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

[Dan Williams]

On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <liran.alon@xxxxxxxxxx> wrote:
>
>
>
> > On 11 Dec 2019, at 23:32, Barret Rhoden <brho@xxxxxxxxxx> 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.
>
> For hugetlbfs pages, tdp_page_fault() -> mapping_level() -> host_mapping_level() -> kvm_host_page_size() -> vma_kernel_pagesize()
> will return the page-size of the hugetlbfs without the need to parse the page-tables.
> See vma->vm_ops->pagesize() callback implementation at hugetlb_vm_ops->pagesize()==hugetlb_vm_op_pagesize().
>
> Only for pages that were originally mapped as small-pages and later merged to larger pages by THP, there is a need to check for PageTransCompound(). Again, instead of parsing page-tables.
>
> Therefore, it seems more logical to me that:
> (a) If DAX-backed files are mapped as large-pages to userspace, it should be reflected in vma->vm_ops->page_size() of that mapping. Causing kvm_host_page_size() to return the right size without the need to parse the page-tables.

A given dax-mapped vma may have mixed page sizes so ->page_size()
can't be used reliably to enumerating the mapping size.

> (b) If DAX-backed files small-pages can be later merged to large-pages by THP, then the âstruct pageâ of these pages should be modified as usual to make PageTransCompound() return true for them. Iâm not highly familiar with this mechanism, but I would expect THP to be able to merge DAX-backed files small-pages to large-pages in case DAX provides âstruct pageâ for the DAX pages.

DAX pages do not participate in THP and do not have the
PageTransCompound accounting. The only mechanism that records the
mapping size for dax is the page tables themselves.


>
> >
> > 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)) {
>
> Doesnât dev_pagemap_mapping_shift() get âstruct pageâ as first parameter?
> Was this changed by a commit I missed?
>
> -Liran
>
> > +     case PMD_SHIFT:
> > +     case PUD_SIZE:
> > +             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
> >
>