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*)". If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache. > > Even some of buffer address is _not_ L1 cache size aligned, this way is > totally broken wrt. DMA to/from this buffer. > > So this issue has to be fixed in slab debug side. > Well, this definitely would increase memory consumption. Many (probably most) of the kmalloc() users doesn't need such alignment, why should they pay the cost?
