In the patch that enables memory hot-remove (commit bbd6ec605c0f ("arm64/mm:
Enable memory hot remove")) for arm64, there’s a notifier put in place that
prevents boot memory from being offlined and removed. The commit text mentions
that boot memory on arm64 cannot be removed. But x86 and other archs doesn’t
seem to do this prevention.
The current logic is that only “new” memory blocks which are hot-added can
later be offlined and removed. The memory that system booted up with cannot be
offlined and removed. But there could be many usercases such as inter-VM memory
sharing where a primary VM could offline and hot-remove a block/section of
memory and lend it to secondary VM where it could hot-add it. And after usecase
is done, the reverse happens where secondary VM hot-removes and gives it back
to primary which can hot-add it back. In such cases, the present logic for
arm64 doesn’t allow this hot-remove in primary to happen.
Also, on systems with movable zone that sort of guarantees pages to be migrated
and isolated so that blocks can be offlined, this logic also defeats the
purpose of having a movable zone which system can rely on memory hot-plugging,
which say virt-io mem also relies on for fully plugged memory blocks.
This patch tries to solve by introducing a new section mem map sit
'SECTION_MARK_HOTPLUGGABLE' which allows the concerned module drivers be able
to mark requried sections as "hotpluggable" by setting this bit. Also this
marking is only allowed for sections which are in movable zone and have
unmovable pages. The arm64 mmu code on receiving the MEM_GOING_OFFLINE
notification, we disallow offlining of any boot memory by checking if
section_early or not. With the introduction of SECTION_MARK_HOTPLUGGABLE, we
allow boot mem sections that are marked as hotpluggable with this bit set to be
offlined and removed. Thereby allowing required bootmem sections to be
offlinable.
Sudarshan Rajagopalan (2):
mm/memory_hotplug: allow marking of memory sections as hotpluggable
arm64: allow hotpluggable sections to be offlined
arch/arm64/mm/mmu.c | 2 +-
include/linux/memory_hotplug.h | 1 +
include/linux/mmzone.h | 9 ++++++++-
mm/memory_hotplug.c | 20 ++++++++++++++++++++
mm/sparse.c | 31 +++++++++++++++++++++++++++++++
5 files changed, 61 insertions(+), 2 deletions(-)
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