Re: [PATCH v1 3/3] memory-hotplug.rst: document the "auto-movable" online policy

David Hildenbrand
2021-10-08T08:33:21Z

>> It's essentially ignored with the auto-movable policy for memory hotplugged
>> after boot (!MEMBLOCK_HOTPLUG). That's why only the description of
>> "contig-zones" below describes how it interacts with the ``movable_node``,
>> and we make it clear here that it's restricted to the "contig-zones" policy
>> as well.
>>
>>
>> Bare metal, where we care about reliably unplugging hotplugged memory
>> usually configures auto-onlining to "online_movable": for example, that's
>> the case on RHEL. auto-movable doesn't make too much sense for bare metal:
>> the nature of "movable_node" is to essentially online anything that might
>> get hotunplugged MOVABLE, especially after hotplugging memory and rebooting:
>> that is highly dangerous especially in virtualized environments.
>>
>> "auto-movable" is valuable in virtualized environments, where we add memory
>> via:
>> * add_memory_driver_managed() like virtio-mem, whereby such memory is
>> never part of the firmware provided memory-map, so the policy is
>> always in control even after a reboot.
>> * Hotplugged virtual DIMMs, such as provided by x86-64/arm64, whereby we
>> don't include these DIMMs in the firmware-provided memory map, but
>> ACPI code adds them after early boot, making it behave similar to
>> add_memory_driver_managed() -- the policy is always in control even
>> after a reboot.
>>
>
> Do you want to put it somewhere in Documentation/ ?
> It's already written anyway ;-)
>

I'll add to the "auto-movable" description:

"This policy ignores the ``movable_node`` kernel command line parameter
and isn't really applicable in environments that require it (e.g., bare
metal with hotunpluggable nodes) where hotplugged memory might be
exposed via the firmware-provided memory map early during boot to the
system instead of getting detected, added and onlined later during boot
(such as done by virito-mem or by some hypervisors implementing emulated
DIMMs)."

Thanks Mike!

--
Thanks,

David / dhildenb