Re: [RFC 4/4] xfs: support dma-buf registered buffers for direct I/O

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 16, 2026 at 02:49:14PM +0530, Anuj Gupta wrote:
> From: Nitesh Shetty <nj.shetty@samsung.com>
> 
> Add the init_dma_buf_io_ctx fop for XFS files.
> XFS picks the device and calls the iomap layer with
> the relevant device.
> 
> Refuse the iterator from every CPU-copy path: DAX, buffered
> fallback from a -ENOTBLK direct write, and read/writes
> without O_DIRECT.
> 
> If a file is switched between the data and realtime devices
> after registration, the block layer cross-device check refuses
> the I/O.
> 
> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
> ---
>  fs/xfs/xfs_file.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 426a67b813a7..f97d5acfaf68 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -340,6 +340,11 @@ xfs_file_read_iter(
>  	if (xfs_is_shutdown(mp))
>  		return -EIO;
>  
> +	/* a device mapping can only be read by a direct I/O bio */
> +	if (iov_iter_is_dmabuf_map(to) &&
> +	    (IS_DAX(inode) || !(iocb->ki_flags & IOCB_DIRECT)))
> +		return -EOPNOTSUPP;

Maybe move this into the actual methods for dax/buffered instead
to avoid extra conditionals?  Same for the write side.

>  
> @@ -1209,6 +1219,9 @@ xfs_file_write_iter(
>  		ret = xfs_file_dio_write(iocb, from);
>  		if (ret != -ENOTBLK)
>  			return ret;
> +		/* the page cache cannot copy from a device mapping */
> +		if (iov_iter_is_dmabuf_map(from))
> +			return -EOPNOTSUPP;

I suspect these random fallbacks are going to cause trouble, but
I'm not really sure what to do about it either.

> +/*
> + * A realtime file's data lives on the realtime device, anything else on the
> + * data device. If a file is switched between them after registration, the
> + * block layer cross-device check refuses the I/O.
> + */
> +static int
> +xfs_file_init_dma_buf_io_ctx(
> +	struct file		*file,
> +	struct dma_buf_io_ctx	*ctx)
> +{
> +	struct xfs_inode	*ip = XFS_I(file_inode(file));
> +
> +	return iomap_file_init_dma_buf_io_ctx(file,
> +			xfs_inode_buftarg(ip)->bt_bdev, ctx);
> +}

Ah, this is where the previous cross check comes from.  What about
simply disallowing toggling the RT flag with outstanding reservations
instead?