On 29.01.20 10:41, David Hildenbrand wrote: > 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()" > Yet another ping. -- Thanks, David / dhildenb
