Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

[Christoph Hellwig]

On Tue, Jun 03, 2025 at 03:14:20PM +0200, Christian König wrote:
> On 6/3/25 15:00, Christoph Hellwig wrote:
> > This is a really weird interface.  No one has yet to explain why dmabuf
> > is so special that we can't support direct I/O to it when we can support
> > it to otherwise exotic mappings like PCI P2P ones.
> 
> With udmabuf you can do direct I/O, it's just inefficient to walk the
> page tables for it when you already have an array of all the folios.

Does it matter compared to the I/O in this case?

Either way there has been talk (in case of networking implementations)
that use a dmabuf as a first class container for lower level I/O.
I'd much rather do that than adding odd side interfaces.  I.e. have
a version of splice that doesn't bother with the pipe, but instead
just uses in-kernel direct I/O on one side and dmabuf-provided folios
on the other.