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

On 09/14/2016 05:42 AM, Paul Mackerras wrote:

On Wed, Sep 14, 2016 at 09:57:48AM +1000, Michael Ellerman wrote:

quoted

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Anshuman Khandual [off-list ref] writes:

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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.

Thanks Paul, so I will not be sending follow up patch on this.