Hotplug + Reboot is crashing HPT guest with HPT resizing enabled

Bharata B Rao bharata.rao at
Fri Mar 2 19:12:49 AEDT 2018

On Fri, Mar 2, 2018 at 7:51 AM, David Gibson <david at>

> 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.

Thanks for the clarification.

> > 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?

I do now and I no longer see the crash that I was observing last week.

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