Re: [RFC] KVM: PPC: Book3S HV: Fall back to same size HPT in allocation ioctl
From: Anshuman Khandual <hidden>
Date: 2016-09-14 03:53:04
On 09/14/2016 05:27 AM, Michael Ellerman wrote:
Anshuman Khandual [off-list ref] writes:quoted
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.
Right and also we have this fallback registered any way. I wonder why we are still debating about the need of a fallback mechanism when we already have got one.
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:Hmm, I think its something the commit missed to accommodate for. But maybe yes, its a bug in the commit.
quoted hunk ↗ jump to hunk
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.
Right and thats what Aneesh's proposed patch (in the other thread) does.