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

From: David Hildenbrand <david@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
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Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes
Date: Tue, 5 Nov 2019 10:49:10 +0100	[thread overview]
Message-ID: <01adb4cb-6092-638c-0bab-e61322be7cf5@redhat.com> (raw)
In-Reply-To: <bbe59155-24ae-f229-e182-107730423c47@redhat.com>

On 05.11.19 10:17, David Hildenbrand wrote:
> On 05.11.19 05:38, Dan Williams wrote:
>> On Thu, Oct 24, 2019 at 5:11 AM David Hildenbrand <david@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@redhat.com>
>>> Cc: "Radim Krčmář" <rkrcmar@redhat.com>
>>> Cc: Michal Hocko <mhocko@kernel.org>
>>> Cc: Dan Williams <dan.j.williams@intel.com>
>>> Cc: KarimAllah Ahmed <karahmed@amazon.de>
>>> Signed-off-by: David Hildenbrand <david@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
> 

I think I know what's going wrong:

Pages that are pinned via gfn_to_pfn() and friends take a references, 
however are often released via 
kvm_release_pfn_clean()/kvm_release_pfn_dirty()/kvm_release_page_clean()...

E.g., in arch/x86/kvm/x86.c:reexecute_instruction()

...
pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
...
kvm_release_pfn_clean(pfn);

void kvm_release_pfn_clean(kvm_pfn_t pfn)
{
	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn))
		put_page(pfn_to_page(pfn));
}

This function makes perfect sense as the counterpart for kvm_get_pfn():

void kvm_get_pfn(kvm_pfn_t pfn)
{
	if (!kvm_is_reserved_pfn(pfn))
		get_page(pfn_to_page(pfn));
}

As all ZONE_DEVICE pages are currently reserved, pages pinned via 
gfn_to_pfn() and friends will often not see a put_page() AFAIKS.

Now, my patch does not change that, the result of 
kvm_is_reserved_pfn(pfn) will be unchanged. A proper fix for that would 
probably be

a) To drop the reference to ZONE_DEVICE pages in gfn_to_pfn() and 
friends, after you successfully pinned the pages. (not sure if that's 
the right thing to do but you're the expert)

b) To not use kvm_release_pfn_clean() and friends on pages that were 
definitely pinned.

-- 

Thanks,

David / dhildenb