Rebooting a hash guest after hotplugging memory to it is crashing the
guest. This is seen only when HPT resizing is enabled. I see guest crashing
at multiple places, but this location is fairly commonly seen:
kernel BUG at mm/slub.c:3912!
Testing with latest guest kernel and ppc-for-2.12 branch of QEMU.
A bit of debugging shows me that when memory is added, the guest kernel
tries to resize HPT to a htab_shift value lesser than the value with which
the guest has booted. For eg. a 8GB guest boots with htab_shift of 26. When
1G is hot-added,
arch/powerpc/mm/hash_utils_64.c:resize_hpt_for_hotplug() ends up assigning
24 to target_hpt_shift. This looks suspicious as we are increasing the
memory, but kernel is asking for shrinking the HPT size. HPT resizing
requests fail though, but next reboot crashes the guest.