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
