Re: [PATCH v3 03/70] RAMBlock/guest_memfd: Enable KVM_GUEST_MEMFD_ALLOW_HUGEPAGE

David Hildenbrand
2023-11-30T17:21:49Z

On 30.11.23 17:01, Sean Christopherson wrote:
> On Thu, Nov 30, 2023, David Hildenbrand wrote:
>> On 30.11.23 08:32, Xiaoyao Li wrote:
>>> On 11/20/2023 5:26 PM, David Hildenbrand wrote:
>>>>
>>>>>> ... did you shamelessly copy that from hw/virtio/virtio-mem.c ? ;)
>>>>>
>>>>> Get caught.
>>>>>
>>>>>> This should be factored out into a common helper.
>>>>>
>>>>> Sure, will do it in next version.
>>>>
>>>> Factor it out in a separate patch. Then, this patch is get small that
>>>> you can just squash it into #2.
>>>>
>>>> And my comment regarding "flags = 0" to patch #2 does no longer apply :)
>>>>
>>>
>>> I see.
>>>
>>> But it depends on if KVM_GUEST_MEMFD_ALLOW_HUGEPAGE will appear together
>>> with initial guest memfd in linux (hopefully 6.8)
>>> https://lore.kernel.org/all/CABgObfa=DH7FySBviF63OS9sVog_wt-AqYgtUAGKqnY5Bizivw@mail.gmail.com/
>>>
>>
>> Doesn't seem to be in -next if I am looking at the right tree:
>>
>> https://git.kernel.org/pub/scm/virt/kvm/kvm.git/log/?h=next
>
> Yeah, we punted on adding hugepage support for the initial guest_memfd merge so
> as not to rush in kludgy uABI. The internal KVM code isn't problematic, we just
> haven't figured out exactly what the ABI should look like, e.g. should hugepages
> be dependent on THP being enabled, and if not, how does userspace discover the
> supported hugepage sizes?

Are we talking about THP or hugetlb? They are two different things, and
"KVM_GUEST_MEMFD_ALLOW_HUGEPAGE" doesn't make it clearer what we are
talking about.

This patch here "get_thp_size()" indicates that we care about THP, not
hugetlb.

THP lives in:
/sys/kernel/mm/transparent_hugepage/
and hugetlb in:
/sys/kernel/mm/hugepages/

THP for shmem+anon currently really only supports PMD-sized THP, that
size can be observed via:
/sys/kernel/mm/transparent_hugepage/hpage_pmd_size

hugetlb sizes can be detected simply by looking at the folders inside
/sys/kernel/mm/hugepages/. "tools/testing/selftests/mm/vm_util.c" in the
kernel has a function "detect_hugetlb_page_sizes()" that uses that
interface to detect the sizes.

But likely we want THP support here. Because for hugetlb, one would
actually have to instruct the kernel which size to use, like we do for
memfd with hugetlb.

Anon support for smaller sizes than PMDs is in the works, and once
upstream, it can then be detected via
/sys/kernel/mm/transparent_hugepage/ as well.

shmem support for smaller sizes is partially in the works: only on the
write() path. Likely, we'll make it configurable/observable in
/sys/kernel/mm/transparent_hugepage/ as well.

So if we are talking about THP for shmem, there really only is
/sys/kernel/mm/transparent_hugepage/hpage_pmd_size.

--
Cheers,

David / dhildenb