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

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-04-09 16:10, Zi Yan wrote:
> 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?

Sure, I can do that.

> 
> Thanks.
> 
> Best Regards,
> Yan, Zi
>