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[91.12.111.174]) by smtp.gmail.com with ESMTPSA id c7sm5823276ede.37.2021.05.04.04.04.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 May 2021 04:04:18 -0700 (PDT) To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20210428133754.10713-1-david@redhat.com> <20210428133754.10713-10-david@redhat.com> <477b3679-1218-87bb-29d6-9b1b6079ab78@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v7 09/15] util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux Message-ID: Date: Tue, 4 May 2021 13:04:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: 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: -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, 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: 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 , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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. I suggest taking a look at the libvirt virito-mem implemetation -- don't think it's upstream yet: https://lkml.kernel.org/r/cover.1615982004.git.mprivozn@redhat.com I'm CCing Michal -- I already gave him a note upfront which additional properties we might see for memory backends (e.g., reserve, managed-size) and virtio-mem devices (e.g., iothread, prealloc, reserve, prot). > > >> 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 ? Sorry, yes exactly; only for the memory backend managed by a virtio-mem device. -- Thanks, David / dhildenb