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

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Date:   Wed, 6 Feb 2019 20:39:59 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Doug Ledford <dledford@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jan Kara <jack@...e.cz>, Ira Weiny <ira.weiny@...el.com>,
        lsf-pc@...ts.linux-foundation.org, linux-rdma@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        John Hubbard <jhubbard@...dia.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP
 usage by RDMA

On Wed, 6 Feb 2019, Matthew Wilcox wrote:

> 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?

We have MMU notifier callbacks that can tell the device to release the
mappings. And an RDMA device may operate in ODP mode which is on demand
paging. With that data may be migrated as usual.