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

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Message-ID: <20190206204954.GS21860@bombadil.infradead.org>
Date:   Wed, 6 Feb 2019 12:49:54 -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 03:47:53PM -0500, Doug Ledford wrote:
> 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:
> > > > Not hot-unplugging the RDMA device but hot-unplugging an NV-DIMM.

^^^ I think you missed this line ^^^

> 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.