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

[Ming Lei]

On Mon, Apr 03, 2023 at 04:38:30PM +0800, Ziyang Zhang wrote:
> On 2023/3/29 18:52, Ming Lei wrote:
> > 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.
> > 
> 
> Hi, Ming
> 
> Sorry, I do not understand... I think UBLK_IO_NEED_GET_DATA is normal IO just like
> UBLK_IO_FUSED_SUBMIT_IO. It is issued from one pthread(ubq_daemon for now) and
> is completed just in time(not queued). So I think we can allow UBLK_IO_NEED_GET_DATA
> to be issued from other context.
 
No, it isn't.

UBLK_IO_FUSED_SUBMIT_IO is actually for handling target IO, and this
command just reads/provides IO buffer meta to io_uring in read-only
approach, and io buffer meta won't be changed, and any io state won't
be changed, so it is fine to call concurrently.

UBLK_IO_NEED_GET_DATA is still part of io commands, in which io->addr
needs to be set, and io->flags is touched, and it can't be done safely
concurrently.

Also after zero-copy is supported, UBLK_IO_NEED_GET_DATA may become
legacy code path, because ublk server can read/write io data directly
in userspace via read()/write(), and there isn't buffer allocation issue
any more.


Thanks,
Ming