linux-kernel - Re: [PATCH v12 01/84] KVM: arm64: Release pfn, i.e. put page, if copying MTE tags hits ZONE

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Message-ID: <87a5hxfs3d.fsf@draig.linaro.org>
Date: Wed, 31 Jul 2024 17:23:34 +0100
From: Alex Bennée <alex.bennee@...aro.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,  Marc Zyngier <maz@...nel.org>,
  Oliver Upton <oliver.upton@...ux.dev>,  Tianrui Zhao
 <zhaotianrui@...ngson.cn>,  Bibo Mao <maobibo@...ngson.cn>,  Huacai Chen
 <chenhuacai@...nel.org>,  Michael Ellerman <mpe@...erman.id.au>,  Anup
 Patel <anup@...infault.org>,  Paul Walmsley <paul.walmsley@...ive.com>,
  Palmer Dabbelt <palmer@...belt.com>,  Albert Ou <aou@...s.berkeley.edu>,
  Christian Borntraeger <borntraeger@...ux.ibm.com>,  Janosch Frank
 <frankja@...ux.ibm.com>,  Claudio Imbrenda <imbrenda@...ux.ibm.com>,
  kvm@...r.kernel.org,  linux-arm-kernel@...ts.infradead.org,
  kvmarm@...ts.linux.dev,  loongarch@...ts.linux.dev,
  linux-mips@...r.kernel.org,  linuxppc-dev@...ts.ozlabs.org,
  kvm-riscv@...ts.infradead.org,  linux-riscv@...ts.infradead.org,
  linux-kernel@...r.kernel.org,  David Matlack <dmatlack@...gle.com>,
  David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v12 01/84] KVM: arm64: Release pfn, i.e. put page, if
 copying MTE tags hits ZONE_DEVICE

Sean Christopherson <seanjc@...gle.com> writes:

> Put the page reference acquired by gfn_to_pfn_prot() if
> kvm_vm_ioctl_mte_copy_tags() runs into ZONE_DEVICE memory.  KVM's less-
> than-stellar heuristics for dealing with pfn-mapped memory means that KVM
> can get a page reference to ZONE_DEVICE memory.
>
> Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/arm64/kvm/guest.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 11098eb7eb44..e1f0ff08836a 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1059,6 +1059,7 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  		page = pfn_to_online_page(pfn);
>  		if (!page) {
>  			/* Reject ZONE_DEVICE memory */
> +			kvm_release_pfn_clean(pfn);

I guess this gets renamed later in the series.

However my main comment is does lack of page always mean a ZONE_DEVICE?
Looking at pfn_to_online_page() I see a bunch of other checks first. Why
isn't it that functions responsibility to clean up after itself if its
returning NULLs?

>  			ret = -EFAULT;
>  			goto out;
>  		}

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro