lkml.org 
[lkml]   [2018]   [Sep]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: block: DMA alignment of IO buffer allocated from slab
From
Date
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*)".
>
> 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.
*/

Thanks,

Bart.

\
 
 \ /
  Last update: 2018-09-24 16:22    [W:0.098 / U:0.156 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site