[RFC 2/2] arm64/mm: Enable THP migration without split

[Anshuman Khandual <anshuman.khandual@arm.com>]

In certain page migration situations a THP page can be migrated without
being split into it's constituent normal pages. This not only saves time
required to split the THP page later to be put back when required but also
saves an wider address range translation from the existing huge TLB entry
reducing future page fault costs.

The previous patch changed the THP helper functions which now complies with
the generic MM semantics clearing the path for THP migration support. Hence
just enable it.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/Kconfig               | 4 ++++
 arch/arm64/include/asm/pgtable.h | 5 +++++
 2 files changed, 9 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 0758d89..27bd8c4 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1589,6 +1589,10 @@ config ARCH_ENABLE_HUGEPAGE_MIGRATION
 	def_bool y
 	depends on HUGETLB_PAGE && MIGRATION
 
+config ARCH_ENABLE_THP_MIGRATION
+	def_bool y
+	depends on TRANSPARENT_HUGEPAGE
+
 menu "Power management options"
 
 source "kernel/power/Kconfig"
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 90d4e24..4860573 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -851,6 +851,11 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
 #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })
 #define __swp_entry_to_pte(swp)	((pte_t) { (swp).val })
 
+#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
+#define __pmd_to_swp_entry(pmd)        ((swp_entry_t) { pmd_val(pmd) })
+#define __swp_entry_to_pmd(swp)        __pmd((swp).val)
+#endif
+
 /*
  * Ensure that there are not more swap files than can be encoded in the kernel
  * PTEs.
-- 
2.7.4