On Sat, Nov 05, 2022 at 12:37:21AM +0100, Bernd Schubert wrote: > > > On 11/4/22 01:44, Ming Lei wrote: > > On Thu, Nov 03, 2022 at 11:28:29PM +0100, Bernd Schubert wrote: > > > > > > > > > On 11/3/22 09:50, Ming Lei wrote: > > > > Pass ublk block IO request pages to kernel backend IO handling code via > > > > pipe, and request page copy can be avoided. So far, the existed > > > > pipe/splice mechanism works for handling write request only. > > > > > > > > The initial idea of using splice for zero copy is from Miklos and Stefan. > > > > > > > > Read request's zero copy requires pipe's change to allow one read end to > > > > produce buffers for another read end to consume. The added SPLICE_F_READ_TO_READ > > > > flag is for supporting this feature. > > > > > > > > READ is handled by sending IORING_OP_SPLICE with SPLICE_F_DIRECT | > > > > SPLICE_F_READ_TO_READ. WRITE is handled by sending IORING_OP_SPLICE with > > > > SPLICE_F_DIRECT. Kernel internal pipe is used for simplifying userspace, > > > > meantime potential info leak could be avoided. > > > > > > > > > Sorry to ask, do you have an ublk branch that gives an example how to use > > > this? > > > > Follows the ublk splice-zc branch: > > > > https://github.com/ming1/ubdsrv/commits/splice-zc > > > > which is mentioned in cover letter, but I guess it should be added to > > here too, sorry for that, so far only ublk-loop supports it by: > > > > ublk add -t loop -f $BACKING -z > > > > without '-z', ublk-loop is created with zero copy disabled. > > Ah, thanks a lot! And sorry, I had missed this part in the cover letter. > > I will take a look on your new zero copy code on Monday. > > > > > > > > > > I still have several things to fix in my branches, but I got basic fuse > > > uring with copies working. Adding back splice would be next after posting > > > rfc patches. My initial assumption was that I needed to duplicate everything > > > splice does into the fuse .uring_cmd handler - obviously there is a better > > > way with your patches. > > > > > > This week I have a few days off, by end of next week or the week after I > > > might have patches in an rfc state (one thing I'm going to ask about is how > > > do I know what is the next CQE in the kernel handler - ublk does this with > > > tags through mq, but I don't understand yet where the tag is increased and > > > what the relation between tag and right CQE order is). > > > > tag is one attribute of io request, which is originated from ublk > > driver, and it is unique for each request among one queue. So ublksrv > > won't change it at all, just use it, and ublk driver guarantees that > > it is unique. > > > > In ublkserv implementation, the tag info is set in cqe->user_data, so > > we can retrieve the io request via tag part of cqe->user_data. > > Yeah, this is the easy part I understood. At least I hope so :) > > > > > Also I may not understand your question of 'the relation between tag and right > > CQE order', io_uring provides IOSQE_IO_DRAIN/IOSQE_IO_LINK for ordering > > SQE, and ublksrv only applies IOSQE_IO_LINK in ublk-qcow2, so care to > > explain it in a bit details about the "the relation between tag and right > > CQE order"? > > > For fuse (kernel) a vfs request comes in and I need to choose a command in > the ring queue. Right now this is just an atomic counter % queue_size > > fuse_request_alloc_ring() > req_cnt = atomic_inc_return(&queue->req_cnt); > tag = req_cnt & (fc->ring.queue_depth - 1); /* cnt % queue_depth */ > > ring_req = &queue->ring_req[tag]; > > > > I might be wrong, but I think that can be compared a bit to ublk_queue_rq(). > Looks like ublk_queue_rq gets called in blk-mq context and blk-mq seems to > provide rq->tag, which then determines the command in the ring queue - > completion of commands is done in tag-order provided by blk-mq? The part I The two are not related, blk-mq tag number means nothing wrt. io handling order: - tag is allocated via sbitmap, which may return tag number in any order, you may think the returned number is just random - blk-mq may re-order requests and dispatch them with any order - once requests are issued to io_uring, userspace may handles these IOs with any order - after backend io is queued via io_uring or libaio or whatever to kernel, it could be completed at any order > didn't figure out yet is where the tag value gets set. > Also interesting is that there is no handler if the ring is already full - > like the ublk_io command is currently busy in ublksrv (user space). Handled > auto-magically with blk-mq? For ublk, the queue has fixed depth, so the pre-allocated io_uring size is enough, and blk-mq can throttle IOs from the beginning if the max queue depth is reached, so ublk needn't to worry about io_uring size/depth. But fuse may have to consider request throttle. Thanks, Ming
