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
