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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 0525EC433E0 for ; Tue, 2 Mar 2021 19:34:41 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6CB2A64F2C for ; Tue, 2 Mar 2021 19:34:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6CB2A64F2C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:43432 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lHAn0-0003Q8-R1 for qemu-devel@archiver.kernel.org; Tue, 02 Mar 2021 14:34:38 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:48076) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lHAHG-0007u1-Hk for qemu-devel@nongnu.org; Tue, 02 Mar 2021 14:01:50 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:50294) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1lHAHE-0000ES-Oe for qemu-devel@nongnu.org; Tue, 02 Mar 2021 14:01:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614711707; 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=1Kq8ImMfn1vW9nImGYK9skXurogm4MKw3gVNZIccGfQ=; b=fmT1Q9M4w6nSj9lZoeIiw/iItXcsWhrgcQsoYoRYavZ2w2B2j1Yy8ANl3kQspQZ+P5RxLH 6rs6sx9iRQaiuPR6vTtWcKpiAlQuyKII2tQQMC9HQb6wefDo6aadd2RDND/m2cflPDPeFK x2UlQKyXK/rapfmx5JN0uKVsV77Nbw0= 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-76-uYiO4hBbOeanzVFNy2AgeA-1; Tue, 02 Mar 2021 14:01:45 -0500 X-MC-Unique: uYiO4hBbOeanzVFNy2AgeA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0E34179EC0; Tue, 2 Mar 2021 19:01:44 +0000 (UTC) Received: from [10.36.114.189] (ovpn-114-189.ams2.redhat.com [10.36.114.189]) by smtp.corp.redhat.com (Postfix) with ESMTP id E7FC061F20; Tue, 2 Mar 2021 19:01:11 +0000 (UTC) To: Peter Xu References: <20210209134939.13083-1-david@redhat.com> <20210209134939.13083-9-david@redhat.com> <20210302175112.GN397383@xz-x1> From: David Hildenbrand Organization: Red Hat GmbH Subject: Re: [PATCH v1 8/9] util/mmap-alloc: support RAM_NORESERVE via MAP_NORESERVE Message-ID: <522c672e-9c16-48ef-24a8-3687b5332b2a@redhat.com> Date: Tue, 2 Mar 2021 20:01:11 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210302175112.GN397383@xz-x1> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Thomas Huth , Cornelia Huck , Eduardo Habkost , "Michael S. Tsirkin" , Stefan Weil , Murilo Opsfelder Araujo , Richard Henderson , "Dr. David Alan Gilbert" , Juan Quintela , qemu-devel@nongnu.org, Halil Pasic , Christian Borntraeger , Greg Kurz , Stefan Hajnoczi , Igor Mammedov , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Igor Kotrasinski Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 02.03.21 18:51, Peter Xu wrote: > On Tue, Feb 09, 2021 at 02:49:38PM +0100, David Hildenbrand wrote: >> +#define OVERCOMMIT_MEMORY_PATH "/proc/sys/vm/overcommit_memory" >> +static bool map_noreserve_effective(int fd, bool shared) >> +{ >> +#if defined(__linux__) >> + gchar *content = NULL; >> + const char *endptr; >> + unsigned int tmp; >> + >> + /* hugetlbfs behaves differently */ >> + if (qemu_fd_getpagesize(fd) != qemu_real_host_page_size) { >> + return true; >> + } >> + >> + /* only private shared mappings are accounted (ignoring /dev/zero) */ >> + if (fd != -1 && shared) { >> + return true; >> + } >> + >> + if (g_file_get_contents(OVERCOMMIT_MEMORY_PATH, &content, NULL, NULL) && >> + !qemu_strtoui(content, &endptr, 0, &tmp) && >> + (!endptr || *endptr == '\n')) { >> + if (tmp == 2) { >> + error_report("Skipping reservation of swap space is not supported: " >> + " \"" OVERCOMMIT_MEMORY_PATH "\" is \"2\""); >> + return false; >> + } >> + return true; >> + } >> + /* this interface has been around since Linux 2.6 */ >> + error_report("Skipping reservation of swap space is not supported: " >> + " Could not read: \"" OVERCOMMIT_MEMORY_PATH "\""); >> + return false; >> +#else >> + return true; >> +#endif >> +} > > I feel like this helper wants to fail gracefully for some conditions. Could > you elaborate one example and attach to the commit log? Sure. The case is "/proc/sys/vm/overcommit_memory == 2" (never overcommit) MAP_NORESERVE is without effect and sparse memory regions are somewhat impossible. > > I'm also wondering whether it would worth to check the global value. Even if > overcommit is globally disabled, do we (as an application process) need to care > about it? I think the MAP_NORESERVE would simply be silently ignored by the > kernel and that seems to be design of it, otherwise would all apps who uses > MAP_NORESERVE would need to do similar things too? Right, I want to catch the "gets silently ignored" part, because someone requested "reserved=off" (!default) but does not actually get what he asked for. As one example, glibc manages heaps via: a) Creating a new heap: mmap(PROT_NONE, MAP_NORESERVE) the maximum size, then mprotect(PROT_READ|PROT_WRITE) the initial heap size. Even if MAP_NORESERVE is ignored, only !PROT_NONE memory ever gets committed ("reserve swap space") in Linux. b) Growing the heap via mprotect(PROT_READ|PROT_WRITE) within the existing mmap. This will commit memory in case MAP_NORESERVE got ignored. c) Shrinking the heap ("discard memory") via MADV_DONTNEED *unless* "/proc/sys/vm/overcommit_memory == 2" - the only way to undo mprotect(PROT_READ|PROT_WRITE) and to un-commit memory is by doing a mmap(PROT_NONE, MAP_FIXED) over the problematic region. If you're interested, you can take a look at: malloc/arena.c sysdeps/unix/sysv/linux/malloc-sysdep.h:check_may_shrink_heap() Thanks! -- Thanks, David / dhildenb