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

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Message-ID: <20190206204128.GR21860@bombadil.infradead.org>
Date:   Wed, 6 Feb 2019 12:41:28 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Doug Ledford <dledford@...hat.com>
Cc:     Christopher Lameter <cl@...ux.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, Feb 06, 2019 at 03:28:35PM -0500, Doug Ledford wrote:
> On Wed, 2019-02-06 at 12:20 -0800, Matthew Wilcox wrote:
> > 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?
> 
> We don't have a means of migration except in the narrowly scoped sense
> of queue pair migration as defined by the IBTA and implemented on some
> dual port IB cards.  This narrowly scoped migration even still involves
> notification of the app.
> 
> Since there's no guarantee that any other port can connect to the same
> machine as any port that's going away, it would always be a
> disconnect/reconnect sequence in the app to support this, not an under
> the covers migration.

I don't understand you.  We're not talking about migrating from one IB
card to another, we're talking about changing the addresses that an STag
refers to.