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,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 F18DAC433B4 for ; Tue, 4 May 2021 10:28:03 +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 9277361182 for ; Tue, 4 May 2021 10:28:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9277361182 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]:53590 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldsHa-0008Uh-Mb for qemu-devel@archiver.kernel.org; Tue, 04 May 2021 06:28:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:56276) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lds0g-0003n5-68 for qemu-devel@nongnu.org; Tue, 04 May 2021 06:10:34 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:50587) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lds0e-0007TX-4r for qemu-devel@nongnu.org; Tue, 04 May 2021 06:10:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620123031; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=wWf0xemB/j9F0y6QEWWJKy+0UNllXLsrvDT0x9z7U1I=; b=fg/3DMUFprYraC2HtDsbfB0WPf/4YEgvAVAsPRaQv047FQLgrSAkq1Ic8uOP2DUOT2K7Ay nsjCuvO60PQWghmEMltXlQBTKaAsUWO7BezOE7xloD/wXWf/GmXsBIVCr+8Y9Et7pfANHA YrrGXB6YqYAN1RfdSBF8zYmI/Ic10co= 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-229-yv3QPmhAMFSQXhGhSUQxkg-1; Tue, 04 May 2021 06:10:25 -0400 X-MC-Unique: yv3QPmhAMFSQXhGhSUQxkg-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 0BCEC8030D5; Tue, 4 May 2021 10:10:24 +0000 (UTC) Received: from redhat.com (ovpn-113-37.ams2.redhat.com [10.36.113.37]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8263C1F051; Tue, 4 May 2021 10:10:01 +0000 (UTC) Date: Tue, 4 May 2021 11:09:58 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: David Hildenbrand Subject: Re: [PATCH v7 09/15] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux Message-ID: References: <20210428133754.10713-1-david@redhat.com> <20210428133754.10713-10-david@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210428133754.10713-10-david@redhat.com> User-Agent: Mutt/2.0.6 (2021-03-06) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=berrange@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.698, 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Marcel Apfelbaum , Eduardo Habkost , "Michael S. Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, Peter Xu , "Dr. David Alan Gilbert" , Greg Kurz , Paolo Bonzini , Stefan Hajnoczi , Murilo Opsfelder Araujo , Igor Mammedov , Nitesh Lal , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Apr 28, 2021 at 03:37:48PM +0200, David Hildenbrand wrote: > Let's support RAM_NORESERVE via MAP_NORESERVE on Linux. The flag has no > effect on most shared mappings - except for hugetlbfs and anonymous memory. > > Linux man page: > "MAP_NORESERVE: Do not reserve swap space for this mapping. When swap > space is reserved, one has the guarantee that it is possible to modify > the mapping. When swap space is not reserved one might get SIGSEGV > upon a write if no physical memory is available. See also the discussion > of the file /proc/sys/vm/overcommit_memory in proc(5). In kernels before > 2.6, this flag had effect only for private writable mappings." > > Note that the "guarantee" part is wrong with memory overcommit in Linux. > > Also, in Linux hugetlbfs is treated differently - we configure reservation > of huge pages from the pool, not reservation of swap space (huge pages > cannot be swapped). > > The rough behavior is [1]: > a) !Hugetlbfs: > > 1) Without MAP_NORESERVE *or* with memory overcommit under Linux > disabled ("/proc/sys/vm/overcommit_memory == 2"), the following > accounting/reservation happens: > For a file backed map > SHARED or READ-only - 0 cost (the file is the map not swap) > PRIVATE WRITABLE - size of mapping per instance > > For an anonymous or /dev/zero map > SHARED - size of mapping > PRIVATE READ-only - 0 cost (but of little use) > PRIVATE WRITABLE - size of mapping per instance > > 2) With MAP_NORESERVE, no accounting/reservation happens. > > b) Hugetlbfs: > > 1) Without MAP_NORESERVE, huge pages are reserved. > > 2) With MAP_NORESERVE, no huge pages are reserved. > > Note: With "/proc/sys/vm/overcommit_memory == 0", we were already able > to configure it for !hugetlbfs globally; this toggle now allows > configuring it more fine-grained, not for the whole system. > > The target use case is virtio-mem, which dynamically exposes memory > inside a large, sparse memory area to the VM. Can you explain this use case in more real world terms, as I'm not understanding what a mgmt app would actually do with this in practice ? Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|