On 13/11/2018 11:02, Pankaj Gupta wrote:
>
>>
>> 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.
The MMU spinlock is taken in kvm_mmu_notifier_invalidate_range_end, so
it should be enough.
Paolo
>
> 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
>>
