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.

Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 mm/huge_memory.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 0f016ea7082d..4117833c53ef 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -115,6 +115,9 @@ 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);
+
 	orders &= supported_orders;
 	if (!orders)
 		return 0;
@@ -122,7 +125,7 @@ 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. */
@@ -805,6 +808,9 @@ 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);
+
 	order = highest_order(orders);
 	while (orders) {
 		thpsize = thpsize_create(order, *hugepage_kobj);
@@ -904,9 +910,6 @@ 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
 	 */
-- 
2.51.1

On 06.11.25 22:28, 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.
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   mm/huge_memory.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f016ea7082d..4117833c53ef 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -115,6 +115,9 @@ 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);
> +

Won't this leave PUD_ORDER set?

Ideally, later we'd also have a

if (!pgtable_has_pud_leaves())
	supported_orders &= ~BIT(PUD_ORDER);


For now you could simply clear PUD_ORDER for now as well.

No PMD leaves -> No PUD leaves, so clear them in all cases to not leave 
PUD_ORDER set.

-- 
Cheers

David

On 2025-11-17 12:47, David Hildenbrand (Red Hat) wrote:
> On 06.11.25 22:28, 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.
>>
>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>> ---
>>   mm/huge_memory.c | 11 +++++++----
>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 0f016ea7082d..4117833c53ef 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -115,6 +115,9 @@ 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);
>> +
> 
> Won't this leave PUD_ORDER set?
> 
> Ideally, later we'd also have a
> 
> if (!pgtable_has_pud_leaves())
>      supported_orders &= ~BIT(PUD_ORDER);
> 
> 
> For now you could simply clear PUD_ORDER for now as well.
> 
> No PMD leaves -> No PUD leaves, so clear them in all cases to not leave PUD_ORDER set.

OK, I'll do it.