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[195.135.220.15]) by mx.google.com with ESMTPS id q25si20375632ejs.275.2019.08.01.00.24.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Aug 2019 00:24:33 -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 C8CFDB022; Thu, 1 Aug 2019 07:24:32 +0000 (UTC) Date: Thu, 1 Aug 2019 09:24:30 +0200 From: Michal Hocko To: David Hildenbrand Cc: Rashmica Gupta , Oscar Salvador , Andrew Morton , Dan Williams , pasha.tatashin@soleen.com, Jonathan.Cameron@huawei.com, anshuman.khandual@arm.com, Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory Message-ID: <20190801072430.GF11627@dhcp22.suse.cz> References: <2750c11a-524d-b248-060c-49e6b3eb8975@redhat.com> <20190626081516.GC30863@linux> <887b902e-063d-a857-d472-f6f69d954378@redhat.com> <9143f64391d11aa0f1988e78be9de7ff56e4b30b.camel@gmail.com> <20190702074806.GA26836@linux> <20190731120859.GJ9330@dhcp22.suse.cz> <4ddee0dd719abd50350f997b8089fa26f6004c0c.camel@gmail.com> <20190801071709.GE11627@dhcp22.suse.cz> <9bcbd574-7e23-5cfe-f633-646a085f935a@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9bcbd574-7e23-5cfe-f633-646a085f935a@redhat.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 Thu 01-08-19 09:18:47, David Hildenbrand wrote: > On 01.08.19 09:17, Michal Hocko wrote: > > On Thu 01-08-19 09:06:40, Rashmica Gupta wrote: > >> On Wed, 2019-07-31 at 14:08 +0200, Michal Hocko wrote: > >>> On Tue 02-07-19 18:52:01, Rashmica Gupta wrote: > >>> [...] > >>>>> 2) Why it was designed, what is the goal of the interface? > >>>>> 3) When it is supposed to be used? > >>>>> > >>>>> > >>>> There is a hardware debugging facility (htm) on some power chips. > >>>> To use > >>>> this you need a contiguous portion of memory for the output to be > >>>> dumped > >>>> to - and we obviously don't want this memory to be simultaneously > >>>> used by > >>>> the kernel. > >>> > >>> How much memory are we talking about here? Just curious. > >> > >> From what I've seen a couple of GB per node, so maybe 2-10GB total. > > > > OK, that is really a lot to keep around unused just in case the > > debugging is going to be used. > > > > I am still not sure the current approach of (ab)using memory hotplug is > > ideal. Sure there is some overlap but you shouldn't really need to > > offline the required memory range at all. All you need is to isolate the > > memory from any existing user and the page allocator. Have you checked > > alloc_contig_range? > > > > Rashmica mentioned somewhere in this thread that the virtual mapping > must not be in place, otherwise the HW might prefetch some of this > memory, leading to errors with memtrace (which checks that in HW). Does anything prevent from unmapping the pfn range from the direct mapping? -- Michal Hocko SUSE Labs