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

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Message-ID: <fbdeccb01f7d0ba2f6ebb69660b7aa3d99690042.camel@redhat.com>
Date:   Wed, 06 Feb 2019 15:47:53 -0500
From:   Doug Ledford <dledford@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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, 2019-02-06 at 12:41 -0800, Matthew Wilcox wrote:
> 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.

You said "now that I think about it, there was a desire to support hot-
unplug which also needed revoke".  For us, hot unplug is done at the
device level and means all connections must be torn down.  So in the
context of this argument, if people want revoke so DAX can migrate from
one NV-DIMM to another, ok.  But revoke does not help RDMA migrate.

If, instead, you mean that you want to support hot unplug of an NV-DIMM
that is currently the target of RDMA transfers, then I believe
Christoph's answer on this is correct.  It all boils down to which
device you are talking about doing the hot unplug on.

-- 
Doug Ledford <dledford@...hat.com>
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