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

[Ziyang Zhang <ZiyangZhang@linux.alibaba.com>]

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.