Re: [PATCH v8 09/14] powerpc: move has_transparent_hugepage() out of THP guard

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 05/10/2026 à 22:40, Luiz Capitulino a écrit :
> 
> 
> On 10/5/26 12:52 AM, Christophe Leroy (CS GROUP) wrote:
>> Hi,
>>
>> Don't forget <linuxppc-dev@lists.ozlabs.org> when you address powerpc 
>> architecture.
> 
> Will do.
> 
>>
>> Le 03/10/2026 à 17:44, Luiz Capitulino a écrit :
>>>
>>>
>>> On 10/2/26 3:28 PM, David Hildenbrand (Arm) wrote:
>>>> On 9/18/26 03:45, Luiz Capitulino wrote:
>>>>> A future commit will introduce a kernel API to allow for checking 
>>>>> if the
>>>>> CPU supports PMD-sized pages. This API will be based on the
>>>>> has_transparent_hugepage() implementation but will be orthogonal to 
>>>>> THP
>>>>> and therefore must work when CONFIG_TRANSPARENT_HUGEPAGE=n.
>>>>>
>>>>> Move its definition out of the THP guard.
>>>>>
>>>>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>>>>> ---
>>>>>   arch/powerpc/include/asm/book3s/64/hash-4k.h  |  2 +-
>>>>>   arch/powerpc/include/asm/book3s/64/hash-64k.h |  2 +-
>>>>>   arch/powerpc/include/asm/book3s/64/pgtable.h  | 18 ++++++++ 
>>>>> +---------
>>>>>   arch/powerpc/include/asm/book3s/64/radix.h    | 14 +++++++-------
>>>>>   arch/powerpc/mm/book3s64/hash_pgtable.c       |  4 ++--
>>>>>   5 files changed, 20 insertions(+), 20 deletions(-)
>>>>>
>>>>> diff --git a/arch/powerpc/include/asm/book3s/64/hash-4k.h b/arch/ 
>>>>> powerpc/include/asm/book3s/64/hash-4k.h
>>>>> index 8e5bd9902bed..79511e6abfca 100644
>>>>> --- a/arch/powerpc/include/asm/book3s/64/hash-4k.h
>>>>> +++ b/arch/powerpc/include/asm/book3s/64/hash-4k.h
>>>>> @@ -165,9 +165,9 @@ extern void 
>>>>> hash__pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
>>>>>   extern pgtable_t hash__pgtable_trans_huge_withdraw(struct 
>>>>> mm_struct *mm, pmd_t *pmdp);
>>>>>   extern pmd_t hash__pmdp_huge_get_and_clear(struct mm_struct *mm,
>>>>>                          unsigned long addr, pmd_t *pmdp);
>>>>> -extern int hash__has_transparent_hugepage(void);
>>>>>   #endif
>>>>> +extern int hash__has_transparent_hugepage(void);
>>>>>   #endif /* !__ASSEMBLER__ */
>>>>>   #endif /* _ASM_POWERPC_BOOK3S_64_HASH_4K_H */
>>>>> diff --git a/arch/powerpc/include/asm/book3s/64/hash-64k.h b/arch/ 
>>>>> powerpc/include/asm/book3s/64/hash-64k.h
>>>>> index 7deb3a66890b..a4a44a112ff9 100644
>>>>> --- a/arch/powerpc/include/asm/book3s/64/hash-64k.h
>>>>> +++ b/arch/powerpc/include/asm/book3s/64/hash-64k.h
>>>>> @@ -278,9 +278,9 @@ extern void 
>>>>> hash__pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
>>>>>   extern pgtable_t hash__pgtable_trans_huge_withdraw(struct 
>>>>> mm_struct *mm, pmd_t *pmdp);
>>>>>   extern pmd_t hash__pmdp_huge_get_and_clear(struct mm_struct *mm,
>>>>>                          unsigned long addr, pmd_t *pmdp);
>>>>> -extern int hash__has_transparent_hugepage(void);
>>>>
>>>> Without some of these helpers in place, I assume actually using PMD 
>>>> leafs
>>>> without THP would require some more work. (which is not the goal of 
>>>> this series,
>>>> just asking).
>>>
>>> Yes, you're right.
>>>
>>>> I do wonder whether the architecture should instead simply say "not 
>>>> supported"
>>>> if !CONFIG_TRANSPARENT_HUGEPAGE?
>>>>
>>>> That should still enable your series: using mTHP without PMD support.
>>>
>>> Yes, it would. However, I think this introduces an inconsistency.
>>
>> powerpc has two types of MMU (HASH and RADIX) with different page 
>> layout. MMU type is selected at boottime based on the capabilities of 
>> the CPU. For transparent page you have:
>>
>> static inline int has_transparent_hugepage(void)
>> {
>>      if (radix_enabled())
>>          return radix__has_transparent_hugepage();
>>      return hash__has_transparent_hugepage();
>> }
> 
> Right and, unless I'm misunderstanding your comment, this supports my
> position that the new arch_has_pmd_leaves() API is all about reporting a
> hardware capability and not tied to THP support.

Yes my comment was in reaction of comment below, I wanted to say that 
CONFIG_TRANSPARENT_HUGEPAGE is not always enough to tell if a powerpc 
actually has transparent hugepages or not.

 >>>> I do wonder whether the architecture should instead simply say "not
 >>>> supported"
 >>>> if !CONFIG_TRANSPARENT_HUGEPAGE?
 >>>>
 >>>> That should still enable your series: using mTHP without PMD support.