Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA

From: Matthew Wilcox
Date: Wed Feb 06 2019 - 15:20:27 EST

On Wed, Feb 06, 2019 at 03:16:02PM -0500, Doug Ledford wrote:
> On Wed, 2019-02-06 at 11:40 -0800, Matthew Wilcox wrote:
> > On Wed, Feb 06, 2019 at 07:16:21PM +0000, Christopher Lameter wrote:
> > >
> > > though? If we only allow this use case then we may not have to worry about
> > > long term GUP because DAX mapped files will stay in the physical location
> > > regardless.
> >
> > ... except for truncate. And now that I think about it, there was a
> > desire to support hot-unplug which also needed revoke.
> We already support hot unplug of RDMA devices. But it is extreme. How
> does hot unplug deal with a program running from the device (something
> that would have returned ETXTBSY)?

Not hot-unplugging the RDMA device but hot-unplugging an NV-DIMM.

It's straightforward to migrate text pages from one DIMM to another;
you remove the PTEs from the CPU's page tables, copy the data over and
pagefaults put the new PTEs in place. We don't have a way to do similar
things to an RDMA device, do we?