Re: [PATCH 05/14] KVM: x86/mmu: Consult max mapping level when zapping collapsible SPTEs

Mike Kravetz
2021-02-18T22:32:57Z

On 2/18/21 8:23 AM, Sean Christopherson wrote:
> On Thu, Feb 18, 2021, Paolo Bonzini wrote:
>> On 13/02/21 01:50, Sean Christopherson wrote:
>>>
>>> pfn = spte_to_pfn(iter.old_spte);
>>> if (kvm_is_reserved_pfn(pfn) ||
>>> - (!PageTransCompoundMap(pfn_to_page(pfn)) &&
>>> - !kvm_is_zone_device_pfn(pfn)))
>>> + iter.level >= kvm_mmu_max_mapping_level(kvm, slot, iter.gfn,
>>> + pfn, PG_LEVEL_NUM))
>>> continue;
>>
>>
>> This changes the test to PageCompound. Is it worth moving the change to
>> patch 1?
>
> Yes? I originally did that in a separate patch, then changed my mind.
>
> If PageTransCompoundMap() also detects HugeTLB pages, then it is the "better"
> option as it checks that the page is actually mapped huge. I dropped the change
> because PageTransCompound() is just a wrapper around PageCompound(), and so I
> assumed PageTransCompoundMap() would detect HugeTLB pages, too. I'm not so sure
> about that after rereading the code, yet again.

I have not followed this thread, but HugeTLB hit my mail filter and I can
help with this question.

No, PageTransCompoundMap() will not detect HugeTLB. hugetlb pages do not
use the compound_mapcount_ptr field. So, that final check/return in
PageTransCompoundMap() will always be false.
--
Mike Kravetz