[RFC V2 2/2] arm64/mm: Enable THP migration

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

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 just enable HP migration
via ARCH_ENABLE_THP_MIGRATION.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
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 | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 31380da53689..01d432dc813e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1874,6 +1874,10 @@ 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 560be593a8dc..892c30129cd2 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -907,6 +907,16 @@ 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) })
+
+/*
+ * pmd_present() must return false for a swap PMD entry.
+ * Just explicitly clear PMD_TABLE_BIT while converting.
+ */
+#define __swp_entry_to_pmd(swp)		__pmd((swp).val & ~PMD_TABLE_BIT)
+#endif /* CONFIG_ARCH_ENABLE_THP_MIGRATION */
+
 /*
  * Ensure that there are not more swap files than can be encoded in the kernel
  * PTEs.
-- 
2.20.1