arm64/mm: Enable THP migration

[kdauer]

In certain page migration situations, a THP page can be migrated without
being split into it's constituent subpages. This saves time required to
split a THP and put it back together when required. But it also saves an
wider address range translation covered by a single TLB entry, reducing
future page fault costs.

A previous patch changed platform THP helpers per generic memory semantics,
clearing the path for THP migration support. This adds two more THP helpers
required to create PMD migration swap entries. Now enable THP migration via
ARCH_ENABLE_THP_MIGRATION.

Signed-off-by: Anshuman Khandual [email protected]
Reviewed-by: Catalin Marinas [email protected]
Cc: Catalin Marinas [email protected]
Cc: Will Deacon [email protected]
Cc: Mark Rutland [email protected]
Cc: Marc Zyngier [email protected]
Cc: Suzuki Poulose [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon [email protected]