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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: <CAPcyv4i5ZaiA+KeraXzz-0vs25UGEmZ2ka9Z-PUT3T_7URAFMA@mail.gmail.com>
Date:   Thu, 12 Dec 2019 20:32:17 +0200
Cc:     Barret Rhoden <brho@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        X86 ML <x86@kernel.org>, KVM list <kvm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Zeng, Jason" <jason.zeng@intel.com>
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References: <20191211213207.215936-1-brho@google.com>
 <20191211213207.215936-3-brho@google.com>
 <376DB19A-4EF1-42BF-A73C-741558E397D4@oracle.com>
 <CAPcyv4gpYF=D323G+69FhFZw4i5W-15_wTRa1xNPdmear0phTw@mail.gmail.com>
 <F19843AB-1974-4E79-A85B-9AE00D58E192@oracle.com>
 <CAPcyv4i5ZaiA+KeraXzz-0vs25UGEmZ2ka9Z-PUT3T_7URAFMA@mail.gmail.com>
To:     Dan Williams <dan.j.williams@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 12 Dec 2019, at 19:59, Dan Williams <dan.j.williams@intel.com> =
wrote:
>=20
> On Thu, Dec 12, 2019 at 9:39 AM Liran Alon <liran.alon@oracle.com> =
wrote:
>>=20
>>=20
>>=20
>>> On 12 Dec 2019, at 18:54, Dan Williams <dan.j.williams@intel.com> =
wrote:
>>>=20
>>> On Thu, Dec 12, 2019 at 4:34 AM Liran Alon <liran.alon@oracle.com> =
wrote:
>>>>=20
>>>>=20
>>>>=20
>>>>> On 11 Dec 2019, at 23:32, 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
>>>>> 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
>>>> For hugetlbfs pages, tdp_page_fault() -> mapping_level() -> =
host_mapping_level() -> kvm_host_page_size() -> vma_kernel_pagesize()
>>>> will return the page-size of the hugetlbfs without the need to =
parse the page-tables.
>>>> See vma->vm_ops->pagesize() callback implementation at =
hugetlb_vm_ops->pagesize()=3D=3Dhugetlb_vm_op_pagesize().
>>>>=20
>>>> Only for pages that were originally mapped as small-pages and later =
merged to larger pages by THP, there is a need to check for =
PageTransCompound(). Again, instead of parsing page-tables.
>>>>=20
>>>> Therefore, it seems more logical to me that:
>>>> (a) If DAX-backed files are mapped as large-pages to userspace, it =
should be reflected in vma->vm_ops->page_size() of that mapping. Causing =
kvm_host_page_size() to return the right size without the need to parse =
the page-tables.
>>>=20
>>> A given dax-mapped vma may have mixed page sizes so ->page_size()
>>> can't be used reliably to enumerating the mapping size.
>>=20
>> Naive question: Why don=E2=80=99t split the VMA in this case to =
multiple VMAs with different results for ->page_size()?
>=20
> Filesystems traditionally have not populated ->pagesize() in their
> vm_operations, there was no compelling reason to go add it and the
> complexity seems prohibitive.

I understand. Though this is technical debt that breaks ->page_size() =
semantics which might cause a complex bug some day...

>=20
>> What you are describing sounds like DAX is breaking this callback =
semantics in an unpredictable manner.
>=20
> It's not unpredictable. vma_kernel_pagesize() returns PAGE_SIZE.

Of course. :) I meant it may be unexpected by the caller.

> Huge
> pages in the page cache has a similar issue.

Ok. I haven=E2=80=99t known that. Thanks for the explanation.

>=20
>>>> (b) If DAX-backed files small-pages can be later merged to =
large-pages by THP, then the =E2=80=9Cstruct page=E2=80=9D of these =
pages should be modified as usual to make PageTransCompound() return =
true for them. I=E2=80=99m not highly familiar with this mechanism, but =
I would expect THP to be able to merge DAX-backed files small-pages to =
large-pages in case DAX provides =E2=80=9Cstruct page=E2=80=9D for the =
DAX pages.
>>>=20
>>> DAX pages do not participate in THP and do not have the
>>> PageTransCompound accounting. The only mechanism that records the
>>> mapping size for dax is the page tables themselves.
>>=20
>> What is the rational behind this? Given that DAX pages can be =
described with =E2=80=9Cstruct page=E2=80=9D (i.e. ZONE_DEVICE), what =
prevents THP from manipulating page-tables to merge multiple DAX PFNs to =
a larger page?
>=20
> THP accounting is a function of the page allocator. ZONE_DEVICE pages
> are excluded from the page allocator. ZONE_DEVICE is just enough
> infrastructure to support pfn_to_page(), page_address(), and
> get_user_pages(). Other page allocator services beyond that are not
> present.

Ok.