[RFC 02/10] mm: introduce pgtable_has_pmd_leaves()

[Luiz Capitulino <luizcap@redhat.com>]

This is a new helper which can be used to check if the architecture
supports PMD-sized pages. It's intended to replace two existing helpers:

- has_transparent_hugepages(): it's used to check for PMD-sized pages
  (not exactly THP support), but it may perform a hardware check so it's
  not intended to be used in fast paths

- thp_disabled_by_hw(): also checks for PMD-sized pages support, but
  uses a cached value

pgtable_has_pmd_leaves() implementation is split in two parts:

1. init_arch_has_pmd_leaves(): runs at boottime as early_initcall as a
   wrapper to has_transparent_hugepages(). The result is cached

2. pgtable_has_pmd_leaves(): just returns the cached value

The next commits will convert users of both has_transparent_hugepages()
and thp_disabled_by_hw() to pgtable_has_pmd_leaves().

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 include/linux/pgtable.h | 10 ++++++++++
 mm/memory.c             | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 32e8457ad535..e4c5f70b0a01 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -2001,6 +2001,16 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
 	}
 }
 
+/*
+ * IMPORTANT: pgtable_has_pmd_leaves() can only be called after
+ * early_initcall, since that's when __arch_has_pmd_leaves is set
+ */
+extern bool __arch_has_pmd_leaves;
+static inline bool pgtable_has_pmd_leaves(void)
+{
+	return __arch_has_pmd_leaves;
+}
+
 #endif /* !__ASSEMBLY__ */
 
 #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
diff --git a/mm/memory.c b/mm/memory.c
index 74b45e258323..7b50f3ec9b37 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -176,6 +176,16 @@ static int __init init_zero_pfn(void)
 }
 early_initcall(init_zero_pfn);
 
+bool __arch_has_pmd_leaves __read_mostly;
+EXPORT_SYMBOL(__arch_has_pmd_leaves);
+
+static int __init init_arch_has_pmd_leaves(void)
+{
+	__arch_has_pmd_leaves = has_transparent_hugepage();
+	return 0;
+}
+early_initcall(init_arch_has_pmd_leaves);
+
 void mm_trace_rss_stat(struct mm_struct *mm, int member)
 {
 	trace_rss_stat(mm, member);
-- 
2.51.1