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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>
References: <20210624035749.4054934-1-stevensd@google.com>
 <1624530624.8jff1f4u11.astroid@bobo.none>
 <1624534759.nj0ylor2eh.astroid@bobo.none>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64
 MMU
Message-ID: <d06a8a55-bb9d-a6ef-21bb-0633b99a50d1@redhat.com>
Date:   Thu, 24 Jun 2021 14:00:26 +0200
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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;
>   }
>   
>   /*
>