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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: <06108004-1720-41EB-BCAB-BFA8FEBF4772@oracle.com>
Date:   Thu, 12 Dec 2019 20:49:52 +0200
Cc:     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>,
        Sean Christopherson <sean.j.christopherson@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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References: <20191212182238.46535-1-brho@google.com>
 <20191212182238.46535-3-brho@google.com>
 <06108004-1720-41EB-BCAB-BFA8FEBF4772@oracle.com>
To:     Barret Rhoden <brho@google.com>
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> On 12 Dec 2019, at 20:47, Liran Alon <liran.alon@oracle.com> wrote:
>=20
>=20
>=20
>> On 12 Dec 2019, at 20:22, Barret Rhoden <brho@google.com> wrote:
>>=20
>> This change allows KVM to map DAX-backed files made of huge pages =
with
>> huge mappings in the EPT/TDP.
>=20
> This change isn=E2=80=99t only relevant for TDP. It also affects when =
KVM use shadow-paging.
> See how FNAME(page_fault)() calls transparent_hugepage_adjust().
>=20
>>=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.
>=20
> I would rephrase =E2=80=9CThe existing check is trying to determine if =
the pfn
> is mapped as part of a transparent huge-page=E2=80=9D.
>=20
>> For
>> non-DAX maps, e.g. hugetlbfs, that means checking PageTransCompound.
>=20
> This is not related to hugetlbfs but rather THP.
>=20
>> 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>
>=20
> I don=E2=80=99t think the right place to change for this functionality =
is transparent_hugepage_adjust()
> which is meant to handle PFNs that are mapped as part of a transparent =
huge-page.
>=20
> For example, this would prevent mapping DAX-backed file page as 1GB.
> As transparent_hugepage_adjust() only handles the case (level =3D=3D =
PT_PAGE_TABLE_LEVEL).
>=20
> As you are parsing the page-tables to discover the page-size the PFN =
is mapped in,
> I think you should instead modify kvm_host_page_size() to parse =
page-tables instead
> of rely on vma_kernel_pagesize() (Which relies on =
vma->vm_ops->pagesize()) in case
> of is_zone_device_page().
> The main complication though of doing this is that at this point you =
don=E2=80=99t yet have the PFN
> that is retrieved by try_async_pf(). So maybe you should consider =
modifying the order of calls
> in tdp_page_fault() & FNAME(page_fault)().
>=20
> -Liran

Or alternatively when thinking about it more, maybe just rename =
transparent_hugepage_adjust()
to not be specific to THP and better handle the case of parsing =
page-tables changing mapping-level to 1GB.
That is probably easier and more elegant.

-Liran