Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes

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On 05.11.19 05:38, Dan Williams wrote:
On Thu, Oct 24, 2019 at 5:11 AM David Hildenbrand <david@xxxxxxxxxx> wrote:

Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
change that.

KVM has this weird use case that you can map anything from /dev/mem
into the guest. pfn_valid() is not a reliable check whether the memmap
was initialized and can be touched. pfn_to_online_page() makes sure
that we have an initialized memmap (and don't have ZONE_DEVICE memory).

Rewrite kvm_is_reserved_pfn() to make sure the function produces the
same result once we stop setting ZONE_DEVICE pages PG_reserved.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: KarimAllah Ahmed <karahmed@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
  virt/kvm/kvm_main.c | 10 ++++++++--
  1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e9eb666eb6e8..9d18cc67d124 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -151,9 +151,15 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,

  bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
  {
-       if (pfn_valid(pfn))
-               return PageReserved(pfn_to_page(pfn));
+       struct page *page = pfn_to_online_page(pfn);

+       /*
+        * We treat any pages that are not online (not managed by the buddy)
+        * as reserved - this includes ZONE_DEVICE pages and pages without
+        * a memmap (e.g., mapped via /dev/mem).
+        */
+       if (page)
+               return PageReserved(page);
         return true;
  }

So after this all the pfn_valid() usage in kvm_main.c is replaced with
pfn_to_online_page()? Looks correct to me.

However, I'm worried that kvm is taking reference on ZONE_DEVICE pages
through some other path resulting in this:

     https://lore.kernel.org/linux-nvdimm/20190919154708.GA24650@xxxxxxxxxx/

I'll see if this patch set modulates or maintains that failure mode.


I'd assume that the behavior is unchanged. Ithink we get a reference to these ZONE_DEVICE pages via __get_user_pages_fast() and friends in hva_to_pfn_fast() and friends in virt/kvm/kvm_main.c

--

Thanks,

David / dhildenb

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