On 09.01.20 14:48, David Hildenbrand wrote: > 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). Friendly ping again: Can I get some feedback on the two important MM changes in this series "[PATCH RFC v4 06/13] mm: Allow to offline unmovable PageOffline() pages via MEM_GOING_OFFLINE" and "[PATCH RFC v4 08/13] mm/memory_hotplug: Introduce offline_and_remove_memory()" -- Thanks, David / dhildenb
