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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
