Re: [PATCH v5 14/14] mm: thp: always enable mTHP support

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-06-03 04:12, Lance Yang wrote:
> Would it makes sense to call khugepaged_enter_vma() for anon mTHP faults
> as well?
> 
> vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> {
> [...]
> 	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
> 		return VM_FAULT_FALLBACK;
> [...]
> 	khugepaged_enter_vma(vma, vma->vm_flags);
> [...]
> }
> 
> Without PMD leaves, do_huge_pmd_anonymous_page() is not reached. Apart
> from MADV_HUGEPAGE, AFAIK, the mm has no chance to enter khugepaged
> from the fault side :)

Hey Lance,

Wouldn't we also skip do_huge_pmd_anonymous_page() page today if
PMD-sized THPs are disabled in sysfs but mTHP is enabled?

In any case, by reading through khugepaged_enter_vma() I see that
khugepaged only supports PMD-size for now. So, shouldn't this case be
addressed in Nico's series [1] ?

[1] https://lore.kernel.org/linux-mm/20260522150009.121603-1-npache@redhat.com/

> 
> Cheers, Lance
> 
> On Fri, May 29, 2026 at 10:55:32AM -0400, 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
>> - Filter out PMD and PUD orders from allowable orders when
>>    PMD-sized pages are not supported by the CPU
>>
>> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
>> ---
>> mm/huge_memory.c | 25 ++++++++++++++++++++-----
>> mm/shmem.c       | 14 +++++++++-----
>> 2 files changed, 29 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 32254febe097..059901a8c6cb 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -126,6 +126,15 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>> 	else
>> 		supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
>>
>> +	if (!pgtable_has_pmd_leaves()) {
>> +		/*
>> +		 * If the CPU does not support PMD leaves, assume for
>> +		 * now that it does not support PUD leaves and disable
>> +		 * both folio orders.
>> +		 */
>> +		supported_orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
>> +	}
>> +
>> 	orders &= supported_orders;
>> 	if (!orders)
>> 		return 0;
>> @@ -133,7 +142,7 @@ 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. */
>> @@ -848,7 +857,7 @@ 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);
>> @@ -870,6 +879,15 @@ 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()) {
>> +		/*
>> +		 * If the CPU does not support PMD leaves, assume for
>> +		 * now that it does not support PUD leaves and disable
>> +		 * both folio orders.
>> +		 */
>> +		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
>> +	}
>> +
>> 	order = highest_order(orders);
>> 	while (orders) {
>> 		thpsize = thpsize_create(order, *hugepage_kobj);
>> @@ -969,9 +987,6 @@ 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
>> 	 */
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 079e299ea789..c15dffd0eb41 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1844,16 +1844,19 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>> 	unsigned long mask = READ_ONCE(huge_shmem_orders_always);
>> 	unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
>> 	vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
>> -	unsigned int global_orders;
>> +	unsigned int global_orders, disabled_orders = 0;
>>
>> -	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;
>>
>> +	if (!pgtable_has_pmd_leaves())
>> +		disabled_orders = BIT(PMD_ORDER);
>> +
>> 	global_orders = shmem_huge_global_enabled(inode, index, write_end,
>> 						  shmem_huge_force, vma, vm_flags);
>> 	/* Tmpfs huge pages allocation */
>> 	if (!vma || !vma_is_anon_shmem(vma))
>> -		return global_orders;
>> +		return global_orders & ~disabled_orders;
>>
>> 	/*
>> 	 * Following the 'deny' semantics of the top level, force the huge
>> @@ -1867,7 +1870,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>> 	 * means non-PMD sized THP can not override 'huge' mount option now.
>> 	 */
>> 	if (shmem_huge == SHMEM_HUGE_FORCE)
>> -		return READ_ONCE(huge_shmem_orders_inherit);
>> +		return READ_ONCE(huge_shmem_orders_inherit) & ~disabled_orders;
>>
>> 	/* Allow mTHP that will be fully within i_size. */
>> 	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
>> @@ -1878,6 +1881,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>> 	if (global_orders > 0)
>> 		mask |= READ_ONCE(huge_shmem_orders_inherit);
>>
>> +	mask &= ~disabled_orders;
>> 	return THP_ORDERS_ALL_FILE_DEFAULT & mask;
>> }
>>
>> @@ -5461,7 +5465,7 @@ void __init shmem_init(void)
>> 	 * Default to setting PMD-sized THP to inherit the global setting and
>> 	 * disable all other multi-size THPs.
>> 	 */
>> -	if (!shmem_orders_configured)
>> +	if (!shmem_orders_configured && pgtable_has_pmd_leaves())
>> 		huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
>> #endif
>> 	return;
>> -- 
>> 2.54.0
>>
>>
>