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Date:   Fri, 25 Jun 2021 10:20:32 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64
 MMU
To:     Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        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>
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Excerpts from Paolo Bonzini's message of June 25, 2021 1:35 am:
> On 24/06/21 14:57, Nicholas Piggin wrote:
>> KVM: Fix page ref underflow for regions with valid but non-refcounted pa=
ges
>=20
> It doesn't really fix the underflow, it disallows mapping them in the=20
> first place.  Since in principle things can break, I'd rather be=20
> explicit, so let's go with "KVM: do not allow mapping valid but=20
> non-reference-counted pages".
>=20
>> It's possible to create a region which maps valid but non-refcounted
>> pages (e.g., tail pages of non-compound higher order allocations). These
>> host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
>> of APIs, which take a reference to the page, which takes it from 0 to 1.
>> When the reference is dropped, this will free the page incorrectly.
>>=20
>> Fix this by only taking a reference on the page if it was non-zero,
>=20
> s/on the page/on valid pages/ (makes clear that invalid pages are fine=20
> without refcounting).

That seems okay, you can adjust the title or changelog as you like.

> Thank you *so* much, I'm awful at Linux mm.

Glad to help. Easy to see why you were taking this approach because the=20
API really does need to be improved and even a pretty intwined with mm=20
subsystem like KVM shouldn't _really_ be doing this kind of trick (and
it should go away when old API is removed).

Thanks,
Nick