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Subject: Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20191213175031.GC31552@linux.intel.com>
Date:   Fri, 13 Dec 2019 20:08:59 +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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 <ED482280-CB47-4AB6-9E7E-EEE7848E0F8B@oracle.com>
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 <20191213175031.GC31552@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 13 Dec 2019, at 19:50, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>=20
> On Fri, Dec 13, 2019 at 07:31:55PM +0200, Liran Alon wrote:
>>=20
>>> On 13 Dec 2019, at 19:19, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>>>=20
>>> Then allowed_hugepage_adjust() would look something like:
>>>=20
>>> static void allowed_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t =
gfn,
>>> 				    kvm_pfn_t *pfnp, int *levelp, int =
max_level)
>>> {
>>> 	kvm_pfn_t pfn =3D *pfnp;
>>> 	int level =3D *levelp;=09
>>> 	unsigned long mask;
>>>=20
>>> 	if (is_error_noslot_pfn(pfn) || !kvm_is_reserved_pfn(pfn) ||
>>> 	    level =3D=3D PT_PAGE_TABLE_LEVEL)
>>> 		return;
>>>=20
>>> 	/*
>>> 	 * mmu_notifier_retry() was successful and mmu_lock is held, so
>>> 	 * the pmd/pud can't be split from under us.
>>> 	 */
>>> 	level =3D host_pfn_mapping_level(vcpu->kvm, gfn, pfn);
>>>=20
>>> 	*levelp =3D level =3D min(level, max_level);
>>> 	mask =3D KVM_PAGES_PER_HPAGE(level) - 1;
>>> 	VM_BUG_ON((gfn & mask) !=3D (pfn & mask));
>>> 	*pfnp =3D pfn & ~mask;
>>=20
>> Why don=E2=80=99t you still need to kvm_release_pfn_clean() for =
original pfn and
>> kvm_get_pfn() for new huge-page start pfn?
>=20
> That code is gone in kvm/queue.  thp_adjust() is now called from
> __direct_map() and FNAME(fetch), and so its pfn adjustment doesn't =
bleed
> back to the page fault handlers.  The only reason the put/get pfn code
> existed was because the page fault handlers called =
kvm_release_pfn_clean()
> on the pfn, i.e. they would have put the wrong pfn.

Ack. Thanks for the explaining this.