Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe

From: Oliver Upton <oliver.upton@linux•dev>
To: Yu Zhao <yuzhao@google•com>
Cc: Andrew Morton <akpm@linux-foundation•org>,
	Paolo Bonzini <pbonzini@redhat•com>,
	Alistair Popple <apopple@nvidia•com>,
	Anup Patel <anup@brainfault•org>, Ben Gardon <bgardon@google•com>,
	Borislav Petkov <bp@alien8•de>,
	Catalin Marinas <catalin.marinas@arm•com>,
	Chao Peng <chao.p.peng@linux•intel.com>,
	Christophe Leroy <christophe.leroy@csgroup•eu>,
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	Fabiano Rosas <farosas@linux•ibm.com>,
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	Ingo Molnar <mingo@redhat•com>, James Morse <james.morse@arm•com>,
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	Zenghui Yu <yuzenghui@huawei•com>,
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Subject: Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe
Date: Wed, 31 May 2023 23:22:53 +0000	[thread overview]
Message-ID: <ZHfWzX04GlcNngdU@linux.dev> (raw)
In-Reply-To: <CAOUHufZrfnfcbrqSzmHkejR5MA2gmGKZ3LMRhbLHV+1427z=Tw@mail.gmail.com>

On Wed, May 31, 2023 at 05:10:52PM -0600, Yu Zhao wrote:
> On Wed, May 31, 2023 at 1:28 PM Oliver Upton <oliver.upton@linux•dev> wrote:
> > On Tue, May 30, 2023 at 02:06:55PM -0600, Yu Zhao wrote:
> > > On Tue, May 30, 2023 at 1:37 PM Oliver Upton <oliver.upton@linux•dev> wrote:
> > > > As it is currently implemented, yes. But, there's potential to fast-path
> > > > the implementation by checking page_count() before starting the walk.
> > >
> > > Do you mind posting another patch? I'd be happy to ack it, as well as
> > > the one you suggested above.
> >
> > I'd rather not take such a patch independent of the test_clear_young
> > series if you're OK with that. Do you mind implementing something
> > similar to the above patch w/ the proposed optimization if you need it?
> 
> No worries. I can take the above together with the following, which
> would form a new series with its own merits, since apparently you
> think the !AF case is important.

Sorry if my suggestion was unclear.

I thought we were talking about ->free_removed_table() being called from
the stage-2 unmap path, in which case we wind up unnecessarily visiting
PTEs on a table known to be empty. You could fast-path that by only
initiating a walk if  page_count() > 1:

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 95dae02ccc2e..766563dc465c 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -1331,7 +1331,8 @@ void kvm_pgtable_stage2_free_removed(struct kvm_pgtable_mm_ops *mm_ops, void *pg
 		.end	= kvm_granule_size(level),
 	};
 
-	WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
+	if (mm_ops->page_count(pgtable) > 1)
+		WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
 
 	WARN_ON(mm_ops->page_count(pgtable) != 1);
 	mm_ops->put_page(pgtable);


A lock-free access fault walker is interesting, but in my testing it hasn't
led to any significant improvements over acquiring the MMU lock for
read. Because of that I hadn't bothered with posting the series upstream.

-- 
Thanks,
Oliver

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