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 9AEF2C5519F for ; Fri, 20 Nov 2020 09:44:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2BBC22222F for ; Fri, 20 Nov 2020 09:44:19 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="gajonlih" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727753AbgKTJn6 (ORCPT ); Fri, 20 Nov 2020 04:43:58 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38580 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727464AbgKTJn5 (ORCPT ); Fri, 20 Nov 2020 04:43:57 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1605865435; 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=ZwdHt+LWlPpn5AnW9j4bNwWOooSLgU3rF8T4LtW1bPs=; b=gajonlih1PfsaIPo4Cvp6gFZ92/HE4hF8wgzoGTLO86BRJ7dMFux8T0hh5Y+HP4TC98h0M Tmeul0jsO4FSBAvgqoQr3/3FH3ZxoUqTgr8SpOq4nVF9ZUYJ9FYIo7eo63EPkKog6NxznN XXluYjjfsO4l5Wh0nKTQE8BgNL0iMMM= 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-415-fR3VtGVkMPKYU73JVMnOig-1; Fri, 20 Nov 2020 04:43:51 -0500 X-MC-Unique: fR3VtGVkMPKYU73JVMnOig-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6B6011835AA2; Fri, 20 Nov 2020 09:43:47 +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 ECAF619D9F; Fri, 20 Nov 2020 09:43:40 +0000 (UTC) Subject: Re: [PATCH v5 00/21] Free some vmemmap pages of hugetlb page To: Michal Hocko Cc: Muchun Song , corbet@lwn.net, mike.kravetz@oracle.com, 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> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Fri, 20 Nov 2020 10:43:40 +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: <20201120093912.GM3200@dhcp22.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Thanks, David / dhildenb