Re: memory hotplug and force_remove

[joeyli <jlee@suse.com>]

On Thu, Mar 30, 2017 at 06:20:34PM +0200, Michal Hocko wrote:
> On Thu 30-03-17 10:47:52, Jiri Kosina wrote:
> > On Tue, 28 Mar 2017, Rafael J. Wysocki wrote:
> > 
> > > > > > 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).
> 
> Another option would keepint the force_remove knob but make the code be
> error handling aware. In other words rather than ignoring offline error
> simply propagate it up the chain and do not consider the offline. Would
> that be acceptable?

Then the only difference between normal mode is that the force_remove mode
doesn't send out uevent for not-offline-yet container.

I vote to remove force_remove not just it ignored offline error and also
it's a acpi global knob that it affect all container devices in system.

Thanks a lot!
Joey Lee