From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Thu, 24 Jun 2021 12:00:26 +0000
Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU
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To: Nicholas Piggin <npiggin@gmail.com>, Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>, Huacai Chen <chenhuacai@kernel.org>, Marc Zyngier <maz@kernel.org>, Paul Mackerras <paulus@ozlabs.org>, David Stevens <stevensd@chromium.org>, Zhenyu Wang <zhenyuw@linux.intel.com>, Zhi Wang <zhi.a.wang@intel.com>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>, dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, intel-gvt-dev@lists.freedesktop.org, James Morse <james.morse@arm.com>, Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>, kvmarm@lists.cs.columbia.edu, kvm-ppc@vger.kernel.org, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Sean Christopherson <seanjc@google.com>, David Stevens <stevensd@google.com>, Suzuki K Poulose <suzuki.poulose@arm.com>, Vitaly Kuznetsov <vkuznets@redhat.com>, Wanpeng Li <wanpengli@tencent.com>, Will Deacon <will@kernel.org>

On 24/06/21 13:42, Nicholas Piggin wrote:
> +static int kvm_try_get_pfn(kvm_pfn_t pfn)
> +{
> +	if (kvm_is_reserved_pfn(pfn))
> +		return 1;

So !pfn_valid would always return true.  Yeah, this should work and is 
certainly appealing!

Paolo


> +	return get_page_unless_zero(pfn_to_page(pfn));
> +}
> +
>   static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   			       unsigned long addr, bool *async,
>   			       bool write_fault, bool *writable,
> @@ -2104,13 +2111,21 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   	 * Whoever called remap_pfn_range is also going to call e.g.
>   	 * unmap_mapping_range before the underlying pages are freed,
>   	 * causing a call to our MMU notifier.
> +	 *
> +	 * Certain IO or PFNMAP mappings can be backed with valid
> +	 * struct pages, but be allocated without refcounting e.g.,
> +	 * tail pages of non-compound higher order allocations, which
> +	 * would then underflow the refcount when the caller does the
> +	 * required put_page. Don't allow those pages here.
>   	 */
> -	kvm_get_pfn(pfn);
> +	if (!kvm_try_get_pfn(pfn))
> +		r = -EFAULT;
>   
>   out:
>   	pte_unmap_unlock(ptep, ptl);
>   	*p_pfn = pfn;
> -	return 0;
> +
> +	return r;
>   }
>   
>   /*
>