Re: [RFC PATCH 3/5] mm/vma: add support for peer to peer to device vma

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Jan 31, 2019 at 09:02:03AM +0100, Christoph Hellwig wrote:
> On Wed, Jan 30, 2019 at 01:50:27PM -0500, Jerome Glisse wrote:
> > I do not see how VMA changes are any different than using struct page
> > in respect to userspace exposure. Those vma callback do not need to be
> > set by everyone, in fact expectation is that only handful of driver
> > will set those.
> > 
> > How can we do p2p between RDMA and GPU for instance, without exposure
> > to userspace ? At some point you need to tell userspace hey this kernel
> > does allow you to do that :)
> 
> To do RDMA on a memory region you need struct page backіng to start
> with..

No you do not with this patchset and there is no reason to tie RDMA to
struct page it does not provide a single feature we would need. So as
it can be done without and they are not benefit of using one i do not
see why we should use one.

Cheers,
Jérôme