Re: memory hotplug and force_remove

On Tue, 28 Mar 2017, Rafael J. Wysocki wrote:

quoted

quoted

quoted

we have been chasing the following BUG() triggering during the memory
hotremove (remove_memory):
	ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
				check_memblock_offlined_cb);
	if (ret)
		BUG();

and it took a while to learn that the issue is caused by
/sys/firmware/acpi/hotplug/force_remove being enabled. I was really
surprised to see such an option because at least for the memory hotplug
it cannot work at all. Memory hotplug fails when the memory is still
in use. Even if we do not BUG() here enforcing the hotplug operation
will lead to problematic behavior later like crash or a silent memory
corruption if the memory gets onlined back and reused by somebody else.

I am wondering what was the motivation for introducing this behavior and
whether there is a way to disallow it for memory hotplug. Or maybe drop
it completely. What would break in such a case?

Honestly, I don't remember from the top of my head and I haven't looked at
that code for several months.

I need some time to recall that.

Did you have any chance to look into this?

Well, yes.

It looks like that was added for some people who depended on the old behavior
at that time.

I guess we can try to drop it and see what happpens. :-)

I'd agree with that; at the same time, udev rule should be submitted to 
systemd folks though. I don't think there is anything existing in this 
area yet (neither do distros ship their own udev rules for this AFAIK).

Thanks,

-- 
Jiri Kosina
SUSE Labs

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