On 02.03.20 14:49, David Hildenbrand wrote: > This series is based on latest linux-next. The patches are located at: > https://github.com/davidhildenbrand/linux.git virtio-mem-v1 > > The basic idea of virtio-mem is to provide a flexible, > cross-architecture memory hot(un)plug solution that avoids many limitations > imposed by existing technologies, architectures, and interfaces. More > details can be found below and in linked material. > > It's currently only enabled for x86-64, however, should theoretically work > on any architecture that supports virtio and implements memory hot(un)plug > under Linux - like s390x, powerpc64, and arm64. On x86-64, it is currently > possible to add/remove memory to the system in >= 4MB granularity. > Memory hotplug works very reliably. For memory unplug, there are no > guarantees how much memory can actually get unplugged, it depends on the > setup (especially: fragmentation of physical memory). > > I am currently getting the QEMU side into shape (which will be posted as > RFC soon, see below for a link to the current state). Experimental Kata > support is in the works [4]. Also, a cloud-hypervisor implementation is > under discussion [5]. > > -------------------------------------------------------------------------- > 1. virtio-mem > -------------------------------------------------------------------------- > > The basic idea behind virtio-mem was presented at KVM Forum 2018. The > slides can be found at [1]. The previous RFC can be found at [2]. The > first RFC can be found at [3]. However, the concept evolved over time. The > KVM Forum slides roughly match the current design. > > Patch #2 ("virtio-mem: Paravirtualized memory hotplug") contains quite some > information, especially in "include/uapi/linux/virtio_mem.h": > > Each virtio-mem device manages a dedicated region in physical address > space. Each device can belong to a single NUMA node, multiple devices > for a single NUMA node are possible. A virtio-mem device is like a > "resizable DIMM" consisting of small memory blocks that can be plugged > or unplugged. The device driver is responsible for (un)plugging memory > blocks on demand. > > Virtio-mem devices can only operate on their assigned memory region in > order to (un)plug memory. A device cannot (un)plug memory belonging to > other devices. > > The "region_size" corresponds to the maximum amount of memory that can > be provided by a device. The "size" corresponds to the amount of memory > that is currently plugged. "requested_size" corresponds to a request > from the device to the device driver to (un)plug blocks. The > device driver should try to (un)plug blocks in order to reach the > "requested_size". It is impossible to plug more memory than requested. > > The "usable_region_size" represents the memory region that can actually > be used to (un)plug memory. It is always at least as big as the > "requested_size" and will grow dynamically. It will only shrink when > explicitly triggered (VIRTIO_MEM_REQ_UNPLUG). > > There are no guarantees what will happen if unplugged memory is > read/written. Such memory should, in general, not be touched. E.g., > even writing might succeed, but the values will simply be discarded at > random points in time. > > It can happen that the device cannot process a request, because it is > busy. The device driver has to retry later. > > Usually, during system resets all memory will get unplugged, so the > device driver can start with a clean state. However, in specific > scenarios (if the device is busy) it can happen that the device still > has memory plugged. The device driver can request to unplug all memory > (VIRTIO_MEM_REQ_UNPLUG) - which might take a while to succeed if the > device is busy. > > -------------------------------------------------------------------------- > 2. Linux Implementation > -------------------------------------------------------------------------- > > Memory blocks (e.g., 128MB) are added/removed on demand. Within these > memory blocks, subblocks (e.g., 4MB) are plugged/unplugged. The sizes > depend on the target architecture, MAX_ORDER, pageblock_order, and > the block size of a virtio-mem device. > > add_memory()/try_remove_memory() is used to add/remove memory blocks. > virtio-mem will not online memory blocks itself. This has to be done by > user space, or configured into the kernel > (CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE). virtio-mem will only unplug memory > that was online to the ZONE_NORMAL. Memory is suggested to be onlined to > the ZONE_NORMAL for now. > > The memory hotplug notifier is used to properly synchronize against > onlining/offlining of memory blocks and to track the states of memory > blocks (including the zone memory blocks are onlined to). > > The set_online_page() callback is used to keep unplugged subblocks > of a memory block fake-offline when onlining the memory block. > generic_online_page() is used to fake-online plugged subblocks. This > handling is similar to the Hyper-V balloon driver. > > PG_offline is used to mark unplugged subblocks as offline, so e.g., > dumping tools (makedumpfile) will skip these pages. This is similar to > other balloon drivers like virtio-balloon and Hyper-V. > > Memory offlining code is extended to allow drivers to drop their reference > to PG_offline pages when MEM_GOING_OFFLINE, so these pages can be skipped > when offlining memory blocks. This allows to offline memory blocks that > have partially unplugged (allocated e.g., via alloc_contig_range()) > subblocks - or are completely unplugged. > > alloc_contig_range()/free_contig_range() [now exposed] is used to > unplug/plug subblocks of memory blocks the are already exposed to Linux. > > offline_and_remove_memory() [new] is used to offline a fully unplugged > memory block and remove it from Linux. > > -------------------------------------------------------------------------- > 3. Changes RFC v4 -> v1 > -------------------------------------------------------------------------- > > Only minor things changed, especially, nothing on the virtio side. > - "virtio-mem: Paravirtualized memory hotplug" > -- Fix compilation without CONFIG_ACPI_NUMA > -- Minor code simplifications > -- Better lockdep handling > -- Fix retry handling when getting a config update while processing work > - "virtio-mem: Paravirtualized memory hotunplug part 1" > - "virtio-mem: Paravirtualized memory hotunplug part 2" > -- Unplug memory from highest to lowest, as we plug from lowest to highest > - "mm: Allow to offline unmovable PageOffline() pages via > MEM_GOING_OFFLINE" > -- Optimized comments/description > - "mm/memory_hotplug: Introduce offline_and_remove_memory()" > -- Rephrased description > - Drop the drop_slab() functionality for now > - Added "MAINTAINERS: Add myself as virtio-mem maintainer" > - Fixed many spelling issues. checkpatch mostly complains about BUG_ONs > and two macros, which is fine. > > -------------------------------------------------------------------------- > 4. Future work > -------------------------------------------------------------------------- > > virtio-mem extensions (via new feature flags): > - Indicate the guest status (e.g., initialized, working, all memory is > busy when unplugging, too many memory blocks are offline when plugging, > etc.) > - Guest-triggered shrinking of the usable region (e.g., whenever the > highest memory block is removed). > - Exchange of plugged<->unplugged block for defragmentation. > > Memory hotplug: > - Reduce the amount of memory resources if that tunes out to be an > issue. Or try to speed up relevant code paths to deal with many > resources. > - Allocate vmemmap from added memory. > > Memory hotunplug: > - Performance improvements: > -- Sense (lockless) if it make sense to try alloc_contig_range() at all > before directly trying to isolate and taking locks. > -- Try to unplug bigger chunks within a memory block first. > - Make unplug more likely to succeed: > -- There are various idea to limit fragmentation on memory block > granularity. (e.g., ZONE_PREFER_MOVABLE and smart balancing) > -- Allocate vmemmap from added memory. > - OOM handling, e.g., via an OOM handler. > - Defragmentation > > -------------------------------------------------------------------------- > 5. Example Usage > -------------------------------------------------------------------------- > > A QEMU implementation (without protection of unplugged memory, but with > resizable memory regions and optimized migration) is available at (kept > updated): > https://github.com/davidhildenbrand/qemu.git virtio-mem > > Start QEMU with two virtio-mem devices (one per NUMA node): > $ qemu-system-x86_64 -m 4G,maxmem=204G \ > -smp sockets=2,cores=2 \ > -numa node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 \ > [...] > -object memory-backend-ram,id=mem0,size=100G,managed-size=on \ > -device virtio-mem-pci,id=vm0,memdev=mem0,node=0,requested-size=0M \ > -object memory-backend-ram,id=mem1,size=100G,managed-size=on \ > -device virtio-mem-pci,id=vm1,memdev=mem1,node=1,requested-size=1G > > Query the configuration: > QEMU 4.2.50 monitor - type 'help' for more information > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 0 > size: 0 > max-size: 107374182400 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x1a40000000 > node: 1 > requested-size: 1073741824 > size: 1073741824 > max-size: 107374182400 > block-size: 2097152 > memdev: /objects/mem1 > > Add some memory to node 0: > QEMU 4.2.50 monitor - type 'help' for more information > (qemu) qom-set vm0 requested-size 1G > > Remove some memory from node 1: > QEMU 4.2.50 monitor - type 'help' for more information > (qemu) qom-set vm1 requested-size 64M > > Query the configuration again: > QEMU 4.2.50 monitor - type 'help' for more information > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 1073741824 > size: 1073741824 > max-size: 107374182400 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x1a40000000 > node: 1 > requested-size: 67108864 > size: 67108864 > max-size: 107374182400 > block-size: 2097152 > memdev: /objects/mem1 > > -------------------------------------------------------------------------- > 6. Q/A > -------------------------------------------------------------------------- > > Q: Why add/remove parts ("subblocks") of memory blocks/sections? > A: Flexibility (section size depends on the architecture) - e.g., some > architectures have a section size of 2GB. Also, the memory block size > is variable (e.g., on x86-64). I want to avoid any such restrictions. > Some use cases want to add/remove memory in smaller granularity to a > VM (e.g., the Hyper-V balloon also implements this) - especially smaller > VMs like used for kata containers. Also, on memory unplug, it is more > reliable to free-up and unplug multiple small chunks instead > of one big chunk. E.g., if one page of a DIMM is either unmovable or > pinned, the DIMM can't get unplugged. This approach is basically a > compromise between DIMM-based memory hot(un)plug and balloon > inflation/deflation, which works mostly on page granularity. > > Q: Why care about memory blocks? > A: They are the way to tell user space about new memory. This way, > memory can get onlined/offlined by user space. Also, e.g., kdump > relies on udev events to reload kexec when memory blocks are > onlined/offlined. Memory blocks are the "real" memory hot(un)plug > granularity. Everything that's smaller has to be emulated "on top". > > Q: Won't memory unplug of subblocks fragment memory? > A: Yes and no. Unplugging e.g., >=4MB subblocks on x86-64 will not really > fragment memory like unplugging random pages like a balloon driver does. > Buddy merging will not be limited. However, any allocation that requires > bigger consecutive memory chunks (e.g., gigantic pages) might observe > the fragmentation. Possible solutions: Allocate gigantic huge pages > before unplugging memory, don't unplug memory, combine virtio-mem with > DIMM based memory or bigger initial memory. Remember, a virtio-mem > device will only unplug on the memory range it manages, not on other > DIMMs. Unplug of single memory blocks will result in similar > fragmentation in respect to gigantic huge pages. > > Q: How reliable is memory unplug? > A: There are no guarantees on how much memory can get unplugged > again. However, it is more likely to find 4MB chunks to unplug than > e.g., 128MB chunks. If memory is terribly fragmented, there is nothing > we can do - for now. I consider memory hotplug the first primary use > of virtio-mem. Memory unplug might usually work, but we want to improve > the performance and the amount of memory we can actually unplug later. > > Q: Why not unplug from the ZONE_MOVABLE? > A: Unplugged memory chunks are unmovable. Unmovable data must not end up > on the ZONE_MOVABLE - similar to gigantic pages - they will never be > allocated from ZONE_MOVABLE. virtio-mem added memory can be onlined > to the ZONE_MOVABLE, but subblocks will not get unplugged from it. > > Q: How big should the initial (!virtio-mem) memory of a VM be? > A: virtio-mem memory will not go to the DMA zones. So to avoid running out > of DMA memory, I suggest something like 2-3GB on x86-64. But many > VMs can most probably deal with less DMA memory - depends on the use > case. > > [1] https://events.linuxfoundation.org/wp-content/uploads/2017/12/virtio-mem-Paravirtualized-Memory-David-Hildenbrand-Red-Hat-1.pdf > [2] https://lkml.kernel.org/r/20190919142228.5483-1-david@xxxxxxxxxx > [3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d@xxxxxxxxxx > [4] https://github.com/kata-containers/documentation/pull/592 > [5] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/837 > > Cc: Sebastien Boeuf <sebastien.boeuf@xxxxxxxxx> > Cc: Samuel Ortiz <samuel.ortiz@xxxxxxxxx> > Cc: Robert Bradford <robert.bradford@xxxxxxxxx> > Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx> > Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx> > Cc: teawater <teawaterz@xxxxxxxxxxxxxxxxx> > Cc: Igor Mammedov <imammedo@xxxxxxxxxx> > Cc: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> > > David Hildenbrand (11): > ACPI: NUMA: export pxm_to_node > virtio-mem: Paravirtualized memory hotplug > virtio-mem: Paravirtualized memory hotunplug part 1 > mm: Export alloc_contig_range() / free_contig_range() > virtio-mem: Paravirtualized memory hotunplug part 2 > mm: Allow to offline unmovable PageOffline() pages via > MEM_GOING_OFFLINE > virtio-mem: Allow to offline partially unplugged memory blocks > mm/memory_hotplug: Introduce offline_and_remove_memory() > virtio-mem: Offline and remove completely unplugged memory blocks > virtio-mem: Better retry handling > MAINTAINERS: Add myself as virtio-mem maintainer As requested by Michal, I will squash some patches. I'll pull out the NID thingies from "virtio-mem: Paravirtualized memory hotplug" and squash "ACPI: NUMA: export pxm_to_node" into that change. Also, I'll squash "mm: Export alloc_contig_range() / free_contig_range()" into "virtio-mem: Paravirtualized memory hotunplug part 2". I'll not squash the other, more involved, MM patches. Thanks! -- Thanks, David / dhildenb
