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

[Paul Mackerras <paulus@ozlabs.org>]

On Wed, Sep 14, 2016 at 09:57:48AM +1000, Michael Ellerman wrote:
> Anshuman Khandual <khandual@linux.vnet.ibm.com> writes:
> 
> > 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.
> 
> It looks like if CMA is not configured we will just fail instantly.
> 
> So this does look like something we should fix.
> 
> But I think it is just a bug in commit 572abd563bef ("KVM: PPC: Book3S
> HV: Don't fall back to smaller HPT size in allocation ioctl"), which did:
> 
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 1f9c0a17f445..10722b1e38b5 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -70,7 +70,8 @@ long kvmppc_alloc_hpt(struct kvm *kvm, u32 *htab_orderp)
>         }
>  
>         /* Lastly try successively smaller sizes from the page allocator */
> -       while (!hpt && order > PPC_MIN_HPT_ORDER) {
> +       /* Only do this if userspace didn't specify a size via ioctl */
> +       while (!hpt && order > PPC_MIN_HPT_ORDER && !htab_orderp) {
>                 hpt = __get_free_pages(GFP_KERNEL|__GFP_ZERO|__GFP_REPEAT|
>                                        __GFP_NOWARN, order - PAGE_SHIFT);
>                 if (!hpt)
> 
> 
> Instead of guarding the loop entry with !htab_orderp, it should have
> allowed the loop to enter, but prevented it from iterating if the
> allocation fails and htab_orderp != 0.

You're right.  I'll fix it.

Paul.