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

[David Hildenbrand <david@redhat.com>]

On 05.11.19 10:49, David Hildenbrand wrote:
> 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.
> 

(talking to myself, sorry)

Thinking again, dropping this patch from this series could effectively 
also fix that issue. E.g., kvm_release_pfn_clean() and friends would 
always do a put_page() if "pfn_valid() and !PageReserved()", so after 
patch 9 also on ZONDE_DEVICE pages.

But it would have side effects that might not be desired. E.g.,:

1. kvm_pfn_to_page() would also return ZONE_DEVICE pages (might even be 
the right thing to do).

2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be 
okay)

-- 

Thanks,

David / dhildenb