Re: [PATCH v3 0/1] mm: introduce put_user_page*(), placeholder versions

[Christoph Hellwig]

On Wed, Mar 13, 2019 at 09:11:13AM +1100, Dave Chinner wrote:
> On Tue, Mar 12, 2019 at 03:39:33AM -0700, Ira Weiny wrote:
> > IMHO I don't think that the copy_file_range() is going to carry us through the
> > next wave of user performance requirements.  RDMA, while the first, is not the
> > only technology which is looking to have direct access to files.  XDP is
> > another.[1]
> 
> Sure, all I doing here was demonstrating that people have been
> trying to get local direct access to file mappings to DMA directly
> into them for a long time. Direct Io games like these are now
> largely unnecessary because we now have much better APIs to do
> zero-copy data transfer between files (which can do hardware offload
> if it is available!).

And that is just the file to file case.  There are tons of other
users of get_user_pages, including various drivers that do large
amounts of I/O like video capture.  For them it makes tons of sense
to transfer directly to/from a mmap()ed file.