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

[Luiz Capitulino <luizcap@redhat.com>]

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.

>> +
>>   	orders &= supported_orders;
>>   	if (!orders)
>>   		return 0;
>> @@ -125,7 +128,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>   	if (!vma->vm_mm)		/* vdso */
>>   		return 0;
>>
>> -	if (!pgtable_has_pmd_leaves() || vma_thp_disabled(vma, vm_flags, forced_collapse))
>> +	if (vma_thp_disabled(vma, vm_flags, forced_collapse))
>>   		return 0;
>>
>>   	/* khugepaged doesn't collapse DAX vma, but page fault is fine. */
>> @@ -787,7 +790,7 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
>>   	 * disable all other sizes. powerpc's PMD_ORDER isn't a compile-time
>>   	 * constant so we have to do this here.
>>   	 */
>> -	if (!anon_orders_configured)
>> +	if (!anon_orders_configured && pgtable_has_pmd_leaves())
>>   		huge_anon_orders_inherit = BIT(PMD_ORDER);
>>
>>   	*hugepage_kobj = kobject_create_and_add("transparent_hugepage", mm_kobj);
>> @@ -809,6 +812,9 @@ static int __init hugepage_init_sysfs(struct kobject **hugepage_kobj)
>>   	}
>>
>>   	orders = THP_ORDERS_ALL_ANON | THP_ORDERS_ALL_FILE_DEFAULT;
>> +	if (!pgtable_has_pmd_leaves())
>> +		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
>> +
> 
> Ditto.
> 
>>   	order = highest_order(orders);
>>   	while (orders) {
>>   		thpsize = thpsize_create(order, *hugepage_kobj);
>> @@ -908,9 +914,6 @@ static int __init hugepage_init(void)
>>   	int err;
>>   	struct kobject *hugepage_kobj;
>>
>> -	if (!pgtable_has_pmd_leaves())
>> -		return -EINVAL;
>> -
>>   	/*
>>   	 * hugepages can't be allocated by the buddy allocator
>>   	 */
> The code after is:
> 
> MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER > MAX_PAGE_ORDER);
> 
> Should this check be removed or only performed when pgtable_has_pmd_leaves()?
> 
> I do not know if there is a possible Kconfig that lowers MAX_PAGE_ORDER
> below PMD_ORDER and enables THP. After this patchset, that might be valid
> if people do not want to use mTHP but not PMD THP.

I need to look into this more carefully to be able to answer this, I'll
get back to you.

>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 613393eae5a9..b49a30475cb0 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1839,7 +1839,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>>   	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
>>   	unsigned int global_orders;
>>
>> -	if (!pgtable_has_pmd_leaves() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
>> +	if (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force))
>>   		return 0;
>>
>>   	global_orders = shmem_huge_global_enabled(inode, index, write_end,
>> @@ -1947,6 +1947,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
>>
>>   	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
>>   		orders = 0;
>> +	else if (!pgtable_has_pmd_leaves())
>> +		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
> 
> Same question as the first one.
> 
>>
>>   	if (orders > 0) {
>>   		suitable_orders = shmem_suitable_orders(inode, vmf,
>> -- 
>> 2.53.0
> 
> 
> Best Regards,
> Yan, Zi
>