Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 31 May 2023 19:28:17 +0000
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>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jonathan Corbet <corbet@lwn.net>,
        Marc Zyngier <maz@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michael Larabel <michael@michaellarabel.com>,
        Mike Rapoport <rppt@kernel.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Peter Xu <peterx@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        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, linuxppc-dev@lists.ozlabs.org,
        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
Message-ID: <ZHef0VsZvZ1Vnz0u@linux.dev>
References: <20230526234435.662652-1-yuzhao@google.com>
 <20230526234435.662652-5-yuzhao@google.com>
 <ZHJHJPBF6euzOFdw@linux.dev>
 <CAOUHufa74CufHziHSquO5bZwbFXz2MNssBzW+AH7=Xo5RCnQ0A@mail.gmail.com>
 <ZHZQdQAApIrw6fBu@linux.dev>
 <CAOUHufZOkBmZJgCU2xW2B8S3P3TWERHezy0xKWY9_TeyV9K7Rg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAOUHufZOkBmZJgCU2xW2B8S3P3TWERHezy0xKWY9_TeyV9K7Rg@mail.gmail.com>
Precedence: bulk

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:
> >
> > 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.
> 
> 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?

-- 
Thanks,
Oliver