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=-3.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 4C842C433E0 for ; Tue, 16 Mar 2021 09:30:16 +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 E727D65010 for ; Tue, 16 Mar 2021 09:30:14 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E727D65010 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]:48144 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lM61l-0003zF-6h for qemu-devel@archiver.kernel.org; Tue, 16 Mar 2021 05:30:13 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:55962) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lM60n-00038D-Se for qemu-devel@nongnu.org; Tue, 16 Mar 2021 05:29:13 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:29459) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lM60k-0002ua-1A for qemu-devel@nongnu.org; Tue, 16 Mar 2021 05:29:13 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615886949; 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=yS4lZ5+7wIHqixQy+6fZz8H26r35rl0C9UO6y7V5Hm4=; b=bYZ3/lvpbgm4H9mHRhgMWQrEHnQ48GwkZFpqxRlSkrq84v7537uSKOyfIN7p6m+Ljz8YJE QaitHUIyVv2qvKUWYwUOnyJBOW/9fITH9MMJrU04mwyLZ77VO10sPTN7mM+3w/zl8c8tXp DiWYAizGdin1ExfwSlgbDYZHhaVSpWY= 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-138-dMXZuizfNL2kn1jC8D3Mdw-1; Tue, 16 Mar 2021 05:29:06 -0400 X-MC-Unique: dMXZuizfNL2kn1jC8D3Mdw-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 2907F100C660; Tue, 16 Mar 2021 09:29:05 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-112-83.phx2.redhat.com [10.3.112.83]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8B7166A045; Tue, 16 Mar 2021 09:29:01 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 093691132C12; Tue, 16 Mar 2021 10:29:00 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Subject: Re: [RFC PATCH] docs/devel: expand style section of memory management References: <20210315165312.22453-1-alex.bennee@linaro.org> Date: Tue, 16 Mar 2021 10:29:00 +0100 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Mon, 15 Mar 2021 17:09:13 +0000") Message-ID: <877dm78n1f.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: Peter Maydell , Thomas Huth , Alex =?utf-8?Q?Benn=C3=A9e?= , QEMU Developers , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Daniel P. Berrang=C3=A9 writes: > On Mon, Mar 15, 2021 at 06:04:10PM +0100, Thomas Huth wrote: >> On 15/03/2021 17.57, Peter Maydell wrote: >> > On Mon, 15 Mar 2021 at 16:53, Alex Benn=C3=A9e wrote: >> > > -Prefer g_new(T, n) instead of g_malloc(sizeof(T) ``*`` n) for the f= ollowing >> > > +Care should be taken to avoid introducing places where the guest co= uld >> > > +trigger an exit. For example using ``g_malloc`` on start-up is fine >> > > +if the result of a failure is going to be a fatal exit anyway. Ther= e >> > > +may be some start-up cases where failing is unreasonable (for examp= le >> > > +speculatively loading debug symbols). >> > > + >> > > +However if we are doing an allocation because of something the gues= t >> > > +has done we should never trigger an exit. The code may deal with th= is >> > > +by trying to allocate less memory and continue or re-designed to al= locate >> > > +buffers on start-up. >> >=20 >> > I think this is overly strong. We want to avoid malloc-or-die for >> > cases where the guest gets to decide how big the allocation is; >> > but if we're doing a single small fixed-size allocation that happens >> > to be triggered by a guest action we should be OK to g_malloc() that >> > I think. >>=20 >> I agree with Peter. If the host is so much out-of-memory that we even ca= n't >> allocate some few bytes anymore (let's say less than 4k), the system is >> pretty much dead anyway and it might be better to terminate the program >> immediately instead of continuing with the out-of-memory situation. > > On a Linux host you're almost certainly not going to see g_malloc > fail for small allocations at least. Instead at some point the host > will be under enough memory pressure that the OOM killer activates > and reaps arbitrary processes based on some criteria it has, freeing > up memory for malloc to succeed (unless OOM killer picked you as the > victim). This happens even for large allocations. In a prior iteration of the "When it's okay to treat OOM as fatal?" discussion[1], I showed that Linux malloc() and g_malloc() happily give me a terabyte of memory I don't have in 1024 chunks of 1 GiB each. I just reran the test, same results. See also [2]. [1] https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg04229.html [2] http://turnoff.us/geek/bad-malloc/