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

[Balbir Singh]

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
the case where guest pages are pinned?

Balbir Singh.