Re: [RFC PATCH 4/4] ublk_drv: support splice based read/write zero copy

[Bernd Schubert <bernd.schubert@fastmail.fm>]

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 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?
This is one of the parts not handled in my fuse code yet and my current 
plan is to have a request queue on top of the (per core) ring queues. 
Similar to the existing fuse request queue, just not one, but per ring 
queue and processed by the ring queue. Unless there is a better way - 
which is another reason to understand how ublk handles this.


Thanks,
Bernd