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 87532C433B4 for ; Tue, 4 May 2021 11:16:36 +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 9B48D61155 for ; Tue, 4 May 2021 11:16:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9B48D61155 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]:42044 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ldt2Y-0006Gj-Bp for qemu-devel@archiver.kernel.org; Tue, 04 May 2021 07:16:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:40020) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldt1L-0005iz-35 for qemu-devel@nongnu.org; Tue, 04 May 2021 07:15:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:44018) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1ldt1H-0002mu-E6 for qemu-devel@nongnu.org; Tue, 04 May 2021 07:15:18 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620126914; 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=iTQeib5gRSUVgPFrMlJogZ1XWbGnOsi2AYh2TljkYcw=; b=L9FSOYi6BGIxz5F+oG9lrvf8X4mFinoSVR2zzIHKgQ+K4xGxqq/YCjjlTTF9LpbijxY83y 3hCMrMPpyykZS/CibSvBZFJvBA7VEjjuipKdM2ide0kt3b2yfIxlAygY4a9hfJJYzdmROa 8P82CCt5XKOwWSp/7v6L8Ka5oKcdwmU= 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-tGQo6JjCOdqGCknknh5bww-1; Tue, 04 May 2021 07:15:13 -0400 X-MC-Unique: tGQo6JjCOdqGCknknh5bww-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 18880800C7A; Tue, 4 May 2021 11:15:12 +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 DC8DB5D9C0; Tue, 4 May 2021 11:14:51 +0000 (UTC) Date: Tue, 4 May 2021 12:14:48 +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: User-Agent: Mutt/2.0.6 (2021-03-06) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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=-0.697, 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" , Michal Privoznik , 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 01:04:17PM +0200, David Hildenbrand wrote: > On 04.05.21 12:32, Daniel P. Berrangé wrote: > > 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 ? > > Roughly, yes. We only special-case memory backends managed by virtio-mem devices. > > An advanced example for a single VM could look like this: > > sudo build/qemu-system-x86_64 \ > ... \ > -m 4G,maxmem=64G \ > -smp sockets=2,cores=2 \ > -object hostmem-memfd,id=bmem0,size=2G,hugetlb=yes,hugetlbsize=2M \ > -numa node,nodeid=0,cpus=0-1,memdev=bmem0 \ > -object hostmem-memfd,id=bmem1,size=2G,hugetlb=yes,hugetlbsize=2M \ > -numa node,nodeid=1,cpus=2-3,memdev=bmem1 \ > ... \ > -object hostmem-memfd,id=mem0,size=30G,hugetlb=yes,hugetlbsize=2M,reserve=off \ > -device virtio-mem-pci,id=vmem0,memdev=mem0,node=0,requested-size=0G \ > -object hostmem-memfd,id=mem1,size=30G,hugetlb=yes,hugetlbsize=2M,reserve=off \ > -device virtio-mem-pci,id=vmem1,memdev=mem1,node=1,requested-size=0G \ > ... \ > > We can request a size change by adjusting the "requested-size" property (e.g., via qom-set) > and observe the current size by reading the "size" property (e.g., qom-get). Think of > it as an advanced device-local memory balloon mixed with the concept of a memory hotplug. Ok, so in this example, the initial GB of RAM has normal reserve=on so if there's insufficient hugepages we'll see the startup failure IIUC. What happens when we set qom-set requested-size=10GB at runtime, but there are only 8 GB of hugepages left available ? 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 :|