Re: [PATCH V5 16/16] block: ublk_drv: apply io_uring FUSED_CMD for supporting zero copy

[Ming Lei]

On Wed, Mar 29, 2023 at 06:01:16PM +0800, Ziyang Zhang wrote:
> On 2023/3/29 17:00, Ming Lei wrote:
> > On Wed, Mar 29, 2023 at 10:57:53AM +0800, Ziyang Zhang wrote:
> >> On 2023/3/28 23:09, Ming Lei wrote:
> >>> Apply io_uring fused command for supporting zero copy:
> >>>
> >>
> >> [...]
> >>
> >>>  
> >>> @@ -1374,7 +1533,12 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
> >>>  	if (!ubq || ub_cmd->q_id != ubq->q_id)
> >>>  		goto out;
> >>>  
> >>> -	if (ubq->ubq_daemon && ubq->ubq_daemon != current)
> >>> +	/*
> >>> +	 * The fused command reads the io buffer data structure only, so it
> >>> +	 * is fine to be issued from other context.
> >>> +	 */
> >>> +	if ((ubq->ubq_daemon && ubq->ubq_daemon != current) &&
> >>> +			(cmd_op != UBLK_IO_FUSED_SUBMIT_IO))
> >>>  		goto out;
> >>>  
> >>
> >> Hi Ming,
> >>
> >> What is your use case that fused io_uring cmd is issued from another thread?
> >> I think it is good practice to operate one io_uring instance in one thread
> >> only.
> > 
> > So far we limit io command has to be issued from the queue context,
> > which is still not friendly from userspace viewpoint, the reason is
> > that we can't get io_uring exit notification and ublk's use case is
> > very special since the queued io command may not be completed forever,
> 
> OK, so UBLK_IO_FUSED_SUBMIT_IO is guaranteed to be completed because it is
> not queued. FETCH_REQ and COMMIT_AMD_FETCH are queued io commands and could
> not be completed forever so they have to be issued from ubq_daemon. Right?

Yeah, any io command should be issued from ubq daemon context.

> 
> BTW, maybe NEED_GET_DATA can be issued from other context...

So far it won't be supported.

As I mentioned in the link, if io_uring can provide io_uring exit
callback, we may relax this limit.

> 
> > see:
> > 
> > https://lore.kernel.org/linux-fsdevel/ZBxTdCj60+s1aZqA@xxxxxxxxxxxxxxxxxxxxxxxxx/
> > 
> > I remember that people raised concern about this implementation.
> > 
> > But for normal IO, it could be issued from io wq simply because of
> > link(dependency) or whatever, and userspace is still allowed to submit
> > io from another pthread via same io_uring ctx.
> 
> Yes, we can submit to the same ctx from different pthread but lock may be required.

Right.

> IMO, users may only choose ubq_daemon as the only submitter.

At least any io command should be issued from ubq daemon now, but normal
io can be issued from any context.


Thanks,
Ming