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Thu, 25 Mar 2021 14:46:25 +0000 (UTC) Received: from [10.36.115.72] (ovpn-115-72.ams2.redhat.com [10.36.115.72]) by smtp.corp.redhat.com (Postfix) with ESMTP id CE92260C2E; Thu, 25 Mar 2021 14:46:23 +0000 (UTC) To: Michal Hocko Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9591a0b8-c000-2f61-67a6-4402678fe50b@redhat.com> <31110e58-c99a-8dee-6f6e-98f456b77759@redhat.com> <062bc5d7-a83c-1c1a-7b77-9f043643f4fa@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: <31c3e6f7-f631-7b00-2c33-518b0f24a75f@redhat.com> Date: Thu, 25 Mar 2021 15:46:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Stat-Signature: e3ghjm34bdu5bjsmwqb4bokgn39iu5e4 X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 670C23C0 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf29; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=170.10.133.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616683589-899169 Content-Transfer-Encoding: quoted-printable 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 25.03.21 15:34, Michal Hocko wrote: > On Thu 25-03-21 15:09:35, David Hildenbrand wrote: >> On 25.03.21 15:08, Michal Hocko wrote: >>> On Thu 25-03-21 13:40:45, David Hildenbrand wrote: >>>> On 25.03.21 13:35, Michal Hocko wrote: >>>>> On Thu 25-03-21 12:08:43, David Hildenbrand wrote: >>>>>> On 25.03.21 11:55, Oscar Salvador wrote: >>> [...] >>>>>>> - When moving the initialization/accounting to hot-add/hot-remove= , >>>>>>> the section containing the vmemmap pages will remain offlin= e. >>>>>>> It might get onlined once the pages get online in online_pa= ges(), >>>>>>> or not if vmemmap pages span a whole section. >>>>>>> I remember (but maybe David rmemeber better) that that was = a problem >>>>>>> wrt. pfn_to_online_page() and hybernation/kdump. >>>>>>> So, if that is really a problem, we would have to care of o= t setting >>>>>>> the section to the right state. >>>>>> >>>>>> Good memory. Indeed, hibernation/kdump won't save the state of the= vmemmap, >>>>>> because the memory is marked as offline and, thus, logically witho= ut any >>>>>> valuable content. >=20 > ^^^^ THIS >=20 >>>>> >>>>> Could you point me to the respective hibernation code please? I alw= ays >>>>> get lost in that area. Anyway, we do have the same problem even if = the >>>>> whole accounting is handled during {on,off}lining, no? >>>> >>>> kernel/power/snapshot.c:saveable_page(). >>> >>> Thanks! So this is as I've suspected. The very same problem is presen= t >>> if the memory block is marked offline. So we need a solution here >>> anyway. One way to go would be to consider these vmemmap pages always >>> online. pfn_to_online_page would have to special case them but we wou= ld >>> need to identify them first. I used to have PageVmemmap or something >>> like that in my early attempt to do this. >>> >>> That being said this is not an argument for one or the other aproach. >>> Both need fixing. >> >> Can you elaborate? What is the issue there? What needs fixing? >=20 > offline section containing vmemmap will be lost during hibernation cycl= e > IIU the above correctly. >=20 Can tell me how that is a problem with Oscars current patch? I only see=20 this being a problem with what you propose - most probably I am missing=20 something important here. Offline memory sections don't have a valid memmap (assumption: garbage).=20 On hibernation, the whole offline memory block won't be saved, including=20 the vmemmap content that resides on the block. This includes the vmemmap=20 of the vmemmap pages, which is itself. When restoring, the whole memory block will contain garbage, including=20 the whole vmemmap - which is marked to be offline and to contain garbage. Oscars patch: Works as expected. Onlining the memory block will properly=20 intiialize the vmemmap (including the vmemmap of the vmemmap), then mark=20 the vmemmap to be valid (by marking the sections to be online). Your porposal: Does not work as expected. Once we online the memory=20 block we don't initialize the vmemmap of the vmemmap pages. There is=20 garbage, which gets exposed to the system as soon as we mark the vmemmap=20 to be valid (by marking the sections to be online). --=20 Thanks, David / dhildenb