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SubjectRe: block: DMA alignment of IO buffer allocated from slab
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On Mon, 2018-09-24 at 17:43 +0300, Andrey Ryabinin wrote:
>
> On 09/24/2018 05:19 PM, Bart Van Assche wrote:
> > On 9/24/18 2:46 AM, Andrey Ryabinin wrote:
> > > On 09/24/2018 01:42 AM, Ming Lei wrote:
> > > > On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
> > > > > Christoph Hellwig <hch@lst.de> writes:
> > > > >
> > > > > > On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote:
> > > > > > > 1) does kmalloc-N slab guarantee to return N-byte aligned buffer? If
> > > > > > > yes, is it a stable rule?
> > > > > >
> > > > > > This is the assumption in a lot of the kernel, so I think if somethings
> > > > > > breaks this we are in a lot of pain.
> > >
> > > This assumption is not correct. And it's not correct at least from the beginning of the
> > > git era, which is even before SLUB allocator appeared. With CONFIG_DEBUG_SLAB=y
> > > the same as with CONFIG_SLUB_DEBUG_ON=y kmalloc return 'unaligned' objects.
> > > The guaranteed arch-and-config-independent alignment of kmalloc() result is "sizeof(void*)".
>
> Correction sizeof(unsigned long long), so 8-byte alignment guarantee.
>
> > >
> > > If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache.
> >
> > Hello Andrey,
> >
> > The above confuses me. Can you explain to me why the following comment is present in include/linux/slab.h?
> >
> > /*
> > * kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> > * pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> > * aligned pointers.
> > */
> >
>
> ARCH_KMALLOC_MINALIGN - guaranteed alignment of the kmalloc() result.
> ARCH_SLAB_MINALIGN - guaranteed alignment of kmem_cache_alloc() result.
>
> If the 'align' argument passed into kmem_cache_create() is bigger than ARCH_SLAB_MINALIGN
> than kmem_cache_alloc() from that cache should return 'align'-aligned pointers.

Hello Andrey,

Do you realize that that comment from <linux/slab.h> contradicts what you
wrote about kmalloc() if ARCH_KMALLOC_MINALIGN > sizeof(unsigned long long)?

Additionally, shouldn't CONFIG_DEBUG_SLAB=y and CONFIG_SLUB_DEBUG_ON=y
provide the same guarantees as with debugging disabled, namely that kmalloc()
buffers are aligned on ARCH_KMALLOC_MINALIGN boundaries? Since buffers
allocated with kmalloc() are often used for DMA, how otherwise is DMA assumed
to work?

Thanks,

Bart.

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