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=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,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 61438C56201 for ; Fri, 20 Nov 2020 18:00:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0DF9E2245D for ; Fri, 20 Nov 2020 18:00:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="YAofb6qX" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729149AbgKTSAg (ORCPT ); Fri, 20 Nov 2020 13:00:36 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:53585 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729059AbgKTSAf (ORCPT ); Fri, 20 Nov 2020 13:00:35 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605895233; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=JxskEXwutt4P1nZSKgawl/xEgyUPfquuR5hR5qwSjZM=; b=YAofb6qXRZb424IaniSlwERtCoNEUtaY7B5h73IDegWhCecykWHJKmr0i/bJPGtBYiFc24 wCKHF4rwNgVk0RMWjBdVXK+uLZumMhWmzgirwZ/n4hzgfRdRT1EO0rfTE9CZ097NtwLmZw MGdEvpKqaQKTdXltp/K68k08lKT3N54= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-245-KXpfjzaZNAWdTW2OD65koQ-1; Fri, 20 Nov 2020 13:00:27 -0500 X-MC-Unique: KXpfjzaZNAWdTW2OD65koQ-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6C42D100C601; Fri, 20 Nov 2020 18:00:23 +0000 (UTC) Received: from [10.36.114.78] (ovpn-114-78.ams2.redhat.com [10.36.114.78]) by smtp.corp.redhat.com (Postfix) with ESMTP id BCE8260862; Fri, 20 Nov 2020 18:00:16 +0000 (UTC) Subject: Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page To: Mike Kravetz , Michal Hocko Cc: Muchun Song , corbet@lwn.net, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org, peterz@infradead.org, viro@zeniv.linux.org.uk, akpm@linux-foundation.org, paulmck@kernel.org, mchehab+huawei@kernel.org, pawan.kumar.gupta@linux.intel.com, rdunlap@infradead.org, oneukum@suse.com, anshuman.khandual@arm.com, jroedel@suse.de, almasrymina@google.com, rientjes@google.com, willy@infradead.org, osalvador@suse.de, song.bao.hua@hisilicon.com, duanxiongchun@bytedance.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org References: <20201120064325.34492-1-songmuchun@bytedance.com> <20201120084202.GJ3200@dhcp22.suse.cz> <6b1533f7-69c6-6f19-fc93-c69750caaecc@redhat.com> <20201120093912.GM3200@dhcp22.suse.cz> <55e53264-a07a-a3ec-4253-e72c718b4ee6@oracle.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Fri, 20 Nov 2020 19:00:15 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <55e53264-a07a-a3ec-4253-e72c718b4ee6@oracle.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20.11.20 18:45, Mike Kravetz wrote: > On 11/20/20 1:43 AM, David Hildenbrand wrote: >> On 20.11.20 10:39, Michal Hocko wrote: >>> On Fri 20-11-20 10:27:05, David Hildenbrand wrote: >>>> On 20.11.20 09:42, Michal Hocko wrote: >>>>> On Fri 20-11-20 14:43:04, Muchun Song wrote: >>>>> [...] >>>>> >>>>> Thanks for improving the cover letter and providing some numbers. I have >>>>> only glanced through the patchset because I didn't really have more time >>>>> to dive depply into them. >>>>> >>>>> Overall it looks promissing. To summarize. I would prefer to not have >>>>> the feature enablement controlled by compile time option and the kernel >>>>> command line option should be opt-in. I also do not like that freeing >>>>> the pool can trigger the oom killer or even shut the system down if no >>>>> oom victim is eligible. >>>>> >>>>> One thing that I didn't really get to think hard about is what is the >>>>> effect of vmemmap manipulation wrt pfn walkers. pfn_to_page can be >>>>> invalid when racing with the split. How do we enforce that this won't >>>>> blow up? >>>> >>>> I have the same concerns - the sections are online the whole time and >>>> anybody with pfn_to_online_page() can grab them >>>> >>>> I think we have similar issues with memory offlining when removing the >>>> vmemmap, it's just very hard to trigger and we can easily protect by >>>> grabbing the memhotplug lock. >>> >>> I am not sure we can/want to span memory hotplug locking out to all pfn >>> walkers. But you are right that the underlying problem is similar but >>> much harder to trigger because vmemmaps are only removed when the >>> physical memory is hotremoved and that happens very seldom. Maybe it >>> will happen more with virtualization usecases. But this work makes it >>> even more tricky. If a pfn walker races with a hotremove then it would >>> just blow up when accessing the unmapped physical address space. For >>> this feature a pfn walker would just grab a real struct page re-used for >>> some unpredictable use under its feet. Any failure would be silent and >>> hard to debug. >> >> Right, we don't want the memory hotplug locking, thus discussions regarding rcu. Luckily, for now I never saw a BUG report regarding this - maybe because the time between memory offlining (offline_pages()) and memory/vmemmap getting removed (try_remove_memory()) is just too long. Someone would have to sleep after pfn_to_online_page() for quite a while to trigger it. >> >>> >>> [...] >>>> To keep things easy, maybe simply never allow to free these hugetlb pages >>>> again for now? If they were reserved during boot and the vmemmap condensed, >>>> then just let them stick around for all eternity. >>> >>> Not sure I understand. Do you propose to only free those vmemmap pages >>> when the pool is initialized during boot time and never allow to free >>> them up? That would certainly make it safer and maybe even simpler wrt >>> implementation. >> >> Exactly, let's keep it simple for now. I guess most use cases of this (virtualization, databases, ...) will allocate hugepages during boot and never free them. > > Not sure if I agree with that last statement. Database and virtualization > use cases from my employer allocate allocate hugetlb pages after boot. It > is shortly after boot, but still not from boot/kernel command line. Right, but the ones that care about this optimization for now could be converted, I assume? I mean we are talking about "opt-in" from sysadmins, so requiring to specify a different cmdline parameter does not sound to weird to me. And it should simplify a first version quite a lot. The more I think about this, the more I believe doing these vmemmap modifications after boot are very dangerous. > > Somewhat related, but not exactly addressing this issue ... > > One idea discussed in a previous patch set was to disable PMD/huge page > mapping of vmemmap if this feature was enabled. This would eliminate a bunch > of the complex code doing page table manipulation. It does not address > the issue of struct page pages going away which is being discussed here, > but it could be a way to simply the first version of this code. If this > is going to be an 'opt in' feature as previously suggested, then eliminating > the PMD/huge page vmemmap mapping may be acceptable. My guess is that > sysadmins would only 'opt in' if they expect most of system memory to be used > by hugetlb pages. We certainly have database and virtualization use cases > where this is true. It sounds like a hack to me, which does not fully solve the problem. But yeah, it's a simplification. -- Thanks, David / dhildenb