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

[Ming Lei <ming.lei@redhat.com>]

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?


Thanks,
Ming