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

[Andrey Ryabinin]

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@xxxxxx> 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?
> 
> /*
> Â* kmalloc and friends return ARCH_KMALLOC_MINALIGN aligned
> Â* pointers. kmem_cache_alloc and friends return ARCH_SLAB_MINALIGN
> Â* aligned pointers.
> Â*/
> 

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.


> Thanks,
> 
> Bart.