linux-kernel - Re: [PATCH v12 54/84] KVM: arm64: Mark "struct page" pfns accessed/dirty before dropping mmu

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Message-ID: <ZrFmool3SKpp_NRZ@linux.dev>
Date: Mon, 5 Aug 2024 23:56:18 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
	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>,
	Fuad Tabba <tabba@...gle.com>
Subject: Re: [PATCH v12 54/84] KVM: arm64: Mark "struct page" pfns
 accessed/dirty before dropping mmu_lock

On Mon, Aug 05, 2024 at 04:53:01PM -0700, Sean Christopherson wrote:
> On Mon, Aug 05, 2024, Oliver Upton wrote:
> > > > ---
> > > >  arch/arm64/kvm/mmu.c | 10 ++++++----
> > > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > > > index 22ee37360c4e..ce13c3d884d5 100644
> > > > --- a/arch/arm64/kvm/mmu.c
> > > > +++ b/arch/arm64/kvm/mmu.c
> > > > @@ -1685,15 +1685,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > > >  	}
> > > >  
> > > >  out_unlock:
> > > > +	if (writable && !ret)
> > > > +		kvm_set_pfn_dirty(pfn);
> > > 
> > > I'm guessing you meant kvm_release_pfn_dirty() here, because this leaks
> > > a reference.
> 
> Doh, I did indeed.  Alternatively, this could be:
> 
> 	if (writable && !ret)
> 		kvm_set_pfn_dirty(pfn);
> 
> 	kvm_release_pfn_clean(pfn);
> 
> It won't matter in the end, because this just becomes:
> 
> 	kvm_release_faultin_page(kvm, page, !!ret, writable);
> 
> So I guess the question is if you prefer to make the switch to an if-else in this
> path, or more implicitly in the conversion to kvm_release_faultin_page().
> 
> I made the same goof for RISC-V, perhaps to prove that I too can copy+paste arm64's
> MMU code ;-)

LOL, whatever way you want to address it is fine by me, just wanted to
make sure this intermediate bug wouldn't bite an unlucky bisection.

-- 
Thanks,
Oliver