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d=suse.com; s=susede1; t=1616686558; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Hpz5bZISM1LPyawPkc+NNNxx8vN1clwZYSZEi0x1MIs=; b=nMpO1UOk1RdHTKFtgRVc34rgrYVPuo2Xk4zaR2ml/ODzH2v0YITYLapp/S3OdEb9hwiHrw c0UQPmQFqRgBr80ox2K/hNZhHrab3Rs/UgPVxX5/GOyY0Q3dEtn4ReFQcunBDyar1FHtp/ l/NdTOawXeAo0AYkdCk53AJSNt57f+k= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 98323AA55; Thu, 25 Mar 2021 15:35:58 +0000 (UTC) Date: Thu, 25 Mar 2021 16:35:57 +0100 From: Michal Hocko To: David Hildenbrand Cc: Oscar Salvador , Andrew Morton , Anshuman Khandual , Vlastimil Babka , Pavel Tatashin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v5 1/5] mm,memory_hotplug: Allocate memmap from the added memory range Message-ID: 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <40fac999-2d28-9205-23f0-516fa9342bbe@redhat.com> X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 0D5BDA0009E8 X-Stat-Signature: jw7zsfqcdu5uyq546g4fradhbr19x5uj Received-SPF: none (suse.com>: No applicable sender policy available) receiver=imf07; identity=mailfrom; envelope-from=""; helo=mx2.suse.de; client-ip=195.135.220.15 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616686562-142696 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 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). -- Michal Hocko SUSE Labs