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From: Ming Lei <[email protected]>
To: Ziyang Zhang <[email protected]>
Cc: [email protected],
	Miklos Szeredi <[email protected]>,
	Xiaoguang Wang <[email protected]>,
	Bernd Schubert <[email protected]>,
	Pavel Begunkov <[email protected]>,
	[email protected], Stefan Hajnoczi <[email protected]>,
	[email protected], Jens Axboe <[email protected]>,
	Dan Williams <[email protected]>,
	[email protected]
Subject: Re: [PATCH V5 16/16] block: ublk_drv: apply io_uring FUSED_CMD for supporting zero copy
Date: Mon, 3 Apr 2023 17:22:09 +0800	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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


      reply	other threads:[~2023-04-03  9:23 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-03-28 15:09 [PATCH V5 00/16] io_uring/ublk: add IORING_OP_FUSED_CMD Ming Lei
2023-03-28 15:09 ` [PATCH V5 01/16] io_uring: increase io_kiocb->flags into 64bit Ming Lei
2023-03-28 15:09 ` [PATCH V5 02/16] io_uring: add IORING_OP_FUSED_CMD Ming Lei
2023-03-28 17:33   ` kernel test robot
2023-03-28 15:09 ` [PATCH V5 03/16] io_uring: support normal SQE for fused command Ming Lei
2023-03-28 15:09 ` [PATCH V5 04/16] io_uring: support OP_READ/OP_WRITE for fused secondary request Ming Lei
2023-03-28 15:09 ` [PATCH V5 05/16] io_uring: support OP_SEND_ZC/OP_RECV " Ming Lei
2023-03-28 15:09 ` [PATCH V5 06/16] block: ublk_drv: add common exit handling Ming Lei
2023-03-28 15:09 ` [PATCH V5 07/16] block: ublk_drv: don't consider flush request in map/unmap io Ming Lei
2023-03-28 15:09 ` [PATCH V5 08/16] block: ublk_drv: add two helpers to clean up map/unmap request Ming Lei
2023-03-28 15:09 ` [PATCH V5 09/16] block: ublk_drv: clean up several helpers Ming Lei
2023-03-28 15:09 ` [PATCH V5 10/16] block: ublk_drv: cleanup 'struct ublk_map_data' Ming Lei
2023-03-28 15:09 ` [PATCH V5 11/16] block: ublk_drv: cleanup ublk_copy_user_pages Ming Lei
2023-03-28 15:09 ` [PATCH V5 12/16] block: ublk_drv: grab request reference when the request is handled by userspace Ming Lei
2023-03-28 15:09 ` [PATCH V5 13/16] block: ublk_drv: support to copy any part of request pages Ming Lei
2023-03-28 15:09 ` [PATCH V5 14/16] block: ublk_drv: add read()/write() support for ublk char device Ming Lei
2023-03-28 15:09 ` [PATCH V5 15/16] block: ublk_drv: don't check buffer in case of zero copy Ming Lei
2023-03-28 15:09 ` [PATCH V5 16/16] block: ublk_drv: apply io_uring FUSED_CMD for supporting " Ming Lei
2023-03-29  2:57   ` Ziyang Zhang
2023-03-29  9:00     ` Ming Lei
2023-03-29 10:01       ` Ziyang Zhang
2023-03-29 10:52         ` Ming Lei
2023-04-03  8:38           ` Ziyang Zhang
2023-04-03  9:22             ` Ming Lei [this message]

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