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[195.135.220.15]) by mx.google.com with ESMTPS id b3si1937967ejv.334.2019.06.25.22.45.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 25 Jun 2019 22:45:56 -0700 (PDT) Received-SPF: softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning mhocko@kernel.org does not designate 195.135.220.15 as permitted sender) smtp.mailfrom=mhocko@kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A851EAF25; Wed, 26 Jun 2019 05:45:55 +0000 (UTC) Date: Wed, 26 Jun 2019 07:45:54 +0200 From: Michal Hocko To: John Hubbard Cc: Jason Gunthorpe , Ira Weiny , Ralph Campbell , "linux-nvdimm@lists.01.org" , "nouveau@lists.freedesktop.org" , "linux-kernel@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , "linux-mm@kvack.org" , =?iso-8859-1?B?Suly9G1l?= Glisse , Ben Skeggs , "linux-pci@vger.kernel.org" , Christoph Hellwig Subject: Re: [PATCH 18/22] mm: mark DEVICE_PUBLIC as broken Message-ID: <20190626054554.GA17798@dhcp22.suse.cz> References: <20190613094326.24093-1-hch@lst.de> <20190613094326.24093-19-hch@lst.de> <20190613194430.GY22062@mellanox.com> <20190613195819.GA22062@mellanox.com> <20190614004314.GD783@iweiny-DESK2.sc.intel.com> <20190619192719.GO9374@mellanox.com> <29f43c79-b454-0477-a799-7850e6571bd3@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <29f43c79-b454-0477-a799-7850e6571bd3@nvidia.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue 25-06-19 20:15:28, John Hubbard wrote: > On 6/19/19 12:27 PM, Jason Gunthorpe wrote: > > On Thu, Jun 13, 2019 at 06:23:04PM -0700, John Hubbard wrote: > >> On 6/13/19 5:43 PM, Ira Weiny wrote: > >>> On Thu, Jun 13, 2019 at 07:58:29PM +0000, Jason Gunthorpe wrote: > >>>> On Thu, Jun 13, 2019 at 12:53:02PM -0700, Ralph Campbell wrote: > >>>>> > >> ... > >>> So I think it is ok. Frankly I was wondering if we should remove the public > >>> type altogether but conceptually it seems ok. But I don't see any users of it > >>> so... should we get rid of it in the code rather than turning the config off? > >>> > >>> Ira > >> > >> That seems reasonable. I recall that the hope was for those IBM Power 9 > >> systems to use _PUBLIC, as they have hardware-based coherent device (GPU) > >> memory, and so the memory really is visible to the CPU. And the IBM team > >> was thinking of taking advantage of it. But I haven't seen anything on > >> that front for a while. > > > > Does anyone know who those people are and can we encourage them to > > send some patches? :) > > > > I asked about this, and it seems that the idea was: DEVICE_PUBLIC was there > in order to provide an alternative way to do things (such as migrate memory > to and from a device), in case the combination of existing and near-future > NUMA APIs was insufficient. This probably came as a follow-up to the early > 2017-ish conversations about NUMA, in which the linux-mm recommendation was > "try using HMM mechanisms, and if those are inadequate, then maybe we can > look at enhancing NUMA so that it has better handling of advanced (GPU-like) > devices". Yes that was the original idea. It sounds so much better to use a common framework rather than awkward special cased cpuless NUMA nodes with a weird semantic. User of the neither of the two has shown up so I guess that the envisioned HW just didn't materialized. Or has there been a completely different approach chosen? > In the end, however, _PUBLIC was never used, nor does anyone in the local > (NVIDIA + IBM) kernel vicinity seem to have plans to use it. So it really > does seem safe to remove, although of course it's good to start with > BROKEN and see if anyone pops up and complains. Well, I do not really see much of a difference. Preserving an unused code which doesn't have any user in sight just adds a maintenance burden whether the code depends on BROKEN or not. We can always revert patches which remove the code once a real user shows up. -- Michal Hocko SUSE Labs