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

[Lance Yang <lance.yang@linux.dev>]

On Wed, Apr 08, 2026 at 04:22:57PM -0400, Luiz Capitulino wrote:
>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();

One more thought here: I don't see why we need boot-time caching.

has_transparent_hugepage() does *not* look expensive on the common
archs. On x86, it is just a CPU feature check. MIPS is different, yes,
only the first call there is more involved ...

But if we *really* want caching, couldn't we just do it lazily instead
of adding another early boot init step?

Something like:

bool pgtable_has_pmd_leaves(void)
{
	static int __arch_has_pmd_leaves = -1;

	if (READ_ONCE(__arch_has_pmd_leaves) < 0)
		WRITE_ONCE(__arch_has_pmd_leaves, has_transparent_hugepage());

	return READ_ONCE(__arch_has_pmd_leaves);
}

That would avoid depending on parse_early_param() / parse_args()
ordering, and it seems much simpler too :)

The boot-time init step looks a bit shaky to me ...

Thanks,
Lance