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

[Harry Yoo (Oracle)]

On Fri, Mar 27, 2026 at 03:25:00PM +0500, Mikhail Gavrilov wrote:
> On Fri, Mar 27, 2026 at 1:43 PM Harry Yoo (Oracle) <harry@xxxxxxxxxx> 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,

Hi Mikhail,

> 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.

Don't worry!

> 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.

Thanks for confirming.

> 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?

Yes please. I think keeping the same behavior regardless of the debug
option will be better in the long term.

> Happy to go whichever direction the maintainers prefer.

Thanks a lot for working on this!

-- 
Cheers,
Harry / Hyeonggon