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 C80CDC433B4 for ; Tue, 4 May 2021 10:38:23 +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 05A25611AB for ; Tue, 4 May 2021 10:38:22 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 05A25611AB 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]:50272 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldsRX-00024m-TC for qemu-devel@archiver.kernel.org; Tue, 04 May 2021 06:38:19 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:60844) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldsMj-0005Yg-Kg for qemu-devel@nongnu.org; Tue, 04 May 2021 06:33:21 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:46855) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldsMg-0003I4-Hy for qemu-devel@nongnu.org; Tue, 04 May 2021 06:33:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620124397; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=4v4ejs3Xmr16ZOCSAi/hIYGNTs7jwOdknDmrkb0oOFY=; b=KOjMR+5DgI8bQINXeXZ5kqIAy2IBdgjrfospANBcdzQ7xkLzRvTahZ2aM4Seu3nv+taX0c llT+5ZP08Vc4xgB3A7wlHiBKWqks0132ChUCEnRjMb7ieiqEn7zn/ogGyW22Zo79tPO35Y 935JY28JfzlIibbMvAcYE0jfp9gMO3U= 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-483-4-E-Z6FqNqeLbGThGqYmWA-1; Tue, 04 May 2021 06:33:13 -0400 X-MC-Unique: 4-E-Z6FqNqeLbGThGqYmWA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8FBF7107ACE8; Tue, 4 May 2021 10:33:12 +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 30F5D60C0F; Tue, 4 May 2021 10:32:50 +0000 (UTC) Date: Tue, 4 May 2021 11:32:47 +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> <477b3679-1218-87bb-29d6-9b1b6079ab78@redhat.com> MIME-Version: 1.0 In-Reply-To: <477b3679-1218-87bb-29d6-9b1b6079ab78@redhat.com> User-Agent: Mutt/2.0.6 (2021-03-06) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 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 Content-Transfer-Encoding: 8bit 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 Tue, May 04, 2021 at 12:21:25PM +0200, David Hildenbrand wrote: > On 04.05.21 12:09, Daniel P. Berrangé wrote: > > 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 ? > > Let's consider huge pages for simplicity. Assume you have 128 free huge > pages in your hypervisor that you want to dynamically assign to VMs. > > Further assume you have two VMs running. A workflow could look like > > 1. Assign all huge pages to VM 0 > 2. Reassign 64 huge pages to VM 1 > 3. Reassign another 32 huge pages to VM 1 > 4. Reasssign 16 huge pages to VM 0 > 5. ... > > Basically what we're used to doing with "ordinary" memory. What does this look like in terms of the memory backend configuration when you boot VM 0 and VM 1 ? Are you saying that we boot both VMs with -object hostmem-memfd,size=128G,hugetlb=yes,hugetlbsize=1G,reserve=off and then we have another property set on 'virtio-mem' to tell it how much/little of that 128 G, to actually give to the guest ? How do we change that at runtime ? > For that to work with virtio-mem, you'll have to disable reservation of huge > pages for the virtio-mem managed memory region. > > (prealloction of huge pages in virtio-mem to protect from user mistakes is a > separate work item) > > reserve=off will be the default for virtio-mem, and actual > reservation/preallcoation will be done within virtio-mem. There could be use > for "reserve=off" for virtio-balloon use cases as well, but I'd like to > exclude that from the discussion for now. The hostmem backend defaults are indepdant of frontend usage, so when you say reserve=off is the default for virtio-mem, are you expecting the mgmt app like libvirt to specify that ? 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 :|