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? > > /* > * 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. > Thanks, > > Bart.
