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

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-04-09 08:26, Lance Yang wrote:
> +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:

This looks like a good suggestion, if nobody opposes I can do this for v4
(although it may delay inclusion a bit).

> 
> 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
>