Re: [PATCH] KVM: PPC: Book3S HV: Don't use compound_order to determine host mapping size

[Nicholas Piggin <npiggin@gmail.com>]

On Tue, 11 Sep 2018 20:01:54 +1000
Paul Mackerras <paulus@ozlabs.org> wrote:

> On Tue, Sep 04, 2018 at 06:16:01PM +1000, Nicholas Piggin wrote:
> > THP paths can defer splitting compound pages until after the actual
> > remap and TLB flushes to split a huge PMD/PUD. This causes radix
> > partition scope page table mappings to get out of synch with the host
> > qemu page table mappings.
> > 
> > This results in random memory corruption in the guest when running
> > with THP. The easiest way to reproduce is use KVM baloon to free up
> > a lot of memory in the guest and then shrink the balloon to give the
> > memory back, while some work is being done in the guest.  
> 
> I'm hitting the WARN_ON you added.  I think I have an old qemu that
> doesn't 2M-align the guest ram and so we get to the level = 0 case
> because of misalignment.  The patch below on top of yours seems to
> work just fine.  In the case where the pte is 2M or 1G but we have
> misalignment, it ORs in address bits from hva into the pte so we get
> to the specific single page we want.
> 
> Care to fold this in and resend?

Thanks for that, I misunderstood the unaligned adjustment case.
Good thing you caught it.

Thanks,
Nick