On Thu 02-03-17 14:53:48, Igor Mammedov wrote:
[...]

When trying to support memory unplug on guest side in RHEL7,
experience shows otherwise. Simplistic udev rule which onlines
added block doesn't work in case one wants to online it as movable.

Hotplugged blocks in current kernel should be onlined in reverse
order to online blocks as movable depending on adjacent blocks zone.

Could you be more specific please? Setting online_movable from the udev
rule should just work regardless of the ordering or the state of other
memblocks. If that doesn't work I would call it a bug.

Which means simple udev rule isn't usable since it gets event from
the first to the last hotplugged block order. So now we would have
to write a daemon that would
 - watch for all blocks in hotplugged memory appear (how would it know)
 - online them in right order (order might also be different depending
   on kernel version)
   -- it becomes even more complicated in NUMA case when there are
      multiple zones and kernel would have to provide user-space
      with information about zone maps

In short current experience shows that userspace approach
 - doesn't solve issues that Vitaly has been fixing (i.e. onlining
   fast and/or under memory pressure) when udev (or something else
   might be killed)

yeah and that is why the patch does the onlining from the kernel.
 

quoted

Can you imagine any situation when somebody actually might want to have
this knob enabled? From what I understand it doesn't seem to be the
case.

For x86:
 * this config option is enabled by default in recent Fedora,

How do you want to support usecases which really want to online memory
as movable? Do you expect those users to disable the option because
unless I am missing something the in kernel auto onlining only supporst
regular onlining.
-- 
Michal Hocko
SUSE Labs

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