Re: [PATCH v2 3/8] kvm: explicitly set FOLL_HONOR_NUMA_FAULT in hva_to_pfn_slow()

David Hildenbrand
2023-08-02T15:32:19Z

On 02.08.23 17:27, Mel Gorman wrote:
> On Tue, Aug 01, 2023 at 02:48:39PM +0200, David Hildenbrand wrote:
>> KVM is *the* case we know that really wants to honor NUMA hinting falls.
>> As we want to stop setting FOLL_HONOR_NUMA_FAULT implicitly, set
>> FOLL_HONOR_NUMA_FAULT whenever we might obtain pages on behalf of a VCPU
>> to map them into a secondary MMU, and add a comment why.
>>
>> Do that unconditionally in hva_to_pfn_slow() when calling
>> get_user_pages_unlocked().
>>
>> kvmppc_book3s_instantiate_page(), hva_to_pfn_fast() and
>> gfn_to_page_many_atomic() are similarly used to map pages into a
>> secondary MMU. However, FOLL_WRITE and get_user_page_fast_only() always
>> implicitly honor NUMA hinting faults -- as documented for
>> FOLL_HONOR_NUMA_FAULT -- so we can limit this change to a single location
>> for now.
>>
>> Don't set it in check_user_page_hwpoison(), where we really only want to
>> check if the mapped page is HW-poisoned.
>>
>> We won't set it for other KVM users of get_user_pages()/pin_user_pages()
>> * arch/powerpc/kvm/book3s_64_mmu_hv.c: not used to map pages into a
>> secondary MMU.
>> * arch/powerpc/kvm/e500_mmu.c: only used on shared TLB pages with userspace
>> * arch/s390/kvm/*: s390x only supports a single NUMA node either way
>> * arch/x86/kvm/svm/sev.c: not used to map pages into a secondary MMU.
>>
>> This is a preparation for making FOLL_HONOR_NUMA_FAULT no longer
>> implicitly be set by get_user_pages() and friends.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>
> Seems sane but I don't know KVM well enough to know if this is the only
> relevant case so didn't ack.

Makes sense, some careful eyes from KVM people would be appreciated.

At least from kvm_main.c POV, I'm pretty confident that that's it.

--
Cheers,

David / dhildenb