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Subject: Re: [PATCH v4 2/2] kvm: Use huge pages for DAX-backed files
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20191212173413.GC3163@linux.intel.com>
Date:   Thu, 12 Dec 2019 19:45:21 +0200
Cc:     Barret Rhoden <brho@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-nvdimm@lists.01.org, x86@kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, jason.zeng@intel.com
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Message-Id: <57D547EE-2627-475D-84AF-0B080433DC52@oracle.com>
References: <20191211213207.215936-1-brho@google.com>
 <20191211213207.215936-3-brho@google.com>
 <20191212173413.GC3163@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On 12 Dec 2019, at 19:34, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>=20
> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> Signed-off-by: Barret Rhoden <brho@google.com>
>> ---
>> arch/x86/kvm/mmu/mmu.c | 36 ++++++++++++++++++++++++++++++++----
>> 1 file changed, 32 insertions(+), 4 deletions(-)
>>=20
>> 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;
>> }
>>=20
>> +static bool pfn_is_huge_mapped(struct kvm *kvm, gfn_t gfn, kvm_pfn_t =
pfn)
>> +{
>> +	struct page *page =3D 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 =3D 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:
>=20
> I assume this means DAX can have 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.

KVM doesn=E2=80=99t handle 1GB correctly for all types of non-hugetlbfs =
pages.
One example we have noticed internally but haven=E2=80=99t submitted an =
upstream patch yet is
for pages without =E2=80=9Cstruct page=E2=80=9D. As in this case, =
hva_to_pfn() will notice vma->vm_flags have VM_PFNMAP set
and call hva_to_pfn_remapped() -> follow_pfn().
However, follow_pfn() currently just calls follow_pte() which use =
__follow_pte_pmd() that doesn=E2=80=99t handle a huge PUD entry.

>=20
> The easiest thing is probably to rebase on kvm/queue.  You'll need to =
do
> that anyways, and I suspect doing so will help shake out any hiccups.
>=20
> [*] =
https://urldefense.proofpoint.com/v2/url?u=3Dhttps-3A__lkml.kernel.org_r_2=
0191206235729.29263-2D1-2Dsean.j.christopherson-40intel.com&d=3DDwIBAg&c=3D=
RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=3DJk6Q8nNzkQ6LJ6g42qARkg6ryI=
DGQr-yKXPNGZbpTx0&m=3DLk-PXE125WU3GWJOV4U4crsSEFx7f5AUmRJhkrfIeAE&s=3DBIo4=
tnL4OfswRQ2QKfTs9VYScLU5lBy2pwzePBnHow8&e=3D=20
>=20
>> +		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 =3D=3D =
PT_PAGE_TABLE_LEVEL &&
>> -	    PageTransCompoundMap(pfn_to_page(pfn)) &&
>> +	    level =3D=3D 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 =3D 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);
>>=20
>> 			if (kvm_available_flush_tlb_with_range())
>> --=20
>> 2.24.0.525.g8f36a354ae-goog
>>=20