Re: block: DMA alignment of IO buffer allocated from slab

[Ming Lei]

On Mon, Sep 24, 2018 at 03:17:16PM +0000, Christopher Lameter wrote:
> On Mon, 24 Sep 2018, Bart Van Assche wrote:
> 
> > /*
> >  * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> >  * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> >  * aligned pointers.
> >  */
> 
> kmalloc alignment is only guaranteed to ARCH_KMALLOC_MINALIGN. That power
> of 2 byte caches (without certain options) are aligned to the power of 2
> is due to the nature that these objects are stored in SLUB. Other
> allocators may behave different and actually different debug options
> result in different alignments. You cannot rely on that.
> 
> ARCH_KMALLOC minalign shows the mininum alignment guarantees. If that is
> not sufficient and you do not want to change the arch guarantees then you
> can open you own slab cache with kmem_cache_create() where you can specify
> different alignment requirements.

Christopher, thank you for clarifying the point!

Then looks it should be reasonable for XFS to switch to kmem_cache_create()
for addressing this issue.

Thanks,
Ming