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=-5.4 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 CA482C4363A for ; Wed, 21 Oct 2020 09:50:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5EF5322249 for ; Wed, 21 Oct 2020 09:50:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Xoo8azTa" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404801AbgJUJu5 (ORCPT ); Wed, 21 Oct 2020 05:50:57 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:34850 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2438182AbgJUJu4 (ORCPT ); Wed, 21 Oct 2020 05:50:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603273855; 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=AXksiluvhYqlJrwQaYz1rE1CqU+tBOKw7Qr8hWu6eF8=; b=Xoo8azTaE0A4dZtrxnY+bRB9oTGiegx9Iy/HcsgNsv4pwRLT/uIyiMplFx+yA/kCnJMzDD dWdhCeSGy+M3mbBFMY7wxL8skV1I1GKt7ZSzXK1SPRxQHPB1sxRJyU8a7ZL7uc8Eb2aXd7 C/o6f43huG1qGQTCg51fYyTd1UvyD0E= 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-383-qzseyaqoPSGju6oe_YyKmA-1; Wed, 21 Oct 2020 05:50:54 -0400 X-MC-Unique: qzseyaqoPSGju6oe_YyKmA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2D85D57088; Wed, 21 Oct 2020 09:50:52 +0000 (UTC) Received: from [10.36.114.138] (ovpn-114-138.ams2.redhat.com [10.36.114.138]) by smtp.corp.redhat.com (Postfix) with ESMTP id 12EDE5B4B2; Wed, 21 Oct 2020 09:50:48 +0000 (UTC) Subject: Re: [PATCH] mm, hugetlb: Avoid double clearing for hugetlb pages To: Michal Hocko , "Guilherme G. Piccoli" Cc: Mike Kravetz , linux-mm@kvack.org, kernel-hardening@lists.openwall.com, linux-hardening@vger.kernel.org, linux-security-module@vger.kernel.org, kernel@gpiccoli.net, cascardo@canonical.com, Alexander Potapenko , James Morris , Kees Cook References: <20201019182853.7467-1-gpiccoli@canonical.com> <20201020082022.GL27114@dhcp22.suse.cz> <9cecd9d9-e25c-4495-50e2-8f7cb7497429@canonical.com> <20201021061538.GA23790@dhcp22.suse.cz> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <0ad2f879-7c72-3eef-5cb6-dee44265eb82@redhat.com> Date: Wed, 21 Oct 2020 11:50:48 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <20201021061538.GA23790@dhcp22.suse.cz> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On 21.10.20 08:15, Michal Hocko wrote: > On Tue 20-10-20 16:19:06, Guilherme G. Piccoli wrote: >> On 20/10/2020 05:20, Michal Hocko wrote: >>> >>> Yes zeroying is quite costly and that is to be expected when the feature >>> is enabled. Hugetlb like other allocator users perform their own >>> initialization rather than go through __GFP_ZERO path. More on that >>> below. >>> >>> Could you be more specific about why this is a problem. Hugetlb pool is >>> usualy preallocatd once during early boot. 24s for 65GB of 2MB pages >>> is non trivial amount of time but it doens't look like a major disaster >>> either. If the pool is allocated later it can take much more time due to >>> memory fragmentation. >>> >>> I definitely do not want to downplay this but I would like to hear about >>> the real life examples of the problem. >> >> Indeed, 24s of delay (!) is not so harmful for boot time, but...64G was >> just my simple test in a guest, the real case is much worse! It aligns >> with Mike's comment, we have complains of minute-like delays, due to a >> very big pool of hugepages being allocated. > > The cost of page clearing is mostly a constant overhead so it is quite > natural to see the time scaling with the number of pages. That overhead > has to happen at some point of time. Sure it is more visible when > allocating during boot time resp. when doing pre-allocation during > runtime. The page fault path would be then faster. The overhead just > moves to a different place. So I am not sure this is really a strong > argument to hold. We have people complaining that starting VMs backed by hugetlbfs takes too long, they would much rather have that initialization be done when booting the hypervisor ... so looks like there is no right or wrong. -- Thanks, David / dhildenb