[RFC] KVM: PPC: Book3S HV: Fall back to same size HPT in allocation ioctl

[Anshuman Khandual]

On 09/12/2016 05:03 PM, Balbir Singh wrote:
> On Mon, Sep 12, 2016 at 9:13 PM, Anshuman Khandual
> <khandual at linux.vnet.ibm.com> wrote:
>> > When the HPT size is explicitly passed on from the userspace, currently
>> > the KVM_PPC_ALLOCATE_HTAB will try to allocate the requested size of HPT
>> > from reserved CMA area and if that is not possible, the allocation just
>> > fails. With the commit 572abd563befd56 ("KVM: PPC: Book3S HV: Don't fall
>> > back to smaller HPT size in allocation ioctl"), it does not even try to
>> > allocate the same order pages from the page allocator before failing for
>> > good. Same order allocation should be attempted from the page allocator
>> > as a fallback option when the CMA allocation attempt fails.
>> >
>> > Signed-off-by: Anshuman Khandual <khandual at linux.vnet.ibm.com>
>> > ---
>> > - This change saves guests from failing to start after migration
>> >
>> >  arch/powerpc/kvm/book3s_64_mmu_hv.c | 8 ++++++++
>> >  1 file changed, 8 insertions(+)
>> >
>> > diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> > index 05f09ae..0a30eb4 100644
>> > --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> > +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> > @@ -78,6 +78,14 @@ long kvmppc_alloc_hpt(struct kvm *kvm, u32 *htab_orderp)
>> >                         --order;
>> >         }
>> >
>> > +       /*
>> > +        * Fallback in case the userspace has provided a size via ioctl.
>> > +        * Try allocating the same order pages from the page allocator.
>> > +        */
>> > +       if (!hpt && order > PPC_MIN_HPT_ORDER && htab_orderp)
>> > +               hpt = __get_free_pages(GFP_KERNEL|__GFP_ZERO|__GFP_REPEAT|
>> > +                       __GFP_NOWARN, order - PAGE_SHIFT);
>> > +
> How often does this succeed? Please provide data. I presume this for

During continuous guest VM migration test from source host to destination host
this patch was able to prevent guest creation failure after migration on the
destination host which was failing after 2-3 days. We have not seen the failure
till now even after 3-4 days.

> the case where guest pages are pinned?

Hmm, need to check that in the test setup. There was nothing running inside the
guests though. IIUC, HPT size of the guest is computed based on the max memory
the guest is ever going to have irrespective of the RAM usage before migration.
How does pinning effect the HPT size ?