[PATCH v1 0/3] mm/memory_hotplug: Make virtio-mem play nicely with kexec-tools

[David Hildenbrand <david@xxxxxxxxxx>]

This series is based on [1]:
        [PATCH v2 00/10] virtio-mem: paravirtualized memory
That will hopefull get picked up soon, rebased to -next.

The following patches were reverted from -next [2]:
        [PATCH 0/3] kexec/memory_hotplug: Prevent removal and accidental use
As discussed in that thread, they should be reverted from -next already.

In theory, if people agree, we could take the first two patches via the
-mm tree now and the last (virtio-mem) patch via MST's tree once picking up
virtio-mem. No strong feelings.


Memory added by virtio-mem is special and might contain logical holes,
especially after memory unplug, but also when adding memory in
sub-section size. While memory in these holes can usually be read, that
memory should not be touched. virtio-mem managed device memory is never
exposed via any firmware memmap (esp., e820). The device driver will
request to plug memory from the hypervisor and add it to Linux.

On a cold start, all memory is unplugged, and the guest driver will first
request to plug memory from the hypervisor, to then add it to Linux. After
a reboot, all memory will get unplugged (except in rare, special cases). In
case the device driver comes up and detects that some memory is still
plugged after a reboot, it will manually request to unplug all memory from
the hypervisor first - to then request to plug memory from the hypervisor
and add to Linux. This is essentially a defragmentation step, where all
logical holes are removed.

As the device driver is responsible for detecting, adding and managing that
memory, also kexec should treat it like that. It is special. We need a way
to teach kexec-tools to not add that memory to the fixed-up firmware
memmap, to not place kexec images onto this memory, but still allow kdump
to dump it. Add a flag to tell memory hotplug code to
not create /sys/firmware/memmap entries and to indicate it via
"System RAM (driver managed)" in /proc/iomem.

Before this series, kexec_file_load() already did the right thing (for
virtio-mem) by not adding that memory to the fixed-up firmware memmap and
letting the device driver handle it. With this series, also kexec_load() -
which relies on user space to provide a fixed up firmware memmap - does
the right thing with virtio-mem memory.

When the virtio-mem device driver(s) come up, they will request to unplug
all memory from the hypervisor first (esp. defragment), to then request to
plug consecutive memory ranges from the hypervisor, and add them to Linux
- just like on a reboot where we still have memory plugged.

[1] https://lore.kernel.org/r/20200311171422.10484-1-david@xxxxxxxxxx/
[2] https://lore.kernel.org/r/20200326180730.4754-1-james.morse@xxxxxxx

David Hildenbrand (3):
  mm/memory_hotplug: Prepare passing flags to add_memory() and friends
  mm/memory_hotplug: Introduce MHP_DRIVER_MANAGED
  virtio-mem: Add memory with MHP_DRIVER_MANAGED

 arch/powerpc/platforms/powernv/memtrace.c     |  2 +-
 .../platforms/pseries/hotplug-memory.c        |  2 +-
 drivers/acpi/acpi_memhotplug.c                |  2 +-
 drivers/base/memory.c                         |  2 +-
 drivers/dax/kmem.c                            |  2 +-
 drivers/hv/hv_balloon.c                       |  2 +-
 drivers/s390/char/sclp_cmd.c                  |  2 +-
 drivers/virtio/virtio_mem.c                   |  3 +-
 drivers/xen/balloon.c                         |  2 +-
 include/linux/memory_hotplug.h                | 15 +++++++--
 mm/memory_hotplug.c                           | 31 +++++++++++++------
 11 files changed, 44 insertions(+), 21 deletions(-)

-- 
2.25.3