Re: [PATCH v3 09/10] mm: thp: always enable mTHP support

Zi Yan
2026-04-09T20:10:32Z

On 9 Apr 2026, at 16:07, Luiz Capitulino wrote:

> On 2026-04-09 11:55, Zi Yan wrote:
>> On 8 Apr 2026, at 16:23, Luiz Capitulino wrote:
>>
>>> If PMD-sized pages are not supported on an architecture (ie. the
>>> arch implements arch_has_pmd_leaves() and it returns false) then the
>>> current code disables all THP, including mTHP.
>>>
>>> This commit fixes this by allowing mTHP to be always enabled for all
>>> archs. When PMD-sized pages are not supported, its sysfs entry won't be
>>> created and their mapping will be disallowed at page-fault time.
>>>
>>> Similarly, this commit implements the following changes for shmem:
>>>
>>> - In shmem_allowable_huge_orders(): drop the pgtable_has_pmd_leaves()
>>> check so that mTHP sizes are considered
>>> - In shmem_alloc_and_add_folio(): don't consider PMD and PUD orders
>>> when PMD-sized pages are not supported by the CPU
>>>
>>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>>> ---
>>> mm/huge_memory.c | 13 ++++++++-----
>>> mm/shmem.c | 4 +++-
>>> 2 files changed, 11 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 86e489c0a150..6de3d8ebc35c 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -118,6 +118,9 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>> else
>>> supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
>>>
>>> + if (!pgtable_has_pmd_leaves())
>>> + supported_orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
>>
>> Why is BIT(PUD_ORDER) also removed? I thought PMD THP support and PUD THP support
>> are separate. Here the code implies PUD THP relies on PMD THP. Is that the case?
>
> This was a suggestion from David to an earlier version:
>
> https://lore.kernel.org/linux-mm/dac20466-adac-4e47-8f50-87f4774fd57b@kernel.org/
>
> My understanding was that if an arch doesn't support PMD pages then it
> probably doesn't support PUD pages either.

Got it. Can you add a comment on “No PMD leaves -> No PUD leaves” for
pgtable_has_pmd_leaves(), so that no one would ask the same question again?

Thanks.

Best Regards,
Yan, Zi