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[209.132.183.28]) by mx.google.com with ESMTPS id t64si39774646qkb.359.2019.08.01.00.31.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Aug 2019 00:31:15 -0700 (PDT) Received-SPF: pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of david@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=david@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 47F593B71F; Thu, 1 Aug 2019 07:31:13 +0000 (UTC) Received: from [10.36.116.245] (ovpn-116-245.ams2.redhat.com [10.36.116.245]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5C4B25D6A7; Thu, 1 Aug 2019 07:31:10 +0000 (UTC) Subject: Re: [PATCH v2 0/5] Allocate memmap from hotadded memory From: David Hildenbrand To: Michal Hocko 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 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> <20190801072430.GF11627@dhcp22.suse.cz> Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Thu, 01 Aug 2019 07:31:13 +0000 (UTC) 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 01.08.19 09:26, David Hildenbrand wrote: > On 01.08.19 09:24, Michal Hocko wrote: >> 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? > > I am not sure about the implications of having > pfn_valid()/pfn_present()/pfn_online() return true but accessing it > results in crashes. (suspend, kdump, whatever other technology touches > online memory) (oneidea: we could of course go ahead and mark the pages PG_offline before unmapping the pfn range to work around these issues) > > (sounds more like a hack to me than just going ahead and > removing/readding the memory via a clean interface we have) > -- Thanks, David / dhildenb