All pages isolated for the migration have an elevated reference count and
therefore seeing a reference count equal to 1 means that the last user of
the page has dropped the reference and the page has became unused and
there doesn't make much sense to migrate it anymore.

This has been done for regular pages and this patch does the same for
hugetlb pages.  Although the likelihood of the race is rather small for
hugetlb pages it makes sense the two code paths in sync.

Link: https://lkml.kernel.org/r/20210115124942.46403-2-songmuchun@bytedance.comSigned-off-by: NMuchun Song <songmuchun@bytedance.com>
Reviewed-by: NMike Kravetz <mike.kravetz@oracle.com>
Acked-by: NYang Shi <shy828301@gmail.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Reviewed-by: NOscar Salvador <osalvador@suse.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>