From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA Date: Wed, 6 Feb 2019 12:41:28 -0800 Message-ID: <20190206204128.GR21860@bombadil.infradead.org> References: <20190205175059.GB21617@iweiny-DESK2.sc.intel.com> <20190206095000.GA12006@quack2.suse.cz> <20190206173114.GB12227@ziepe.ca> <20190206175233.GN21860@bombadil.infradead.org> <47820c4d696aee41225854071ec73373a273fd4a.camel@redhat.com> <01000168c43d594c-7979fcf8-b9c1-4bda-b29a-500efe001d66-000000@email.amazonses.com> <20190206194055.GP21860@bombadil.infradead.org> <20190206202021.GQ21860@bombadil.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Doug Ledford Cc: Christopher Lameter , Jason Gunthorpe , Jan Kara , Ira Weiny , lsf-pc@lists.linux-foundation.org, linux-rdma@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, John Hubbard , Jerome Glisse , Dan Williams , Dave Chinner , Michal Hocko List-Id: linux-rdma@vger.kernel.org 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.