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>,
	Dave Hansen <dave.hansen@linux•intel.com>,
	Fabiano Rosas <farosas@linux•ibm.com>,
	Gaosheng Cui <cuigaosheng1@huawei•com>,
	Gavin Shan <gshan@redhat•com>, "H. Peter Anvin" <hpa@zytor•com>,
	Ingo Molnar <mingo@redhat•com>, James Morse <james.morse@arm•com>,
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	Steven Rostedt <rostedt@goodmis•org>,
	Suzuki K Poulose <suzuki.poulose@arm•com>,
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	Thomas Huth <thuth@redhat•com>, Will Deacon <will@kernel•org>,
	Zenghui Yu <yuzenghui@huawei•com>,
	kvmarm@lists•linux.dev, kvm@vger•kernel.org,
	linux-arm-kernel@lists•infradead.org, linux-doc@vger•kernel.org,
	linux-kernel@vger•kernel.org, linux-mm@kvack•org,
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	linux-trace-kernel@vger•kernel.org, x86@kernel•org,
	linux-mm@google•com
Subject: Re: [PATCH mm-unstable v2 04/10] kvm/arm64: make stage2 page tables RCU safe
Date: Tue, 30 May 2023 19:37:25 +0000	[thread overview]
Message-ID: <ZHZQdQAApIrw6fBu@linux.dev> (raw)
In-Reply-To: <CAOUHufa74CufHziHSquO5bZwbFXz2MNssBzW+AH7=Xo5RCnQ0A@mail.gmail.com>

Hi Yu,

On Sat, May 27, 2023 at 02:13:07PM -0600, Yu Zhao wrote:
> On Sat, May 27, 2023 at 12:08 PM Oliver Upton <oliver.upton@linux•dev> wrote:
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 3d61bd3e591d..bfbebdcb4ef0 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -1019,7 +1019,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
> >                                                kvm_granule_size(ctx->level));
> >
> >         if (childp)
> > -               mm_ops->put_page(childp);
> > +               mm_ops->free_removed_table(childp, ctx->level);
> 
> Thanks, Oliver.
> 
> A couple of things I haven't had the chance to verify -- I'm hoping
> you could help clarify:
> 1. For unmapping, with free_removed_table(), wouldn't we have to look
> into the table we know it's empty unnecessarily?

As it is currently implemented, yes. But, there's potential to fast-path
the implementation by checking page_count() before starting the walk.

> 2. For remapping and unmapping, how does free_removed_table() put the
> final refcnt on the table passed in? (Previously we had
> put_page(childp) in stage2_map_walk_table_post(). So I'm assuming we'd
> have to do something equivalent with free_removed_table().)

Heh, that's a bug, and an embarrassing one at that!

Sent out a fix for that, since it would appear we leak memory on
table->block transitions. PTAL if you have a chance.

https://lore.kernel.org/all/20230530193213.1663411-1-oliver.upton@linux.dev/

-- 
Thanks,
Oliver

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