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

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Date:   Wed, 31 May 2023 23:22:53 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alistair Popple <apopple@...dia.com>,
        Anup Patel <anup@...infault.org>,
        Ben Gardon <bgardon@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        Gaosheng Cui <cuigaosheng1@...wei.com>,
        Gavin Shan <gshan@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Morse <james.morse@....com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Corbet <corbet@....net>,
        Marc Zyngier <maz@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Larabel <michael@...haellarabel.com>,
        Mike Rapoport <rppt@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Paul Mackerras <paulus@...abs.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Thomas Huth <thuth@...hat.com>, Will Deacon <will@...nel.org>,
        Zenghui Yu <yuzenghui@...wei.com>, kvmarm@...ts.linux.dev,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-trace-kernel@...r.kernel.org, x86@...nel.org,
        linux-mm@...gle.com
Subject: Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables
 RCU safe

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@...ux.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@...ux.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