| Message ID | 20200302134941.315212-1-david@redhat.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | virtio-mem: paravirtualized memory | expand |
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@redhat.com > [3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d@redhat.com > [4] https://github.com/kata-containers/documentation/pull/592 > [5] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/837 > > Cc: Sebastien Boeuf <sebastien.boeuf@intel.com> > Cc: Samuel Ortiz <samuel.ortiz@intel.com> > Cc: Robert Bradford <robert.bradford@intel.com> > Cc: Luiz Capitulino <lcapitulino@redhat.com> > Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com> > Cc: teawater <teawaterz@linux.alibaba.com> > Cc: Igor Mammedov <imammedo@redhat.com> > Cc: Dr. David Alan Gilbert <dgilbert@redhat.com> > > 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!
On Mon 02-03-20 19:15:09, David Hildenbrand wrote:
[...]
> As requested by Michal, I will squash some patches.
Just to clarify. If I am the only one to care then do not bother.
Btw. I still have patch 6 on the todo list to review. I just didn't find
time for it today.
On 02.03.20 19:29, Michal Hocko wrote: > On Mon 02-03-20 19:15:09, David Hildenbrand wrote: > [...] >> As requested by Michal, I will squash some patches. > > Just to clarify. If I am the only one to care then do not bother. Oh, I do bother about your review comments a lot :) And pulling out the NID part from patch #2 makes a lot of sense (so I can properly squash patch #1 into that). Looks much cleaner. > Btw. I still have patch 6 on the todo list to review. I just didn't find > time for it today. Perfect, I'll definitely wait until I resend a v2 - thanks. Cheers!
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@redhat.com [3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d@redhat.com [4] https://github.com/kata-containers/documentation/pull/592 [5] https://github.com/cloud-hypervisor/cloud-hypervisor/pull/837 Cc: Sebastien Boeuf <sebastien.boeuf@intel.com> Cc: Samuel Ortiz <samuel.ortiz@intel.com> Cc: Robert Bradford <robert.bradford@intel.com> Cc: Luiz Capitulino <lcapitulino@redhat.com> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com> Cc: teawater <teawaterz@linux.alibaba.com> Cc: Igor Mammedov <imammedo@redhat.com> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com> 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 MAINTAINERS | 7 + drivers/acpi/numa/srat.c | 1 + drivers/virtio/Kconfig | 18 + drivers/virtio/Makefile | 1 + drivers/virtio/virtio_mem.c | 1906 +++++++++++++++++++++++++++++++ include/linux/memory_hotplug.h | 1 + include/linux/page-flags.h | 10 + include/uapi/linux/virtio_ids.h | 1 + include/uapi/linux/virtio_mem.h | 208 ++++ mm/memory_hotplug.c | 81 +- mm/page_alloc.c | 26 + mm/page_isolation.c | 9 + 12 files changed, 2259 insertions(+), 10 deletions(-) create mode 100644 drivers/virtio/virtio_mem.c create mode 100644 include/uapi/linux/virtio_mem.h