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

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-04-17 05:57, David Hildenbrand (Arm) wrote:
> On 4/10/26 10:19, Lance Yang wrote:
>>
>> 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);
> 
> Would look easier when just using two booleans: "has_pmd_leaves" and
> "probed".
> 
> I do like static keys, though. And wonder whether just putting that into
> hugepage_init() would be sufficient?

Do you mean the static key initialization? I think it's sufficient today,
but we'd be tying the new API to THP and the initialization ordering still
occurs although it's indirect.