[PATCH v4 3/3] nvme/ioctl: move fixed buffer lookup to nvme_uring_cmd_io()

[Kanchan Joshi]

On 3/28/2025 9:16 PM, Caleb Sander Mateos wrote:
> For NVMe passthru operations with fixed buffers, the fixed buffer lookup
> happens in io_uring_cmd_import_fixed(). But nvme_uring_cmd_io() can
> return -EAGAIN first from nvme_alloc_user_request() if all tags in the
> tag set are in use. This ordering difference is observable when using
> UBLK_U_IO_{,UN}REGISTER_IO_BUF SQEs to modify the fixed buffer table. If
> the NVMe passthru operation is followed by UBLK_U_IO_UNREGISTER_IO_BUF
> to unregister the fixed buffer and the NVMe passthru goes async, the
> fixed buffer lookup will fail because it happens after the unregister.

while the patch looks fine, I wonder what setup is required to 
trigger/test this. Given that io_uring NVMe passthru is on the char 
device node, and ublk does not take char device as the backing file. 
Care to explain?