powerpc/THP: Enable THP on PPC64

We enable only if the we support 16MB page size.
Reviewed-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pgtable-ppc64.h

@@ -444,8 +444,7 @@ static inline int pmd_trans_splitting(pmd_t pmd)
 	return 0;
 }
 
-/* We will enable it in the last patch */
-#define has_transparent_hugepage() 0
+extern int has_transparent_hugepage(void);
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
 
 static inline pte_t pmd_pte(pmd_t pmd)

arch/powerpc/mm/pgtable_64.c

@@ -831,4 +831,33 @@ pmd_t pmdp_get_and_clear(struct mm_struct *mm,
 	memset(pgtable, 0, PTE_FRAG_SIZE);
 	return old_pmd;
 }
+
+int has_transparent_hugepage(void)
+{
+	if (!mmu_has_feature(MMU_FTR_16M_PAGE))
+		return 0;
+	/*
+	 * We support THP only if PMD_SIZE is 16MB.
+	 */
+	if (mmu_psize_defs[MMU_PAGE_16M].shift != PMD_SHIFT)
+		return 0;
+	/*
+	 * We need to make sure that we support 16MB hugepage in a segement
+	 * with base page size 64K or 4K. We only enable THP with a PAGE_SIZE
+	 * of 64K.
+	 */
+	/*
+	 * If we have 64K HPTE, we will be using that by default
+	 */
+	if (mmu_psize_defs[MMU_PAGE_64K].shift &&
+	    (mmu_psize_defs[MMU_PAGE_64K].penc[MMU_PAGE_16M] == -1))
+		return 0;
+	/*
+	 * Ok we only have 4K HPTE
+	 */
+	if (mmu_psize_defs[MMU_PAGE_4K].penc[MMU_PAGE_16M] == -1)
+		return 0;
+
+	return 1;
+}
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */