actually allows overlapping cachelines, no?
> 
> Hi Harry,
> 
> On x86_64, dma_get_cache_alignment() returns L1_CACHE_BYTES (both
> are 64). The condition (dma_get_cache_alignment() < L1_CACHE_BYTES)
> would be false, so the check wouldn't suppress the warning.

How does dma_get_cache_alignment() return L1_CACHE_BYTES when
x86_64 doesn't define ARCH_HAS_DMA_MINALIGN?

> The problem isn't that the architecture allows overlapping --

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."

> it's that kmalloc returns 8-byte aligned buffers that happen to land in
> the same 64-byte cacheline.
> The DMA debug code correctly identifies that two DMA mappings share
> a cacheline, but on coherent platforms this is harmless.

That happens only when the architecture can live with that.

> Adding a dev_is_dma_coherent() check in dma-debug would fix x86
> but would also silence the warning for any coherent device, including
> ones behind IOMMUs that might have non-coherent paths.

Sorry, I don't understand where the idea of adding a
dma_is_dma_coherent() check comes from ...

> That's why Alan's conclusion was that fixing the allocator side is safer --
> it doesn't weaken any debug checks, it just ensures the situation
> never arises.

-- 
Cheers,
Harry / Hyeonggon