Re: [PATCH] nvme/io_uring: enable dma-buf backed passthrough I/O

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 16, 2026 at 04:04:20PM +0530, Anuj Gupta wrote:
> From: Anuj Gupta <anuj20.g@samsung.com>
> 
> Allow registering a dma-buf against an NVMe namespace character device
> and using it as the data buffer of fixed uring_cmd passthrough I/O.
> 
> io_uring_cmd_import_fixed() now accepts dma-buf buffers and attaches
> the current map. blk_rq_map_user_iov() maps dma-buf iterators directly,
> since they cannot fall back to a bounce copy.

This really needs to be split into blk, io_uring and nvme parts, with
a good commit log for each part.

> +/* Prepare a bio for passthrough I/O from a directly mapped iterator. */
>  static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter)

And adding a comment to an otherwise unrelated function probably also
wants a comment..

> +	if (iov_iter_is_dmabuf_map(iter)) {
> +		if (map_data || (iov_iter_alignment(iter) & align))
> +			return -EINVAL;
> +		ret = blk_rq_map_user_bvec(rq, iter);
> +		return ret == -EREMOTEIO ? -EINVAL : ret;

The error value remapping could probably also use a comment (and be
turned into an if to stand out).

> +static int nvme_ns_chr_init_dma_buf_io_ctx(struct file *file,
> +					   struct dma_buf_io_ctx *ctx)
> +{
> +	struct nvme_ns *ns = container_of(file_inode(file)->i_cdev,
> +					  struct nvme_ns, cdev);
	struct nvme_ns *ns =

>  }
>  
> +void io_uring_cmd_detach_dmabuf(struct io_uring_cmd *ioucmd)
> +{
> +	io_detach_dmabuf_map(cmd_to_io_kiocb(ioucmd));
> +}
> +EXPORT_SYMBOL_GPL(io_uring_cmd_detach_dmabuf);

Add a comment on when this should be used?

>  		return -EINVAL;
>  
> -	return io_import_reg_buf(req, iter, ubuf, len, rw, issue_flags);
> +	ret = __io_import_reg_buf(req, iter, ubuf, len, rw, issue_flags,
> +				  IO_REGBUF_IMPORT_ALLOW_DMABUF);
> +	if (ret)
> +		return ret;
> +	return io_attach_dmabuf_map(req, iter, issue_flags);

No new here, but maybe io_import_reg_buf should take the flags argument
instead of the spurious __variant given how few users there are?