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

[Jan Kara <jack@suse.cz>]

On Wed 13-03-19 19:21:37, Christopher Lameter wrote:
> On Wed, 13 Mar 2019, Christoph Hellwig wrote:
> 
> > 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.
> 
> That is very similar to the RDMA case and DAX etc. We need to have a way
> to tell a filesystem that this is going to happen and that things need to
> be setup for this to work properly.

The way to tell filesystem what's happening is exactly what we are working
on with these patches...

> But if that has not been done then I think its proper to fail a long term
> pin operation on page cache pages. Meaning the regular filesystems
> maintain control of whats happening with their pages.

And as I mentioned in my other email, we cannot just fail the pin for
pagecache pages as that would regress existing applications.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR