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Thu, 25 Mar 2021 15:40:48 +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 6C77D59471; Thu, 25 Mar 2021 15:40:46 +0000 (UTC) Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range To: Michal Hocko Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <31110e58-c99a-8dee-6f6e-98f456b77759@redhat.com> <062bc5d7-a83c-1c1a-7b77-9f043643f4fa@redhat.com> <31c3e6f7-f631-7b00-2c33-518b0f24a75f@redhat.com> <40fac999-2d28-9205-23f0-516fa9342bbe@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <8e825d6a-bed5-d2ac-fc69-ae50ba787fe2@redhat.com> Date: Thu, 25 Mar 2021 16:40:45 +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 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Stat-Signature: 3cy8tb9wr6wrdkjkdxjbnqurpnragcuy X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: C201EC0042CF Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf22; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616686853-870758 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 16:35, Michal Hocko wrote: > On Thu 25-03-21 16:19:36, David Hildenbrand wrote: >> On 25.03.21 16:12, Michal Hocko wrote: >>> On Thu 25-03-21 15:46:22, David Hildenbrand wrote: >>>> 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 offline. >>>>>>>>>>> It might get onlined once the pages get online in online_pages(), >>>>>>>>>>> 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 ot 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 without any >>>>>>>>>> valuable content. >>>>> >>>>> ^^^^ THIS >>>>> >>>>>>>>> >>>>>>>>> Could you point me to the respective hibernation code please? I always >>>>>>>>> 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 present >>>>>>> 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 would >>>>>>> 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? >>>>> >>>>> offline section containing vmemmap will be lost during hibernation cycle >>>>> IIU the above correctly. >>>>> >>>> >>>> Can tell me how that is a problem with Oscars current patch? I only see this >>>> being a problem with what you propose - most probably I am missing something >>>> important here. >>>> >>>> Offline memory sections don't have a valid memmap (assumption: garbage). On >>>> hibernation, the whole offline memory block won't be saved, including the >>>> vmemmap content that resides on the block. This includes the vmemmap of the >>>> vmemmap pages, which is itself. >>>> >>>> When restoring, the whole memory block will contain garbage, including the >>>> whole vmemmap - which is marked to be offline and to contain garbage. >>> >>> Hmm, so I might be misunderstanding the restoring part. But doesn't that >>> mean that the whole section/memory block won't get restored because it >>> is offline and therefore the vmemmap would be pointing to nowhere? >> >> AFAIU, only the content of the memory block won't be restored - whatever >> memory content existed before the restore operation is kept. >> >> The structures that define how the vmemmap should look like - the memory >> sections and the page tables used for describing the vmemmap should properly >> get saved+restored, as these are located on online memory. >> >> So the vmemmap layout should look after restoring just like before saving. > > OK, makes sense. Thanks for the clarification. > > So there is indeed a difference. One way around that would be to mark > vmemmap pages (e.g. PageReserved && magic value stored somewhere in the > struct page - resembling bootmem vmemmaps) or mark section fully backing > vmemmaps as online (ugly). I'm sorry Michal, but then we are hacking around the online section size limitation just in another (IMHO more ugly) way, then what Oscar and I came up with when discussing this in the past. Your first approach would require us to look at potential garbage (pfn_to_online_page() == NULL) and filter out what might still be useful. The second approach exposes garbage to the rest of the system as initialized memmap. I honestly cannot say that I prefer either over what we have here. -- Thanks, David / dhildenb