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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 12 Sep 2016 17:35:28 +0530

Anshuman Khandual <[email-masked]> 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.

IMO we should fix the reason for these CMA allocation failure. We are just
doing work around here.

>
> Signed-off-by: Anshuman Khandual <[email-masked]>
> ---
> - 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);
> +
>  	if (!hpt)
>  		return -ENOMEM;

A better way to do that would be (not even compile tested) ?

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 65b2b00d93d7..3f8995f27339 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -68,16 +68,18 @@ long kvmppc_alloc_hpt(struct kvm *kvm, u32 *htab_orderp)
 		memset((void *)hpt, 0, (1ul << order));
 		kvm->arch.hpt_cma_alloc = 1;
 	}
-
-	/* Lastly try successively smaller sizes from the page allocator */
-	/* Only do this if userspace didn't specify a size via ioctl */
-	while (!hpt && order > PPC_MIN_HPT_ORDER && !htab_orderp) {
+	/*
+	 * Try successively smaller sizes from the page allocator.
+	 * If a size was specified via an ioctl, we just try that
+	 * specific size
+	 */
+-	while (!hpt && order > PPC_MIN_HPT_ORDER) {
 		hpt = __get_free_pages(GFP_KERNEL|__GFP_ZERO|__GFP_REPEAT|
 				       __GFP_NOWARN, order - PAGE_SHIFT);
-		if (!hpt)
-			--order;
+		if (htab_orderp)
+			break;
+		--order;
 	}
-
 	if (!hpt)
 		return -ENOMEM;