kvm: explicitly set FOLL_HONOR_NUMA_FAULT in hva_to_pfn_slow()

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.

Link: https://lkml.kernel.org/r/20230803143208.383663-4-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: liubo <liubo254@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

virt/kvm/kvm_main.c

@@ -2517,7 +2517,18 @@ static bool hva_to_pfn_fast(unsigned long addr, bool write_fault,
 static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
 			   bool interruptible, bool *writable, kvm_pfn_t *pfn)
 {
-	unsigned int flags = FOLL_HWPOISON;
+	/*
+	 * When a VCPU accesses a page that is not mapped into the secondary
+	 * MMU, we lookup the page using GUP to map it, so the guest VCPU can
+	 * make progress. We always want to honor NUMA hinting faults in that
+	 * case, because GUP usage corresponds to memory accesses from the VCPU.
+	 * Otherwise, we'd not trigger NUMA hinting faults once a page is
+	 * mapped into the secondary MMU and gets accessed by a VCPU.
+	 *
+	 * Note that get_user_page_fast_only() and FOLL_WRITE for now
+	 * implicitly honor NUMA hinting faults and don't need this flag.
+	 */
+	unsigned int flags = FOLL_HWPOISON | FOLL_HONOR_NUMA_FAULT;
 	struct page *page;
 	int npages;