Re: [PATCH V6 00/17] io_uring/ublk: add generic IORING_OP_FUSED_CMD

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

On Tue, Apr 18, 2023 at 07:38:03PM +0000, Bernd Schubert wrote:
> On 3/30/23 13:36, Ming Lei wrote:
> [...]
> > V6:
> > 	- re-design fused command, and make it more generic, moving sharing buffer
> > 	as one plugin of fused command, so in future we can implement more plugins
> > 	- document potential other use cases of fused command
> > 	- drop support for builtin secondary sqe in SQE128, so all secondary
> > 	  requests has standalone SQE
> > 	- make fused command as one feature
> > 	- cleanup & improve naming
> 
> Hi Ming, et al.,
> 
> I started to wonder if fused SQE could be extended to combine multiple 
> syscalls, for example open/read/close.  Which would be another solution 
> for the readfile syscall Miklos had proposed some time ago.
> 
> https://lore.kernel.org/lkml/CAJfpegusi8BjWFzEi05926d4RsEQvPnRW-w7My=ibBHQ8NgCuw@mail.gmail.com/
> 
> If fused SQEs could be extended, I think it would be quite helpful for 
> many other patterns. Another similar examples would open/write/close, 
> but ideal would be also to allow to have it more complex like 
> "open/write/sync_file_range/close" - open/write/close might be the 
> fastest and could possibly return before sync_file_range. Use case for 
> the latter would be a file server that wants to give notifications to 
> client when pages have been written out.

The above pattern needn't fused command, and it can be done by plain
SQEs chain, follows the usage:

1) suppose you get one command from /dev/fuse, then FUSE daemon
needs to handle the command as open/write/sync/close
2) get sqe1, prepare it for open syscall, mark it as IOSQE_IO_LINK;
3) get sqe2, prepare it for write syscall, mark it as IOSQE_IO_LINK;
4) get sqe3, prepare it for sync file range syscall, mark it as IOSQE_IO_LINK;
5) get sqe4, prepare it for close syscall
6) io_uring_enter();	//for submit and get events

Then all the four OPs are done one by one by io_uring internal
machinery, and you can choose to get successful CQE for each OP.

Is the above what you want to do?

The fused command proposal is actually for zero copy(but not limited to zc).

If the above write OP need to write to file with in-kernel buffer
of /dev/fuse directly, you can get one sqe0 and prepare it for primary command
before 1), and set sqe2->addr to offet of the buffer in 3).

However, fused command is usually used in the following way, such as FUSE daemon
gets one READ request from /dev/fuse, FUSE userspace can handle the READ request
as io_uring fused command:

1) get sqe0 and prepare it for primary command, in which you need to
provide info for retrieving kernel buffer/pages of this READ request

2) suppose this READ request needs to be handled by translating it to
READs to two files/devices, considering it as one mirror:

- get sqe1, prepare it for read from file1, and set sqe->addr to offset
  of the buffer in 1), set sqe->len as length for read; this READ OP
  uses the kernel buffer in 1) directly 

- get sqe2, prepare it for read from file2, and set sqe->addr to offset
  of buffer in 1), set sqe->len as length for read;  this READ OP
  uses the kernel buffer in 1) directly 

3) submit the three sqe by io_uring_enter()

sqe1 and sqe2 can be submitted concurrently or be issued one by one
in order, fused command supports both, and depends on user requirement.
But io_uring linked OPs is usually slower.

Also file1/file2 needs to be opened beforehand in this example, and FD is
passed to sqe1/sqe2, another choice is to use fixed File; Also you can
add the open/close() OPs into above steps, which need these open/close/READ
to be linked in order, usually slower tnan non-linked OPs.


Thanks, 
Ming