Messages in this thread | | | Subject | Re: block: DMA alignment of IO buffer allocated from slab | From | Bart Van Assche <> | Date | Mon, 24 Sep 2018 07:19:17 -0700 |
| |
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.
| |