Re: memory hotplug and force_remove

[joeyli <jlee@suse.com>]

On Thu, Mar 30, 2017 at 10:15:04PM +0200, Rafael J. Wysocki wrote:
> On Friday, March 31, 2017 12:57:29 AM joeyli wrote:
> > 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.
> 
> Which would be rather confusing.
> 
> The whole point of the thing was the "remove no matter what" behavior and
> there's not much point in keeping it around without that.
>

OK~ Understood.

Then back the "remove no matter waht" behavior, the point is
force_remove knob causes that acpi_scan_try_to_offline() ignored
the offline error of parent/children devices in the second pass:

drivers/acpi/scan.c
static int acpi_scan_try_to_offline(struct acpi_device *device)
{  
...
	/* first pass to call bus offline for parent */
	acpi_bus_offline(handle, 0, (void *)false, (void **)&errdev);
	/* if failed, then second pass */
	if (errdev) { 
		errdev = NULL;
		/* children devices, second pass */
		acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX,
				    NULL, acpi_bus_offline, (void *)true,
				    (void **)&errdev);
		/* ignored children's offline error here */
		if (!errdev || acpi_force_hot_remove)
			/* ignored parent's offline error */
			acpi_bus_offline(handle, 0, (void *)true,
					 (void **)&errdev);

		/* will not set devices back to online */
		if (errdev && !acpi_force_hot_remove) { 
	...
	}
	return 0;
}

Then acpi_scan_try_to_offline() returns 0 and go to _remove_ stage, then
memory subsystem raises BUG() because the device offline state doesn't sync
with memory block state.


Thanks a lot!
Joey Lee

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