[PATCH v3 02/10] mm: introduce pgtable_has_pmd_leaves()

Luiz Capitulino
2026-04-08T20:24:46Z

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

- has_transparent_hugepage(): the name suggests it checks if THP is
enabled, but when CONFIG_TRANSPARENT_HUGEPAGE=y and the architecture
implements this helper, it actually checks if the CPU supports
PMD-sized pages

- 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_hugepage(). This helper is used in fast paths

This commit introduces a new helper called pgtable_has_pmd_leaves()
which is intended to replace both has_transparent_hugepage() and
thp_disabled_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 which is performed by
init_arch_has_pmd_leaves(). We call init_arch_has_pmd_leaves() early
during boot in start_kernel() right after parse_early_param() but before
parse_args(). This allows early_param() handlers to change CPU flags if
needed (eg. parse_memopt() in x86-32) while also allowing users to use
the API from __setup() handlers.

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

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
include/linux/pgtable.h | 15 +++++++++++++++
init/main.c | 1 +
mm/memory.c | 8 ++++++++
3 files changed, 24 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index a50df42a893f..c4c5282f795c 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -2192,6 +2192,21 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
}
}

+#ifdef CONFIG_MMU
+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);
+#else
+static inline bool pgtable_has_pmd_leaves(void)
+{
+ return false;
+}
+static inline void __init init_arch_has_pmd_leaves(void) { }
+#endif
+
#endif /* !__ASSEMBLY__ */

#if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
diff --git a/init/main.c b/init/main.c
index 1cb395dd94e4..07f2ddbf9677 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1044,6 +1044,7 @@ void start_kernel(void)
print_kernel_cmdline(saved_command_line);
/* parameters may set static keys */
parse_early_param();
+ init_arch_has_pmd_leaves();
after_dashes = parse_args("Booting kernel",
static_command_line, __start___param,
__stop___param - __start___param,
diff --git a/mm/memory.c b/mm/memory.c
index c65e82c86fed..5ee312c30f2d 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);
--
2.53.0