On 12.12.19 18:11, David Hildenbrand wrote: > This series is based on latest linux-next. The patches are located at: > https://github.com/davidhildenbrand/linux.git virtio-mem-rfc-v4 > > 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. > > This RFC is limited to 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 (unmovable) memory). I have plans to > improve that in the future. > > -------------------------------------------------------------------------- > 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). > > Memory in the usable region can usually be read, however, there are no > guarantees. 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 > -------------------------------------------------------------------------- > > This RFC reuses quite some existing MM infrastructure, however, has to > expose some additional functionality. > > 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. > > > A lot of additional information can be found in the separate patches and > as comments in the code itself. > > -------------------------------------------------------------------------- > 3. Changes RFC v2 -> v3 > -------------------------------------------------------------------------- > > A lot of things changed, especially also on the QEMU + virtio side. The > biggest changes on the Linux driver side are: > - Onlining/offlining of subblocks is now emulated on top of memory blocks. > set_online_page()+alloc_contig_range()+free_contig_range() is now used > for that. Core MM does not have to be modified and will continue to > online/offline full memory blocks. > - Onlining/offlining of memory blocks is no longer performed by virtio-mem. > - Pg_offline is upstream and can be used. It is also used to allow > offlining of partially unplugged memory blocks. > - Memory block states + subblocks are now tracked more space-efficient. > - Proper kexec(), kdump(), driver unload, driver reload, ZONE_MOVABLE, ... > handling. > > -------------------------------------------------------------------------- > 4. Changes RFC v3 -> RFC v4 > -------------------------------------------------------------------------- > > Only minor things changed, especially nothing on the QEMU + virtio side. > Interresting changes on the Linux driver side are: > - "mm: Allow to offline unmovable PageOffline() pages via > MEM_GOING_OFFLINE" > -- Rework to Michals suggestion (allow to isolate all PageOffline() pages > by skipping all PageOffline() pages in has_unmovable_pages(). Fail > offlining later if the pages cannot be offlined/migrated). > - "virtio-mem: Allow to offline partially unplugged memory blocks" > -- Adapt to Michals suggestion on core-mm part. > - "virtio-mem: Better retry handling" > -- Optimize retry intervals > - "virtio-mem: Drop slab objects when unplug continues to fail" > -- Call drop_slab()/drop_slab_node() when unplug keeps failing for a longer > time. > - Multiple cleanups and fixes. > > -------------------------------------------------------------------------- > 5. Future work > -------------------------------------------------------------------------- > > The separate patches contain a lot of future work items. One of the next > steps is to make memory unplug more likely to succeed - currently, there > are no guarantees on how much memory can get unplugged again. I have > various ideas on how to limit fragmentation of all memory blocks that > virtio-mem added. > > Memory hotplug: > - Reduce the amount of memory resources if that turnes out to be an > issue. Or try to speed up relevant code paths to deal with many > resources. > - Allocate the vmemmap from the added memory. Makes hotplug more likely > to succeed, the vmemmap is stored on the same NUMA node and that > unmovable memory will later not hinder unplug. > > 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 if possible first. > -- Identify free areas first, that don't have to be evacuated. > - 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 memmap from added memory. This way, less unmovable data can > end up on the memory blocks. > - OOM handling, e.g., via an OOM handler. > - Defragmentation > -- Will require a new virtio-mem CMD to exchange plugged<->unplugged blocks > > -------------------------------------------------------------------------- > 6. Example Usage > -------------------------------------------------------------------------- > > A very basic QEMU prototype (kept updated) is available at: > https://github.com/davidhildenbrand/qemu.git virtio-mem > > It lacks various features, however, works to test the guest driver side: > - No support for resizable memory regions / memory backends yet > - No protection of unplugged memory (esp., userfaultfd-wp) yet > - No dump/migration/XXX optimizations to skip unplugged memory (and avoid > touching it) > > Start QEMU with two virtio-mem devices (one per NUMA node): > $ qemu-system-x86_64 -m 4G,maxmem=20G \ > -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=8G \ > -device virtio-mem-pci,id=vm0,memdev=mem0,node=0,requested-size=128M \ > -object memory-backend-ram,id=mem1,size=8G \ > -device virtio-mem-pci,id=vm1,memdev=mem1,node=1,requested-size=80M > > Query the configuration: > QEMU 4.1.95 monitor - type 'help' for more information > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 134217728 > size: 134217728 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x340000000 > node: 1 > requested-size: 83886080 > size: 83886080 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem1 > > Add some memory to node 1: > QEMU 4.1.95 monitor - type 'help' for more information > (qemu) qom-set vm1 requested-size 1G > > Remove some memory from node 0: > QEMU 4.1.95 monitor - type 'help' for more information > (qemu) qom-set vm0 requested-size 64M > > Query the configuration again: > (qemu) info memory-devices > Memory device [virtio-mem]: "vm0" > memaddr: 0x140000000 > node: 0 > requested-size: 67108864 > size: 67108864 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem0 > Memory device [virtio-mem]: "vm1" > memaddr: 0x340000000 > node: 1 > requested-size: 1073741824 > size: 1073741824 > max-size: 8589934592 > block-size: 2097152 > memdev: /objects/mem1 > > -------------------------------------------------------------------------- > 7. 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 granularities 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. I ahve plans for a > virtio-mem defragmentation feature in the future. > > 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 > > Cc: Sebastien Boeuf <sebastien.boeuf@xxxxxxxxx> > Cc: Samuel Ortiz <samuel.ortiz@xxxxxxxxx> > Cc: Robert Bradford <robert.bradford@xxxxxxxxx> > Cc: Luiz Capitulino <lcapitulino@xxxxxxxxxx> > > David Hildenbrand (13): > 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 > mm/vmscan: Move count_vm_event(DROP_SLAB) into drop_slab() > mm/vmscan: Export drop_slab() and drop_slab_node() > virtio-mem: Drop slab objects when unplug continues to fail Ping, I'd love to get some feedback on a) The remaining MM bits from MM folks (especially, patch #6 and #8). b) The general virtio infrastructure (esp. uapi in patch #2) from virtio folks. I'm planning to send a proper v1 (!RFC) once I have all necessary MM acks. In the meanwhile, I will do more testing and minor reworks (e.g., fix !CONFIG_NUMA compilation). -- Thanks, David / dhildenb
