Re: memory hotplug and force_remove

[joeyli <jlee@suse.com>]

On Mon, Mar 20, 2017 at 10:24:42PM +0100, Rafael J. Wysocki wrote:
> On Monday, March 20, 2017 03:29:39 PM Michal Hocko wrote:
> > Hi Rafael,
> 
> Hi,
> 
> > 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.
>

IMHO. 
In the second pass offline in acpi_scan_try_to_offline(), when force_remove flag
enabled, it's still run offline on the parent device even there have any child
device offline failed. And it doesn't return the error from acpi_bus_offline() to
caller. 

	errdev = NULL;
	acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX, 
			    NULL, acpi_bus_offline, (void *)true,
			    (void **)&errdev);
	if (!errdev || acpi_force_hot_remove)                 
		acpi_bus_offline(handle, 0, (void *)true, 
				 (void **)&errdev);

In this situation, the parent device or any child device may not really
offline successfully. But acpi_scan_hot_remove, the caller doesn't know that.
Then it cause the later acpi_bus_trim() process failed.

acpi_bus_trim()
	-> handler->detach()
		-> acpi_memory_device_remove()
			-> remove_memory() -> BUG()  

because some memory doesn't really offline. 

Thanks a lot!
Joey Lee

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