linux-kernel - Re: [PATCH v7 3/8] KVM: Make __kvm_follow_pfn not imply FOLL

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Message-ID: <CAD=HUj4E84MxZn8=U3wkBtx-VYdeD1mqOH9BUYpG=Y2POvY6sg@mail.gmail.com>
Date:   Thu, 6 Jul 2023 15:09:17 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Xu <peterx@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v7 3/8] KVM: Make __kvm_follow_pfn not imply FOLL_GET

On Wed, Jul 5, 2023 at 8:56 PM Yu Zhang <yu.c.zhang@...ux.intel.com> wrote:
>
> On Tue, Jul 04, 2023 at 04:50:48PM +0900, David Stevens wrote:
> > From: David Stevens <stevensd@...omium.org>
> >
> > Make it so that __kvm_follow_pfn does not imply FOLL_GET. This allows
> > callers to resolve a gfn when the associated pfn has a valid struct page
> > that isn't being actively refcounted (e.g. tail pages of non-compound
> > higher order pages). For a caller to safely omit FOLL_GET, all usages of
> > the returned pfn must be guarded by a mmu notifier.
> >
> > This also adds a is_refcounted_page out parameter to kvm_follow_pfn that
> > is set when the returned pfn has an associated struct page with a valid
> > refcount. Callers that don't pass FOLL_GET should remember this value
> > and use it to avoid places like kvm_is_ad_tracked_page that assume a
> > non-zero refcount.
> >
> > Signed-off-by: David Stevens <stevensd@...omium.org>
> > ---
> >  include/linux/kvm_host.h | 10 ++++++
> >  virt/kvm/kvm_main.c      | 67 +++++++++++++++++++++-------------------
> >  virt/kvm/pfncache.c      |  2 +-
> >  3 files changed, 47 insertions(+), 32 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index ef2763c2b12e..a45308c7d2d9 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1157,6 +1157,9 @@ unsigned long gfn_to_hva_memslot_prot(struct kvm_memory_slot *slot, gfn_t gfn,
> >  void kvm_release_page_clean(struct page *page);
> >  void kvm_release_page_dirty(struct page *page);
> >
> > +void kvm_set_page_accessed(struct page *page);
> > +void kvm_set_page_dirty(struct page *page);
> > +
> >  struct kvm_follow_pfn {
> >       const struct kvm_memory_slot *slot;
> >       gfn_t gfn;
> > @@ -1164,10 +1167,17 @@ struct kvm_follow_pfn {
> >       bool atomic;
> >       /* Allow a read fault to create a writeable mapping. */
> >       bool allow_write_mapping;
> > +     /*
> > +      * Usage of the returned pfn will be guared by a mmu notifier. Must
> > +      * be true if FOLL_GET is not set.
> > +      */
> > +     bool guarded_by_mmu_notifier;
>
> And how? Any place to check the invalidate seq?

kvm_follow_pfn can't meaningfully validate the seq number, since the
mmu notifier locking is handled by the caller. This is more of a
sanity check that the API is being used properly, as proposed here
[1]. I did deviate from the proposal with a bool instead of some type
of integer, since the exact value of mmu_seq wouldn't be useful.

[1] https://lore.kernel.org/all/ZGvUsf7lMkrNDHuE@google.com/#t

-David