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

[David Hildenbrand]

On 05.11.19 05:38, Dan Williams wrote:
> On Thu, Oct 24, 2019 at 5:11 AM David Hildenbrand <david at redhat.com> 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 at redhat.com>
>> Cc: "Radim Krčmář" <rkrcmar at redhat.com>
>> Cc: Michal Hocko <mhocko at kernel.org>
>> Cc: Dan Williams <dan.j.williams at intel.com>
>> Cc: KarimAllah Ahmed <karahmed at amazon.de>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
>> ---
>>   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@angband.pl/
> 
> 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