Re: [PATCH V6 8/8] ublk: support provide io buffer

[Uday Shankar <ushankar@purestorage.com>]

On Thu, Sep 12, 2024 at 06:49:28PM +0800, Ming Lei wrote:
> +static int ublk_provide_io_buf(struct io_uring_cmd *cmd,
> +		struct ublk_queue *ubq, int tag)
> +{
> +	struct ublk_device *ub = cmd->file->private_data;
> +	struct ublk_rq_data *data;
> +	struct request *req;
> +
> +	if (!ub)
> +		return -EPERM;
> +
> +	req = __ublk_check_and_get_req(ub, ubq, tag, 0);
> +	if (!req)
> +		return -EINVAL;
> +
> +	pr_devel("%s: qid %d tag %u request bytes %u\n",
> +			__func__, tag, ubq->q_id, blk_rq_bytes(req));
> +
> +	data = blk_mq_rq_to_pdu(req);
> +
> +	/*
> +	 * io_uring guarantees that the callback will be called after
> +	 * the provided buffer is consumed, and it is automatic removal
> +	 * before this uring command is freed.
> +	 *
> +	 * This request won't be completed unless the callback is called,
> +	 * so ublk module won't be unloaded too.
> +	 */
> +	return io_uring_cmd_provide_kbuf(cmd, data->buf);
> +}

We did some testing with this patchset and saw some panics due to
grp_kbuf_ack being a garbage value. Turns out that's because we forgot
to set the UBLK_F_SUPPORT_ZERO_COPY flag on the device. But it looks
like the UBLK_IO_PROVIDE_IO_BUF command is still allowed for such
devices. Should this function test that the device has zero copy
configured and fail if it doesn't?