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

[Pankaj Gupta]

> 
> On 09.11.18 21:39, 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 (grabbed before checking
> > the mmu seq).
> 
> I wonder if the KVM mmu spinlock is enough for walking (not KVM
> exclusive) host page tables. Can you elaborate?

As this patch is dependent on PageReserved patch(which is in progress), just 
wondering if we are able to test the code path for hugepage with DAX.

Thanks,
Pankaj 
 
> 
> > 
> > Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu.c | 34 ++++++++++++++++++++++++++++++++--
> >  1 file changed, 32 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index cf5f572f2305..2df8c459dc6a 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -3152,6 +3152,36 @@ 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, map_shift;
> > +
> > +	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.
> > +	 */
> > +	map_shift = dev_pagemap_mapping_shift(hva, current->mm);
> 
> You could get rid of that local variable map_shift.
> 
> > +	switch (map_shift) {
> > +	case PMD_SHIFT:
> > +	case PUD_SIZE:
> > +		return true;
> > +	}
> > +	return false;
> > +}
> > +
> >  static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
> >  					gfn_t *gfnp, kvm_pfn_t *pfnp,
> >  					int *levelp)
> > @@ -3168,7 +3198,7 @@ static void transparent_hugepage_adjust(struct
> > kvm_vcpu *vcpu,
> >  	 */
> >  	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
> >  	    level == PT_PAGE_TABLE_LEVEL &&
> > -	    PageTransCompoundMap(pfn_to_page(pfn)) &&
> > +	    pfn_is_huge_mapped(vcpu->kvm, gfn, pfn) &&
> >  	    !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
> >  		unsigned long mask;
> >  		/*
> > @@ -5678,7 +5708,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm
> > *kvm,
> >  		 */
> >  		if (sp->role.direct &&
> >  			!kvm_is_reserved_pfn(pfn) &&
> > -			PageTransCompoundMap(pfn_to_page(pfn))) {
> > +			pfn_is_huge_mapped(kvm, sp->gfn, pfn)) {
> >  			pte_list_remove(rmap_head, sptep);
> >  			need_tlb_flush = 1;
> >  			goto restart;
> > 
> 
> This looks surprisingly simple to me :)
> 
> --
> 
> Thanks,
> 
> David / dhildenb
>