Re: [PATCH v9 0/6] MAP_DIRECT for DAX userspace flush

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Mon, Oct 16, 2017 at 12:44:31PM -0700, Dan Williams wrote:
> > While I agree with the need for a per-MR notification mechanism, one
> > thing we lose by walking away from MAP_DIRECT is a way for a
> > hypervisor to coordinate pass through of a DAX mapping to an RDMA
> > device in a guest. That will remain a case where we will still need to
> > use device-dax. I'm fine if that's the answer, but just want to be
> > clear about all the places we need to protect a DAX mapping against
> > RDMA from a non-ODP device.
> 
> For this specific issue perhaps we promote FL_LAYOUT as a lease-type
> that can be set by fcntl().

I don't think it is a good userspace interface, mostly because it
is about things that don't matter for userspace (block mappings).

It makes sense as a kernel interface for callers that want to pin
down a memory long-term, but for userspace the fact that the block
mapping changes doesn't matter - it matters that their long term
pin is broken by something.