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

Mel Gorman
2023-08-02T15:29:32Z

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.

--
Mel Gorman
SUSE Labs