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

On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:

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.

I don't follow this logic, the purpose of the invalidation is also to clear out anything that may be mirroring this VA, and "the page hasn't changed" doesn't seem to rule out that case?

I'm also not sure I follow where the zero page came from?

Jason