On Thu, Aug 22, 2019 at 05:14:40PM -0700, Christoph Hellwig wrote: > On Thu, Aug 22, 2019 at 06:20:00PM +0800, Ming Lei wrote: > > In theory, fs bio shouldn't care any DMA limits, which should have been done > > on splitted bio for doing IO to device. > > > > Also .dma_alignment isn't considered in blk_stack_limits(), so in case > > of DM, MD or other stacking drivers, fs code won't know the accurate > > .dma_alignment of underlying queues at all, and the stacking driver's > > queue dma alignment is still 512. > > Trying to handling alignment lower down means bounce buffering, so I > don't think trying to hndle it is a sane idea. I'd be much happier to > say non-passthrough bios need 512 byte alignment, period. That should > cover all the sane cases and we can easily check for it. The occasional > device that would need larger alignment just needs to deal with it. Yeah, I agree we need to avoid bounce buffer, and it is fine to check 512 simply. Also we should consider the interface/protocol between fs and block layer, it could make both sides happy to always align offset & length with logical block size. And that is reasonable for fs bio. Thanks, Ming
