Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files

[Liran Alon]

> On 12 Dec 2019, at 19:34, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> 
> On Wed, Dec 11, 2019 at 04:32:07PM -0500, Barret Rhoden wrote:
>> This change allows KVM to map DAX-backed files made of huge pages with
>> huge mappings in the EPT/TDP.
>> 
>> DAX pages are not PageTransCompound.  The existing check is trying to
>> determine if the mapping for the pfn is a huge mapping or not.  For
>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
>> For DAX, we can check the page table itself.
>> 
>> Note that KVM already faulted in the page (or huge page) in the host's
>> page table, and we hold the KVM mmu spinlock.  We grabbed that lock in
>> kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq.
>> 
>> Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx>
>> ---
>> arch/x86/kvm/mmu/mmu.c | 36 ++++++++++++++++++++++++++++++++----
>> 1 file changed, 32 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 6f92b40d798c..cd07bc4e595f 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -3384,6 +3384,35 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn)
>> 	return -EFAULT;
>> }
>> 
>> +static bool pfn_is_huge_mapped(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
>> +{
>> +	struct page *page = pfn_to_page(pfn);
>> +	unsigned long hva;
>> +
>> +	if (!is_zone_device_page(page))
>> +		return PageTransCompoundMap(page);
>> +
>> +	/*
>> +	 * DAX pages do not use compound pages.  The page should have already
>> +	 * been mapped into the host-side page table during try_async_pf(), so
>> +	 * we can check the page tables directly.
>> +	 */
>> +	hva = gfn_to_hva(kvm, gfn);
>> +	if (kvm_is_error_hva(hva))
>> +		return false;
>> +
>> +	/*
>> +	 * Our caller grabbed the KVM mmu_lock with a successful
>> +	 * mmu_notifier_retry, so we're safe to walk the page table.
>> +	 */
>> +	switch (dev_pagemap_mapping_shift(hva, current->mm)) {
>> +	case PMD_SHIFT:
>> +	case PUD_SIZE:
> 
> I assume this means DAX can have 1GB pages?  I ask because KVM's THP logic
> has historically relied on THP only supporting 2MB.  I cleaned this up in
> a recent series[*], which is in kvm/queue, but I obviously didn't actually
> test whether or not KVM would correctly handle 1GB non-hugetlbfs pages.

KVM doesnât handle 1GB correctly for all types of non-hugetlbfs pages.
One example we have noticed internally but havenât submitted an upstream patch yet is
for pages without âstruct pageâ. As in this case, hva_to_pfn() will notice vma->vm_flags have VM_PFNMAP set
and call hva_to_pfn_remapped() -> follow_pfn().
However, follow_pfn() currently just calls follow_pte() which use __follow_pte_pmd() that doesnât handle a huge PUD entry.

> 
> The easiest thing is probably to rebase on kvm/queue.  You'll need to do
> that anyways, and I suspect doing so will help shake out any hiccups.
> 
> [*] https://urldefense.proofpoint.com/v2/url?u=https-3A__lkml.kernel.org_r_20191206235729.29263-2D1-2Dsean.j.christopherson-40intel.com&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=Lk-PXE125WU3GWJOV4U4crsSEFx7f5AUmRJhkrfIeAE&s=BIo4tnL4OfswRQ2QKfTs9VYScLU5lBy2pwzePBnHow8&e= 
> 
>> +		return true;
>> +	}
>> +	return false;
>> +}
>> +
>> static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
>> 					gfn_t gfn, kvm_pfn_t *pfnp,
>> 					int *levelp)
>> @@ -3398,8 +3427,8 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
>> 	 * here.
>> 	 */
>> 	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
>> -	    !kvm_is_zone_device_pfn(pfn) && level == PT_PAGE_TABLE_LEVEL &&
>> -	    PageTransCompoundMap(pfn_to_page(pfn)) &&
>> +	    level == PT_PAGE_TABLE_LEVEL &&
>> +	    pfn_is_huge_mapped(vcpu->kvm, gfn, pfn) &&
>> 	    !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
>> 		unsigned long mask;
>> 		/*
>> @@ -6015,8 +6044,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
>> 		 * mapping if the indirect sp has level = 1.
>> 		 */
>> 		if (sp->role.direct && !kvm_is_reserved_pfn(pfn) &&
>> -		    !kvm_is_zone_device_pfn(pfn) &&
>> -		    PageTransCompoundMap(pfn_to_page(pfn))) {
>> +		    pfn_is_huge_mapped(kvm, sp->gfn, pfn)) {
>> 			pte_list_remove(rmap_head, sptep);
>> 
>> 			if (kvm_available_flush_tlb_with_range())
>> -- 
>> 2.24.0.525.g8f36a354ae-goog
>>