Re: [PATCH V3 00/16] io_uring/ublk: add IORING_OP_FUSED_CMD

[Jens Axboe <axboe@kernel.dk>]

On 3/17/23 2:14?AM, Ming Lei wrote:
> On Tue, Mar 14, 2023 at 08:57:11PM +0800, Ming Lei wrote:
>> Hello,
>>
>> Add IORING_OP_FUSED_CMD, it is one special URING_CMD, which has to
>> be SQE128. The 1st SQE(master) is one 64byte URING_CMD, and the 2nd
>> 64byte SQE(slave) is another normal 64byte OP. For any OP which needs
>> to support slave OP, io_issue_defs[op].fused_slave needs to be set as 1,
>> and its ->issue() can retrieve/import buffer from master request's
>> fused_cmd_kbuf. The slave OP is actually submitted from kernel, part of
>> this idea is from Xiaoguang's ublk ebpf patchset, but this patchset
>> submits slave OP just like normal OP issued from userspace, that said,
>> SQE order is kept, and batching handling is done too.
>>
>> Please see detailed design in commit log of the 2th patch, and one big
>> point is how to handle buffer ownership.
>>
>> With this way, it is easy to support zero copy for ublk/fuse device.
>>
>> Basically userspace can specify any sub-buffer of the ublk block request
>> buffer from the fused command just by setting 'offset/len'
>> in the slave SQE for running slave OP. This way is flexible to implement
>> io mapping: mirror, stripped, ...
>>
>> The 3th & 4th patches enable fused slave support for the following OPs:
>>
>> 	OP_READ/OP_WRITE
>> 	OP_SEND/OP_RECV/OP_SEND_ZC
>>
>> The other ublk patches cleans ublk driver and implement fused command
>> for supporting zero copy.
>>
>> Follows userspace code:
>>
>> https://github.com/ming1/ubdsrv/tree/fused-cmd-zc-v2
>>
>> All three(loop, nbd and qcow2) ublk targets have supported zero copy by passing:
>>
>> 	ublk add -t [loop|nbd|qcow2] -z .... 
>>
>> Basic fs mount/kernel building and builtin test are done, and also not
>> observe regression on xfstest test over ublk-loop with zero copy.
>>
>> Also add liburing test case for covering fused command based on miniublk
>> of blktest:
>>
>> https://github.com/ming1/liburing/commits/fused_cmd_miniublk
>>
>> Performance improvement is obvious on memory bandwidth
>> related workloads, such as, 1~2X improvement on 64K/512K BS
>> IO test on loop with ramfs backing file.
>>
>> Any comments are welcome!
>>
>> V3:
>> 	- fix build warning reported by kernel test robot
>> 	- drop patch for checking fused flags on existed drivers with
>> 	  ->uring_command(), which isn't necessary, since we do not do that
>>       when adding new ioctl or uring command
>>     - inline io_init_rq() for core code, so just export io_init_slave_req
>> 	- return result of failed slave request unconditionally since REQ_F_CQE_SKIP
>> 	will be cleared
>> 	- pass xfstest over ublk-loop
> 
> Hello Jens and Guys,
> 
> I have been working on io_uring zero copy support for ublk/fuse for a while, and
> I appreciate you may share any thoughts on this patchset or approach?

I'm a bit split on this one, as I really like (and want) the feature.
ublk has become popular pretty quickly, and it makes a LOT of sense to
support zero copy for it. At the same time, I'm not really a huge fan of
the fused commands... They seem too specialized to be useful for other
things, and it'd be a shame to do something like that only for it later
to be replaced by a generic solution. And then we're stuck with
supporting fused commands forever, not sure I like that prospect.

Both Pavel and Xiaoguang voiced similar concerns, and I think it may be
worth spending a bit more time on figuring out if splice can help us
here. David Howells currently has a lot going on in that area too.

So while I'd love to see this feature get queued up right now, I also
don't want to prematurely do so. Can we split out the fixes from this
series into a separate series that we can queue up now? That would also
help shrink the patchset, which is always a win for review.

-- 
Jens Axboe