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

[Ming Lei]

On Fri, Mar 21, 2025 at 08:24:43PM +0000, Pavel Begunkov wrote:
> 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.

I agree, the order should be provided from UAPI/syscall level.

SQE group does address this order issue, and now it can work with
fixed buffer registering OP together.

If no one objects, I will post out the patch for review.


Thanks, 
Ming