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

[David Hildenbrand <david@redhat.com>]

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?

> 
> Signed-off-by: Barret Rhoden <brho@google.com>
> ---
>  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