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

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-04-10 04:19, 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 ...

The caching is not being introduced by this series. thp_disabled_by_hw()
(which is used in hot paths) is checking a cached value too and this
cached value is also set at boot-time by hugepage_init(). So, we keep
the same behavior while extending the use of the cached value in the API
(which makes sense for consistency).

> 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 ...

I understand that caching on first access as you suggest above is an
alternative way of doing this, but having to properly order
initialization is very common in start_kernel(). So, unless we identify
a case where the ordering is tricky or undoable, I don't see an issue
with it. Also, note that this ordering exists today but it's implicity:
thp_disabled_by_hw() can only be called after hugepage_init() runs.

In terms of simplicity, look at how the resulting code (using an
untested version of your static key suggestion) looks in comparison to
caching at first use:

DEFINE_STATIC_KEY_TRUE(__arch_has_pmd_leaves_key);

static inline bool pgtable_has_pmd_leaves(void)
{
	return static_branch_likely(&__arch_has_pmd_leaves_key);
}

void __init init_arch_has_pmd_leaves(void)
{
	if (!arch_has_pmd_leaves())
		static_branch_disable(&__arch_has_pmd_leaves_key);
}