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

[Lance Yang <lance.yang@linux.dev>]

On 2026/6/4 01:40, Luiz Capitulino wrote:
> 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,

Hey

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

Right. I got mixed up here :P

> 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] ?

Yep. Anyway, not a big deal. No need to pile more onto Nico's series
(enough rounds already ... ). Best left as a follow-up after it lands :)

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

Cheers, Lance

>>
>> 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
>>>
>>>
>>
>