Re: [RFC 00/12] io_uring dmabuf read/write support

On 03.07.25 16:23, Christoph Hellwig wrote:

[Note: it would be really useful to Cc all relevant maintainers]

On Fri, Jun 27, 2025 at 04:10:27PM +0100, Pavel Begunkov wrote:

quoted

This series implements it for read/write io_uring requests. The uAPI
looks similar to normal registered buffers, the user will need to
register a dmabuf in io_uring first and then use it as any other
registered buffer. On registration the user also specifies a file
to map the dmabuf for.

Just commenting from the in-kernel POV here, where the interface
feels wrong.

You can't just expose 'the DMA device' up file operations, because
there can be and often is more than one.  Similarly stuffing a
dma_addr_t into an iovec is rather dangerous.

The model that should work much better is to have file operations
to attach to / detach from a dma_buf, and then have an iter that
specifies a dmabuf and offsets into.  That way the code behind the
file operations can forward the attachment to all the needed
devices (including more/less while it remains attached to the file)
and can pick the right dma address for each device.

I also remember some discussion that new dma-buf importers should
use the dynamic imported model for long-term imports, but as I'm
everything but an expert in that area I'll let the dma-buf folks
speak.

Completely correct.

As long as you don't have a really good explanation and some mechanism to prevent abuse long term pinning of DMA-bufs should be avoided.

Regards,
Christian.