Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files

[Liran Alon <liran.alon@oracle.com>]

> On 12 Dec 2019, at 20:22, Barret Rhoden <brho@google.com> wrote:
> 
> This change allows KVM to map DAX-backed files made of huge pages with
> huge mappings in the EPT/TDP.

This change isn’t only relevant for TDP. It also affects when KVM use shadow-paging.
See how FNAME(page_fault)() calls transparent_hugepage_adjust().

> 
> DAX pages are not PageTransCompound.  The existing check is trying to
> determine if the mapping for the pfn is a huge mapping or not.

I would rephrase “The existing check is trying to determine if the pfn
is mapped as part of a transparent huge-page”.

> For
> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.

This is not related to hugetlbfs but rather THP.

> 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@google.com>

I don’t think the right place to change for this functionality is transparent_hugepage_adjust()
which is meant to handle PFNs that are mapped as part of a transparent huge-page.

For example, this would prevent mapping DAX-backed file page as 1GB.
As transparent_hugepage_adjust() only handles the case (level == PT_PAGE_TABLE_LEVEL).

As you are parsing the page-tables to discover the page-size the PFN is mapped in,
I think you should instead modify kvm_host_page_size() to parse page-tables instead
of rely on vma_kernel_pagesize() (Which relies on vma->vm_ops->pagesize()) in case
of is_zone_device_page().
The main complication though of doing this is that at this point you don’t yet have the PFN
that is retrieved by try_async_pf(). So maybe you should consider modifying the order of calls
in tdp_page_fault() & FNAME(page_fault)().

-Liran

> ---
> arch/x86/kvm/mmu/mmu.c | 31 +++++++++++++++++++++++++++----
> 1 file changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7269130ea5e2..ea8f6951398b 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3328,6 +3328,30 @@ static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
> 	__direct_pte_prefetch(vcpu, sp, sptep);
> }
> 
> +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.
> +	 */
> +	return dev_pagemap_mapping_shift(hva, current->mm) > PAGE_SHIFT;
> +}
> +
> static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
> 					gfn_t gfn, kvm_pfn_t *pfnp,
> 					int *levelp)
> @@ -3342,8 +3366,8 @@ 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)) {
> 		unsigned long mask;
> 
> 		/*
> @@ -5957,8 +5981,7 @@ 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
>