[PATCH 02/11] mm: introduce pgtable_has_pmd_leaves()

[Luiz Capitulino <luizcap@redhat.com>]

Currently, we have two helpers that check for PMD-sized pages but have
different names and slightly different semantics:

- has_transparent_hugepages(): the name suggests it checks if THP is
  enabled, but it actually checks if the CPU supports PMD-sized pages.
  It may perform a hardware check, so it can't be used in fast paths

- thp_disabled_by_hw(): the name suggests it checks if THP is disabled
  by the hardware, but it just returns a cached value acquired with
  has_transparent_hugepages(). This way, this helper can be called from
  fast paths

This commit introduces a new helper called pgtable_has_pmd_leaves()
which is intended to replace both has_transparent_hugepages() and
thp_disable_by_hw(). pgtable_has_pmd_leaves() has very clear semantics:
it returns true if the CPU supports PMD-sized pages and false otherwise.
It always returns a cached value, so it can be used in fast paths.

The new helper requires an initialization step that needs to be
performed very early during boot because there are
pgtable_has_pmd_leaves() users that want to call it from __setup()
handlers. This initialization is performed by init_arch_has_pmd_leaves(),
called early from start_kernel().

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 | 7 +++++++
 init/main.c             | 1 +
 mm/memory.c             | 8 ++++++++
 3 files changed, 16 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 652f287c1ef6..6733f90a1da4 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -2017,6 +2017,13 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
 	}
 }
 
+extern bool __arch_has_pmd_leaves;
+static inline bool pgtable_has_pmd_leaves(void)
+{
+	return __arch_has_pmd_leaves;
+}
+void __init init_arch_has_pmd_leaves(void);
+
 #endif /* !__ASSEMBLY__ */
 
 #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
diff --git a/init/main.c b/init/main.c
index b84818ad9685..ad1209fffcde 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1036,6 +1036,7 @@ void start_kernel(void)
 	smp_prepare_boot_cpu();	/* arch-specific boot-cpu hooks */
 	early_numa_node_init();
 	boot_cpu_hotplug_init();
+	init_arch_has_pmd_leaves();
 
 	print_kernel_cmdline(saved_command_line);
 	/* parameters may set static keys */
diff --git a/mm/memory.c b/mm/memory.c
index 2a55edc48a65..79bd59d5243f 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -177,6 +177,14 @@ 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);
+
+void __init init_arch_has_pmd_leaves(void)
+{
+	__arch_has_pmd_leaves = has_transparent_hugepage();
+}
+
 void mm_trace_rss_stat(struct mm_struct *mm, int member)
 {
 	trace_rss_stat(mm, member);
-- 
2.52.0