Received-SPF: pass (google.com: domain of linux-kernel+bounces-132751-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Precedence: bulk
MIME-Version: 1.0
References: <20240320005024.3216282-1-seanjc@google.com> <4d04b010-98f3-4eae-b320-a7dd6104b0bf@redhat.com>
 <CALzav=eLH+V_5Y6ZWsRkmnbEb6fxPa55B7xyWxP3o6qsrs_nHA@mail.gmail.com>
 <a2fff462-dfe6-4979-a7b2-131c6e0b5017@redhat.com> <ZgygGmaEuddZGKyX@google.com>
 <ca1f320b-dc06-48e0-b4f5-ce860a72f0e2@redhat.com> <Zg3V-M3iospVUEDU@google.com>
 <42dbf562-5eab-4f82-ad77-5ee5b8c79285@redhat.com> <Zg7j2D6WFqcPaXFB@google.com>
 <b3ea925f-bd47-4f54-bede-3f0d7471e3d7@redhat.com> <Zg8jip0QIBbOCgpz@google.com>
 <36e1592a-e590-48a0-9a79-eeac6b41314b@redhat.com>
In-Reply-To: <36e1592a-e590-48a0-9a79-eeac6b41314b@redhat.com>
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Fri, 5 Apr 2024 11:37:47 +0200
Message-ID: <CABgObfbykeRXv2r2tULe6_SwD7DkWPaMTdc6PkAUb3JmTodf4w@mail.gmail.com>
Subject: Re: [RFC PATCH 0/4] KVM: x86/mmu: Rework marking folios dirty/accessed
To: David Hildenbrand <david@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>, David Matlack <dmatlack@google.com>, kvm@vger.kernel.org, 
	linux-kernel@vger.kernel.org, David Stevens <stevensd@chromium.org>, 
	Matthew Wilcox <willy@infradead.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On Fri, Apr 5, 2024 at 8:53=E2=80=AFAM David Hildenbrand <david@redhat.com>=
 wrote:
> >       mmu_notifier_invalidate_range_start(&range);
> >       tlb_start_vma(&tlb, vma);
> >       walk_page_range(vma->vm_mm, range.start, range.end,
> >                       &madvise_free_walk_ops, &tlb);
> >       tlb_end_vma(&tlb, vma);
> >       mmu_notifier_invalidate_range_end(&range);
> >
>
> Indeed, we do setup the MMU notifier invalidation. We do the start/end
> ... I was looking for PTE notifications.
>
> I spotted the set_pte_at(), not a set_pte_at_notify() like we do in
> other code. Maybe that's not required here (digging through
> documentation I'm still left clueless). [...]
> Absolutely unclear to me when we *must* to use it, or if it is. Likely
> its a pure optimization when we're *changing* a PTE.

Yes, .change_pte() is just an optimization. The original point of it
was for KSM, so that KVM could flip the sPTE to a new location without
first zapping it. At the time there was also an .invalidate_page()
callback, and both of them were *not* bracketed by calls to
mmu_notifier_invalidate_range_{start,end}()

Later on, both callbacks were changed to occur within an
invalidate_range_start/end() block.

Commit 6bdb913f0a70 ("mm: wrap calls to set_pte_at_notify with
invalidate_range_start and invalidate_range_end", 2012-10-09) did so
for .change_pte(). The reason to do so was a bit roundabout, namely to
allow sleepable .invalidate_page() hooks (because .change_pte() is not
sleepable and at the time .invalidate_page() was used as a fallback
for .change_pte()).

This however made KVM's usage of the .change_pte() callback completely
moot, because KVM unmaps the sPTEs during .invalidate_range_start()
and therefore .change_pte() has no hope of succeeding.

(Commit 369ea8242c0f ("mm/rmap: update to new mmu_notifier semantic
v2", 2017-08-31) is where the other set of non-bracketed calls to MMU
notifier callbacks was changed; calls to
mmu_notifier_invalidate_page() were replaced by calls to
mmu_notifier_invalidate_range(), bracketed by calls to
mmu_notifier_invalidate_range_{start,end}()).

Since KVM is the only user of .change_pte(), we can remove
change_pte() and set_pte_at_notify() completely.

Paolo

> __replace_page() uses it with __replace_page() when replacing the mapped
> page. And migrate_vma_insert_page() uses it with MMU_NOTIFY_MIGRATE.
>
> Other code (e.g., khugepaged) doesn't seem to use it as well.
>
> ... so I guess it's fine? Confusing.
>
> --
> Cheers,
>
> David / dhildenb
>