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

[Lance Yang]

+Cc THP

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.

Set once at boot and then read in hot paths, would a static key be a
better fit there instead? Something like:

DEFINE_STATIC_KEY_FALSE(arch_has_pmd_leaves_key);
 
static inline bool pgtable_has_pmd_leaves(void)
{
	return static_branch_unlikely(&arch_has_pmd_leaves_key);
}
 
void __init init_arch_has_pmd_leaves(void)
{
	if (has_transparent_hugepage())
		static_branch_enable(&arch_has_pmd_leaves_key);
}

Cheers,
Lance