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List:       linux-virtualization
Subject:    Re: [PATCH RFC v4 00/13] virtio-mem: paravirtualized memory
From:       David Hildenbrand <david () redhat ! com>
Date:       2020-01-29 9:41:26
Message-ID: d07c6b4a-46e3-3f81-04db-ea1475fe48c2 () redhat ! com
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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@redhat.com
> > [3] https://lkml.kernel.org/r/547865a9-d6c2-7140-47e2-5af01e7d761d@redhat.com
> > 
> > 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>
> > 
> > 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

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