Re: [PATCH 0/3] Consistently look up fixed buffers before going async

[Pavel Begunkov]

On 3/21/25 18:48, Caleb Sander Mateos wrote:
> To use ublk zero copy, an application submits a sequence of io_uring
> operations:
> (1) Register a ublk request's buffer into the fixed buffer table
> (2) Use the fixed buffer in some I/O operation
> (3) Unregister the buffer from the fixed buffer table
>
> The ordering of these operations is critical; if the fixed buffer lookup
> occurs before the register or after the unregister operation, the I/O
> will fail with EFAULT or even corrupt a different ublk request's buffer.
> It is possible to guarantee the correct order by linking the operations,
> but that adds overhead and doesn't allow multiple I/O operations to
> execute in parallel using the same ublk request's buffer. Ideally, the
> application could just submit the register, I/O, and unregister SQEs in
> the desired order without links and io_uring would ensure the ordering.
> This mostly works, leveraging the fact that each io_uring SQE is prepped
> and issued non-blocking in order (barring link, drain, and force-async
> flags). But it requires the fixed buffer lookup to occur during the
> initial non-blocking issue.

In other words, leveraging internal details that is not a part
of the uapi, should never be relied upon by the user and is fragile.
Any drain request or IOSQE_ASYNC and it'll break, or for any reason
why it might be desirable to change the behaviour in the future.

Sorry, but no, we absolutely can't have that, it'll be an absolute
nightmare to maintain as basically every request scheduling decision
now becomes a part of the uapi.

There is an api to order requests, if you want to order them you
either have to use that or do it in user space. In your particular
case you can try to opportunistically issue them without ordering
by making sure the reg buffer slot is not reused in the meantime
and handling request failures.

--
Pavel Begunkov