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

[Keith Busch <kbusch@kernel.org>]

On Mon, Mar 31, 2025 at 12:16:58PM +0530, Kanchan Joshi wrote:
> 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?

Not sure I understand the question. A ublk daemon can use anything it
wants on the backend. Are you just referring to the public ublksrv
implementation? That's not used here, if that's what you mean.