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

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Message-ID: <df6eb767-0ae0-84a3-3f05-1ece4cb9ce22@oracle.com>
Date:   Thu, 18 Feb 2021 14:30:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>
Subject: Re: [PATCH 05/14] KVM: x86/mmu: Consult max mapping level when
 zapping collapsible SPTEs

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