From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E9487C4363D for ; Wed, 23 Sep 2020 14:31:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A5E9721D92 for ; Wed, 23 Sep 2020 14:31:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726572AbgIWOb2 (ORCPT ); Wed, 23 Sep 2020 10:31:28 -0400 Received: from mx2.suse.de ([195.135.220.15]:36730 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726130AbgIWOb2 (ORCPT ); Wed, 23 Sep 2020 10:31:28 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 190C2B2C7; Wed, 23 Sep 2020 14:32:04 +0000 (UTC) Subject: Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation To: David Hildenbrand , osalvador@suse.de Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-hyperv@vger.kernel.org, xen-devel@lists.xenproject.org, linux-acpi@vger.kernel.org, Andrew Morton , Alexander Duyck , Dave Hansen , Haiyang Zhang , "K. Y. Srinivasan" , Mel Gorman , Michael Ellerman , Michal Hocko , Mike Rapoport , Scott Cheloha , Stephen Hemminger , Wei Liu , Wei Yang References: <5c0910c2cd0d9d351e509392a45552fb@suse.de> From: Vlastimil Babka Message-ID: <67928cbd-950a-3279-bf9b-29b04c87728b@suse.cz> Date: Wed, 23 Sep 2020 16:31:25 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On 9/16/20 9:31 PM, David Hildenbrand wrote: > > >> Am 16.09.2020 um 20:50 schrieb osalvador@suse.de: >> >> On 2020-09-16 20:34, David Hildenbrand wrote: >>> When adding separate memory blocks via add_memory*() and onlining them >>> immediately, the metadata (especially the memmap) of the next block will be >>> placed onto one of the just added+onlined block. This creates a chain >>> of unmovable allocations: If the last memory block cannot get >>> offlined+removed() so will all dependant ones. We directly have unmovable >>> allocations all over the place. >>> This can be observed quite easily using virtio-mem, however, it can also >>> be observed when using DIMMs. The freshly onlined pages will usually be >>> placed to the head of the freelists, meaning they will be allocated next, >>> turning the just-added memory usually immediately un-removable. The >>> fresh pages are cold, prefering to allocate others (that might be hot) >>> also feels to be the natural thing to do. >>> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when >>> adding separate, successive memory blocks, each memory block will have >>> unmovable allocations on them - for example gigantic pages will fail to >>> allocate. >>> While the ZONE_NORMAL doesn't provide any guarantees that memory can get >>> offlined+removed again (any kind of fragmentation with unmovable >>> allocations is possible), there are many scenarios (hotplugging a lot of >>> memory, running workload, hotunplug some memory/as much as possible) where >>> we can offline+remove quite a lot with this patchset. >> >> Hi David, >> > > Hi Oscar. > >> I did not read through the patchset yet, so sorry if the question is nonsense, but is this not trying to fix the same issue the vmemmap patches did? [1] > > Not nonesense at all. It only helps to some degree, though. It solves the dependencies due to the memmap. However, it‘s not completely ideal, especially for single memory blocks. > > With single memory blocks (virtio-mem, xen-balloon, hv balloon, ppc dlpar) you still have unmovable (vmemmap chunks) all over the physical address space. Consider the gigantic page example after hotplug. You directly fragmented all hotplugged memory. > > Of course, there might be (less extreme) dependencies due page tables for the identity mapping, extended struct pages and similar. > > Having that said, there are other benefits when preferring other memory over just hotplugged memory. Think about adding+onlining memory during boot (dimms under QEMU, virtio-mem), once the system is up you will have most (all) of that memory completely untouched. > > So while vmemmap on hotplugged memory would tackle some part of the issue, there are cases where this approach is better, and there are even benefits when combining both. I see the point, but I don't think the head/tail mechanism is great for this. It might sort of work, but with other interfering activity there are no guarantees and it relies on a subtle implementation detail. There are better mechanisms possible I think, such as preparing a larger MIGRATE_UNMOVABLE area in the existing memory before we allocate those long-term management structures. Or onlining a bunch of blocks as zone_movable first and only later convert to zone_normal in a controlled way when existing normal zone becomes depeted? I guess it's an issue that the e.g. 128M block onlines are so disconnected from each other it's hard to employ a strategy that works best for e.g. a whole bunch of GB onlined at once. But I noticed some effort towards new API, so maybe that will be solved there too? > Thanks! > > David > >> >> I was about to give it a new respin now that thw hwpoison stuff has been settled. >> >> [1] https://patchwork.kernel.org/cover/11059175/ >> >