Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by huge_pte_offset()

[Sean Christopherson]

On Sun, Mar 22, 2020 at 07:54:32PM -0700, Mike Kravetz wrote:
> On 3/22/20 7:03 PM, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> > 
> > On 2020/3/22 7:38, Mike Kravetz wrote:
> >> On 2/21/20 7:33 PM, Longpeng(Mike) wrote:
> >>> From: Longpeng <longpeng2@xxxxxxxxxx>
> I have not looked closely at the generated code for lookup_address_in_pgd.
> It appears that it would dereference p4d, pud and pmd multiple times.  Sean
> seemed to think there was something about the calling context that would
> make issues like those seen with huge_pte_offset less likely to happen.  I
> do not know if this is accurate or not.

Only for KVM's calls to lookup_address_in_mm(), I can't speak to other
calls that funnel into to lookup_address_in_pgd().

KVM uses a combination of tracking and blocking mmu_notifier calls to ensure
PTE changes/invalidations between gup() and lookup_address_in_pgd() cause a
restart of the faulting instruction, and that pending changes/invalidations
are blocked until installation of the pfn in KVM's secondary MMU completes.

kvm_mmu_page_fault():

	mmu_seq = kvm->mmu_notifier_seq;
	smp_rmb();

	pfn = gup(hva);

	spin_lock(&kvm->mmu_lock);
	smp_rmb();
	if (kvm->mmu_notifier_seq != mmu_seq)
		goto out_unlock: // Restart guest, i.e. retry the fault

	lookup_address_in_mm(hva, ...);

	...

  out_unlock:
	spin_unlock(&kvm->mmu_lock);


kvm_mmu_notifier_change_pte() / kvm_mmu_notifier_invalidate_range_end():

	spin_lock(&kvm->mmu_lock);
	kvm->mmu_notifier_seq++;
	smp_wmb();
	spin_unlock(&kvm->mmu_lock);


> Let's remove the two READ_ONCE calls and move this patch forward.  We can
> look closer at lookup_address_in_pgd and generate another patch if that needs
> to be fixed as well.
> 
> Thanks
> -- 
> Mike Kravetz