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

[Dan Williams <dan.j.williams@intel.com>]

On Thu, Dec 12, 2019 at 9:34 AM Sean Christopherson
<sean.j.christopherson@intel.com> 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@google.com>
> > ---
> >  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?

Correct, it can. Not in the filesystem-dax case, but device-dax
supports 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.

Yeah, since device-dax is the only path to support longterm page
pinning for vfio device assignment, testing with device-dax + 1GB
pages would be a useful sanity check.