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

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Message-ID: <CAOUHufZOkBmZJgCU2xW2B8S3P3TWERHezy0xKWY9_TeyV9K7Rg@mail.gmail.com>
Date:   Tue, 30 May 2023 14:06:55 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
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 Tue, May 30, 2023 at 1:37 PM Oliver Upton <oliver.upton@...ux.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@...ux.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.

> > 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/

Awesome.