Hotplug + Reboot is crashing HPT guest with HPT resizing enabled

[David Gibson]

On Fri, Feb 23, 2018 at 03:02:40PM +0530, Bharata B Rao wrote:
> 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.

Ugh.  We had several bugs along these lines, but I thought I'd fixed
them.  I wonder what this one is.

> 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.

So the shrink-HPT-on-add-memory is actually expected and should be
harmless.  It occurs because qemu estimates HPT size on the
traditional HPT == RAM size / 64 formular, which was devised with 4k
pages in mind.  The kernel on the other hand, knows it is using 64k
pages and so estimates a smaller HPT size.  Hot plugging memory always
prompts the guest to re-estimate the required HPT size, but if the
added memory is small enough, that size can still be smaller than
qemu's initial guess.

> HPT resizing
> requests fail though, but next reboot crashes the guest.

As noted the shrink is expected, so we need to debug the crash
separately.  Do you have 9478956794c11239b7c1c3ef9ce95c883bb839a3 in
your tree?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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