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

[David Hildenbrand]

>>> 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.
> 
> Assuming gup() takes a reference for ZONE_DEVICE pages, yes, this is a
> KVM bug.

Yes, it does take a reference AFAIKs. E.g.,

mm/gup.c:gup_pte_range():
...
		if (pte_devmap(pte)) {
			if (unlikely(flags & FOLL_LONGTERM))
				goto pte_unmap;

			pgmap = get_dev_pagemap(pte_pfn(pte), pgmap);
			if (unlikely(!pgmap)) {
				undo_dev_pagemap(nr, nr_start, pages);
				goto pte_unmap;
			}
		} else if (pte_special(pte))
			goto pte_unmap;

		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
		page = pte_page(pte);

		head = try_get_compound_head(page, 1);

try_get_compound_head() will increment the reference count.

> 
>>> 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.
> 
> This is already KVM's intent, i.e. the purpose of the PageReserved() check
> is simply to avoid putting a non-existent reference.  The problem is that
> KVM assumes pages with PG_reserved set are never pinned, which AFAICT was
> true when the code was first added.
> 
>> (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.
> 
> Yeah, this appears to be the correct fix.
> 
>> 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).
> 
> This should be ok, at least on x86.  There are only three users of
> kvm_pfn_to_page().  Two of those are on allocations that are controlled by
> KVM and are guaranteed to be vanilla MAP_ANONYMOUS.  The third is on guest
> memory when running a nested guest, and in that case supporting ZONE_DEVICE
> memory is desirable, i.e. KVM should play nice with a guest that is backed
> by ZONE_DEVICE memory.
> 
>> 2. kvm_set_pfn_dirty() would also set ZONE_DEVICE pages dirty (might be
>> okay)
> 
> This is ok from a KVM perspective.

What about

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

Is a pure get_page() sufficient in case of ZONE_DEVICE?
(asking because of the live references obtained via 
get_dev_pagemap(pte_pfn(pte), pgmap) in mm/gup.c:gup_pte_range() 
somewhat confuse me :) )


> 
> The scarier code (for me) is transparent_hugepage_adjust() and
> kvm_mmu_zap_collapsible_spte(), as I don't at all understand the
> interaction between THP and _PAGE_DEVMAP.

The x86 KVM MMU code is one of the ugliest code I know (sorry, but it 
had to be said :/ ). Luckily, this should be independent of the 
PG_reserved thingy AFAIKs.

-- 

Thanks,

David / dhildenb