Re: [RFC 3/4] iomap: add dma-buf direct I/O guards and registration helper

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 16, 2026 at 02:49:13PM +0530, Anuj Gupta wrote:
> From: Nitesh Shetty <nj.shetty@samsung.com>
> 
> Direct I/O copies data from the CPU in three places, zero-filling
> holes, inline data, and bounce pages. A dma-buf iterator has no
> CPU-visible pages, so refuse it.
> 
> Reads that encounter a hole or unwritten extent therefore return
> -EOPNOTSUPP for the entire operation, even if I/O for an earlier mapped
> extent was already submitted.
> 
> Also add iomap_file_init_dma_buf_io_ctx(), so filesystems can register
> without open-coding the DAX check. The filesystem picks the device,
> iomap does the rest.
> 
> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> ---
>  fs/iomap/direct-io.c  | 31 ++++++++++++++++++++++++++++++-
>  include/linux/iomap.h |  3 +++
>  2 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 8b4039d16ce8..fc40f4367ee7 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -427,6 +427,10 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>  	unsigned int alignment;
>  	ssize_t ret = 0;
>  
> +	if (iov_iter_is_dmabuf_map(dio->submit.iter) &&
> +	    (iomap->flags & IOMAP_F_INTEGRITY))
> +		return -EOPNOTSUPP;
> +
>  	/*
>  	 * File systems that write out of place and always allocate new blocks
>  	 * need each bio to be block aligned as that's the unit of allocation.
> @@ -595,7 +599,13 @@ static int iomap_dio_bio_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>  
>  static int iomap_dio_hole_iter(struct iomap_iter *iter, struct iomap_dio *dio)
>  {
> -	loff_t length = iov_iter_zero(iomap_length(iter), dio->submit.iter);
> +	loff_t length;
> +
> +	/* a device mapping cannot be zeroed from the CPU */
> +	if (iov_iter_is_dmabuf_map(dio->submit.iter))
> +		return -EOPNOTSUPP;
> +
> +	length = iov_iter_zero(iomap_length(iter), dio->submit.iter);
>  
>  	dio->size += length;
>  	if (!length)
> @@ -612,6 +622,10 @@ static int iomap_dio_inline_iter(struct iomap_iter *iomi, struct iomap_dio *dio)
>  	loff_t pos = iomi->pos;
>  	u64 copied;
>  
> +	/* inline data is copied from the CPU, which a device mapping bypasses */

Overly lone line here.

> +	/* bouncing copies the data from the CPU, which a device mapping bypasses */

.. and here.  Note that could still copy to/from the device memory in
either case, but that's probably rather pointless at least for actual
device dmabufs.  It might be useful for udmabuf or other kernel
allocations, though.