Re: [PATCHv2 0/6] ublk zero-copy support

Pavel Begunkov
2025-02-12T16:05:58Z

On 2/12/25 15:28, Keith Busch wrote:
> On Wed, Feb 12, 2025 at 10:29:32AM +0800, Ming Lei wrote:
>> It is explained in the following links:
>>
>> https://lore.kernel.org/linux-block/b6211101-3f74-4dea-a880-81bb75575dbd@gmail.com/
>>
>> - node kbuffer is registered in ublk uring_cmd's ->issue(), but lookup
>> in RW_FIXED OP's ->prep(), and ->prep() is always called before calling
>> ->issue() when the two are submitted in same io_uring_enter(), so you
>> need to move io_rsrc_node_lookup() & buffer importing from RW_FIXED's ->prep()
>> to ->issue() first.
>
> I don't think that's accurate, at least in practice. In a normal flow,
> we'll have this sequence:
>
> io_submit_sqes
> io_submit_sqe (uring_cmd ublk register)
> io_init_req
> ->prep()
> io_queue_sqe
> ->issue()
> io_submit_sqe (read/write_fixed)
> io_init_req
> ->prep()
> io_queue_sqe
> ->issue()
>
> The first SQE is handled in its entirety before even looking at the
> subsequent SQE. Since the register is first, then the read/write_fixed's
> prep will have a valid index. Testing this patch series appears to show
> this reliably works.

Ming describes how it works for links. This one is indeed how
non links are normally executed. Though I'd repeat it's an
implementation detail and not a part of the uapi. Interestingly,
Keith, you sent some patches changing the ordering here quite a
while ago, just as an example of how it can change.

>> - secondly, ->issue() order is only respected by IO_LINK, and io_uring
>> can't provide such guarantee without using IO_LINK:
>>
>> Pavel explained it in the following link:
>>
>> https://lore.kernel.org/linux-block/68256da6-bb13-4498-a0e0-dce88bb32242@gmail.com/
>>
>> There are also other examples, such as, register buffer stays in one
>> link chain, and the consumer OP isn't in this chain, the consumer OP
>> can still be issued before issuing register_buffer.
>
> Yep, I got that. Linking is just something I was hoping to avoid. I
> understand there are conditions that can break the normal flow I'm
> relying on regarding the ordering. This hasn't appeared to be a problem
> in practice, but I agree this needs to be handled.

--
Pavel Begunkov