On Thu, 2018-10-18 at 07:03 -0700, Matthew Wilcox wrote: +AD4 On Thu, Oct 18, 2018 at 09:18:12PM +-0800, Ming Lei wrote: +AD4 +AD4 Filesystems may allocate io buffer from slab, and use this buffer to +AD4 +AD4 submit bio. This way may break storage drivers if they have special +AD4 +AD4 requirement on DMA alignment. +AD4 +AD4 Before we go down this road, could we have a discussion about what +AD4 hardware actually requires this? Storage has this weird assumption that +AD4 I/Os must be at least 512 byte aligned in memory, and I don't know where +AD4 this idea comes from. Network devices can do arbitrary byte alignment. +AD4 Even USB controllers can do arbitrary byte alignment. Sure, performance +AD4 is going to suck and there are definite risks on some architectures +AD4 with doing IOs that are sub-cacheline aligned, but why is storage such a +AD4 special snowflake that we assume that host controllers are only capable +AD4 of doing 512-byte aligned DMAs? +AD4 +AD4 I just dragged out the NCR53c810 data sheet from 1993, and it's capable of +AD4 doing arbitrary alignment of DMA. NVMe is capable of 4-byte aligned DMA. +AD4 What hardware needs this 512 byte alignment? How about starting with modifying the queue+AF8-dma+AF8-alignment() function? The current implementation of that function is as follows: static inline int queue+AF8-dma+AF8-alignment(struct request+AF8-queue +ACo-q) +AHs return q ? q-+AD4-dma+AF8-alignment : 511+ADs +AH0 In other words, for block drivers that do not set the DMA alignment explicitly it is assumed that these drivers need 512 byte alignment. I think the +ACI-512 byte alignment as default+ACI was introduced in 2002. From Thomas Gleixner's history tree, commit ad519c6902fb: +-static inline int queue+AF8-dma+AF8-alignment(request+AF8-queue+AF8-t +ACo-q) +-+AHs +- int retval +AD0 511+ADs +- +- if (q +ACYAJg q-+AD4-dma+AF8-alignment) +- retval +AD0 q-+AD4-dma+AF8-alignment+ADs +- +- return retval+ADs +-+AH0 +- +-static inline int bdev+AF8-dma+AF8-aligment(struct block+AF8-device +ACo-bdev) +-+AHs +- return queue+AF8-dma+AF8-alignment(bdev+AF8-get+AF8-queue(bdev))+ADs +-+AH0 +- Bart.
