[PATCH v4 6/6] mm/migrate: remove range invalidation in migrate_vma_pages()

[Ralph Campbell]

When migrating the special zero page, migrate_vma_pages() calls
mmu_notifier_invalidate_range_start() before replacing the zero page
PFN in the CPU page tables. This is unnecessary since the range was
invalidated in migrate_vma_setup() and the page table entry is checked
to be sure it hasn't changed between migrate_vma_setup() and
migrate_vma_pages(). Therefore, remove the redundant invalidation.
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