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 <h...@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.
