Hotplug + Reboot is crashing HPT guest with HPT resizing enabled

[Bharata B Rao]

Hi,

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.

Regards,
Bharata.
-- 
http://raobharata.wordpress.com/
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