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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 757B7C3A5A6 for ; Mon, 23 Sep 2019 11:43:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4C07D20B7C for ; Mon, 23 Sep 2019 11:43:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732238AbfIWLnt (ORCPT ); Mon, 23 Sep 2019 07:43:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41994 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726146AbfIWLns (ORCPT ); Mon, 23 Sep 2019 07:43:48 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 77850300CA4B; Mon, 23 Sep 2019 11:43:48 +0000 (UTC) Received: from [10.36.116.207] (ovpn-116-207.ams2.redhat.com [10.36.116.207]) by smtp.corp.redhat.com (Postfix) with ESMTP id E23345D9E2; Mon, 23 Sep 2019 11:43:44 +0000 (UTC) Subject: Re: [PATCH v1 0/3] mm/memory_hotplug: Export generic_online_page() From: David Hildenbrand To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Souptick Joarder , linux-hyperv@vger.kernel.org, Andrew Morton , Dan Williams , Haiyang Zhang , "K. Y. Srinivasan" , Oscar Salvador , Pavel Tatashin , Qian Cai , Sasha Levin , Stephen Hemminger , Wei Yang References: <20190909114830.662-1-david@redhat.com> <20190923085807.GD6016@dhcp22.suse.cz> <20190923111559.GK6016@dhcp22.suse.cz> <88ac3511-4ad8-d5c8-8e6a-0cca0a0f0989@redhat.com> Openpgp: preference=signencrypt Autocrypt: addr=david@redhat.com; prefer-encrypt=mutual; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwX4EEwECACgFAljj9eoCGwMFCQlmAYAGCwkI BwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJEE3eEPcA/4Na5IIP/3T/FIQMxIfNzZshIq687qgG 8UbspuE/YSUDdv7r5szYTK6KPTlqN8NAcSfheywbuYD9A4ZeSBWD3/NAVUdrCaRP2IvFyELj xoMvfJccbq45BxzgEspg/bVahNbyuBpLBVjVWwRtFCUEXkyazksSv8pdTMAs9IucChvFmmq3 jJ2vlaz9lYt/lxN246fIVceckPMiUveimngvXZw21VOAhfQ+/sofXF8JCFv2mFcBDoa7eYob s0FLpmqFaeNRHAlzMWgSsP80qx5nWWEvRLdKWi533N2vC/EyunN3HcBwVrXH4hxRBMco3jvM m8VKLKao9wKj82qSivUnkPIwsAGNPdFoPbgghCQiBjBe6A75Z2xHFrzo7t1jg7nQfIyNC7ez MZBJ59sqA9EDMEJPlLNIeJmqslXPjmMFnE7Mby/+335WJYDulsRybN+W5rLT5aMvhC6x6POK z55fMNKrMASCzBJum2Fwjf/VnuGRYkhKCqqZ8gJ3OvmR50tInDV2jZ1DQgc3i550T5JDpToh dPBxZocIhzg+MBSRDXcJmHOx/7nQm3iQ6iLuwmXsRC6f5FbFefk9EjuTKcLMvBsEx+2DEx0E UnmJ4hVg7u1PQ+2Oy+Lh/opK/BDiqlQ8Pz2jiXv5xkECvr/3Sv59hlOCZMOaiLTTjtOIU7Tq 7ut6OL64oAq+zsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCghCj/CA/lc/LMthqQ773ga uB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseBfDXHA6m4B3mUTWo13nid 0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts6TZ+IrPOwT1hfB4WNC+X 2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiuQmt3yqrmN63V9wzaPhC+ xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKBTccu2AXJXWAE1Xjh6GOC 8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvFFFyAS0Nk1q/7EChPcbRb hJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh2YmnmLRTro6eZ/qYwWkC u8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRkF3TwgucpyPtcpmQtTkWS gDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0LLH63+BrrHasfJzxKXzqg rW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4vq7oFCPsOgwARAQABwsFl BBgBAgAPBQJVy5+RAhsMBQkJZgGAAAoJEE3eEPcA/4NagOsP/jPoIBb/iXVbM+fmSHOjEshl KMwEl/m5iLj3iHnHPVLBUWrXPdS7iQijJA/VLxjnFknhaS60hkUNWexDMxVVP/6lbOrs4bDZ NEWDMktAeqJaFtxackPszlcpRVkAs6Msn9tu8hlvB517pyUgvuD7ZS9gGOMmYwFQDyytpepo YApVV00P0u3AaE0Cj/o71STqGJKZxcVhPaZ+LR+UCBZOyKfEyq+ZN311VpOJZ1IvTExf+S/5 lqnciDtbO3I4Wq0ArLX1gs1q1XlXLaVaA3yVqeC8E7kOchDNinD3hJS4OX0e1gdsx/e6COvy qNg5aL5n0Kl4fcVqM0LdIhsubVs4eiNCa5XMSYpXmVi3HAuFyg9dN+x8thSwI836FoMASwOl C7tHsTjnSGufB+D7F7ZBT61BffNBBIm1KdMxcxqLUVXpBQHHlGkbwI+3Ye+nE6HmZH7IwLwV W+Ajl7oYF+jeKaH4DZFtgLYGLtZ1LDwKPjX7VAsa4Yx7S5+EBAaZGxK510MjIx6SGrZWBrrV TEvdV00F2MnQoeXKzD7O4WFbL55hhyGgfWTHwZ457iN9SgYi1JLPqWkZB0JRXIEtjd4JEQcx +8Umfre0Xt4713VxMygW0PnQt5aSQdMD58jHFxTk092mU+yIHj5LeYgvwSgZN4airXk5yRXl SE+xAvmumFBY Organization: Red Hat GmbH Message-ID: <388fdfa4-6cd9-bfcd-3f7b-546e54c11a97@redhat.com> Date: Mon, 23 Sep 2019 13:43:43 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <88ac3511-4ad8-d5c8-8e6a-0cca0a0f0989@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.42]); Mon, 23 Sep 2019 11:43:48 +0000 (UTC) Sender: linux-hyperv-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org On 23.09.19 13:34, David Hildenbrand wrote: > On 23.09.19 13:15, Michal Hocko wrote: >> On Mon 23-09-19 11:31:30, David Hildenbrand wrote: >>> On 23.09.19 10:58, Michal Hocko wrote: >>>> On Fri 20-09-19 10:17:54, David Hildenbrand wrote: >>>>> On 09.09.19 13:48, David Hildenbrand wrote: >>>>>> Based on linux/next + "[PATCH 0/3] Remove __online_page_set_limits()" >>>>>> >>>>>> Let's replace the __online_page...() functions by generic_online_page(). >>>>>> Hyper-V only wants to delay the actual onlining of un-backed pages, so we >>>>>> can simpy re-use the generic function. >>>>>> >>>>>> Only compile-tested. >>>>>> >>>>>> Cc: Souptick Joarder >>>>>> >>>>>> David Hildenbrand (3): >>>>>> mm/memory_hotplug: Export generic_online_page() >>>>>> hv_balloon: Use generic_online_page() >>>>>> mm/memory_hotplug: Remove __online_page_free() and >>>>>> __online_page_increment_counters() >>>>>> >>>>>> drivers/hv/hv_balloon.c | 3 +-- >>>>>> include/linux/memory_hotplug.h | 4 +--- >>>>>> mm/memory_hotplug.c | 17 ++--------------- >>>>>> 3 files changed, 4 insertions(+), 20 deletions(-) >>>>>> >>>>> >>>>> Ping, any comments on this one? >>>> >>>> Unification makes a lot of sense to me. You can add >>>> Acked-by: Michal Hocko >>>> >>>> I will most likely won't surprise if I asked for more here though ;) >>> >>> I'm not surprised, but definitely not in a negative sense ;) I was >>> asking myself if we could somehow rework this, too. >>> >>>> I have to confess I really detest the whole concept of a hidden callback >>>> with a very weird API. Is this something we can do about? I do realize >>>> that adding a callback would require either cluttering the existing APIs >>>> but maybe we can come up with something more clever. Or maybe existing >>>> external users of online callback can do that as a separate step after >>>> the online is completed - or is this impossible due to locking >>>> guarantees? >>>> >>> >>> The use case of this (somewhat special) callback really is to avoid >>> selected (unbacked in the hypervisor) pages to get put to the buddy just >>> now, but instead to defer that (sometimes, defer till infinity ;) ). >>> Especially, to hinder these pages from getting touched at all. Pages >>> that won't be put to the buddy will usually get PG_offline set (e.g., >>> Hyper-V and XEN) - the only two users I am aware of. >>> >>> For Hyper-V (and also eventually virtio-mem), it is important to set >>> PG_offline before marking the section to be online (SECTION_IS_ONLINE). >>> Only this way, PG_offline is properly set on all pfn_to_online_page() >>> pages, meaning "don't touch this page" - e.g., used to skip over such >>> pages when suspending or by makedumpfile to skip over such offline pages >>> when creating a memory dump. >> >> Thanks for the clarification. I have never really studied what those >> callbacks are doing really. >> >>> So if we would e.g., try to piggy-back onto the memory_notify() >>> infrastructure, we could >>> 1. Online all pages to the buddy (dropping the callback) >>> 2. E.g., memory_notify(MEM_ONLINE_PAGES, &arg); >>> -> in the notifier, pull pages from the buddy, mark sections online >>> 3. Set all involved sections online (online_mem_sections()) >> >> This doesn't really sound any better. For one pages are immediately >> usable when they hit the buddy allocator so this is racy and thus not >> reliable. >> >>> However, I am not sure what actually happens after 1. - we are only >>> holding the device hotplug lock and the memory hotplug lock, so the >>> pages can just get allocated. Also, it sounds like more work and code >>> for the same end result (okay, if the rework is really necessary, though). >>> >>> So yeah, while the current callback might not be optimal, I don't see an >>> easy and clean way to rework this. With the change in this series we are >>> at least able to simply defer doing what would have been done without >>> the callback - not perfect but better. >>> >>> Do you have anything in mind that could work out and make this nicer? >> >> I am wondering why those pages get onlined when they are, in fact, >> supposed to be offline. >> > > It's the current way of emulating sub-memory-block hotplug on top of the > memory bock device API we have. Hyper-V and XEN have been using that for > a long time. > So one idea would be to let clients set pages to PG_offline during MEM_GOING_ONLINE. We could then skip any PG_offline pages when onlining pages, not onlining them to the buddy. But then, there still has to be a way to online pages when required - e.g., generic_online_page(). At least the callback could go. -- Thanks, David / dhildenb