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

Ziyang Zhang
2023-03-29T10:01:33Z

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?

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

> see:
>
> https://lore.kernel.org/linux-fsdevel/ZBxTdCj60+s1aZqA@ovpn-8-16.pek2.redhat.com/
>
> 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.
IMO, users may only choose ubq_daemon as the only submitter.

Regards,
Zhang