Re: [PATCH 0/6] ublk zero-copy support

Pavel Begunkov
2025-02-08T21:40:23Z

On 2/8/25 20:13, Keith Busch wrote:
> On Sat, Feb 08, 2025 at 02:16:15PM +0000, Pavel Begunkov wrote:
>> On 2/8/25 05:44, Ming Lei wrote:
>>> On Fri, Feb 07, 2025 at 07:06:54AM -0700, Keith Busch wrote:
>>>> On Fri, Feb 07, 2025 at 11:51:49AM +0800, Ming Lei wrote:
>>>>> On Mon, Feb 03, 2025 at 07:45:11AM -0800, Keith Busch wrote:
>>>>>>
>>>>>> The previous version from Ming can be viewed here:
>>>>>>
>>>>>> https://lore.kernel.org/linux-block/20241107110149.890530-1-ming.lei@redhat.com/
>>>>>>
>>>>>> Based on the feedback from that thread, the desired io_uring interfaces
>>>>>> needed to be simpler, and the kernel registered resources need to behave
>>>>>> more similiar to user registered buffers.
>>>>>>
>>>>>> This series introduces a new resource node type, KBUF, which, like the
>>>>>> BUFFER resource, needs to be installed into an io_uring buf_node table
>>>>>> in order for the user to access it in a fixed buffer command. The
>>>>>> new io_uring kernel API provides a way for a user to register a struct
>>>>>> request's bvec to a specific index, and a way to unregister it.
>>>>>>
>>>>>> When the ublk server receives notification of a new command, it must
>>>>>> first select an index and register the zero copy buffer. It may use that
>>>>>> index for any number of fixed buffer commands, then it must unregister
>>>>>> the index when it's done. This can all be done in a single io_uring_enter
>>>>>> if desired, or it can be split into multiple enters if needed.
>>>>>
>>>>> I suspect it may not be done in single io_uring_enter() because there
>>>>> is strict dependency among the three OPs(register buffer, read/write,
>>>>> unregister buffer).
>>>>
>>>> The registration is synchronous. io_uring completes the SQE entirely
>>>> before it even looks at the read command in the next SQE.
>>>
>>> Can you explain a bit "synchronous" here?
>>
>> I'd believe synchronous here means "executed during submission from
>> the submit syscall path". And I agree that we can't rely on that.
>> That's an implementation detail and io_uring doesn't promise that,
>
> The commands are processed in order under the ctx's uring_lock. What are
> you thinking you might do to make this happen in any different order?

Bunch of stuff. IOSQE_ASYNC will reorder them. Drain can push it to
a different path with no guarantees what happens there, even when you
only used drain only for some past requests. Or it can get reordered
by racing with draining. Someone floated (not merged) idea before of
hybrid task / sqpoll execution, things like that might be needed at
some point, and that will reorder requests. Or you might want to
offload more aggressively, e.g. to already waiting tasks or the
thread pool.

--
Pavel Begunkov