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

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-04-11 03:22, Baolin Wang wrote:
> 
> 
> On 4/9/26 4:23 AM, 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));
>> +
>>       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));
>> +
>>       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
>>        */
>> 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));
> Sorry, I still don't like the changes here, because shmem_allowable_huge_orders() is meant to determine which large orders are allowed. Something like below (untested):

Sure, I can do something like this.

> 
> @@ -1834,6 +1834,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>                                  struct vm_area_struct *vma, pgoff_t index,
>                                  loff_t write_end, bool shmem_huge_force)
>   {
> +       unsigned int filter_orders = pgtable_has_pmd_leaves() ? (BIT(PMD_ORDER) | BIT(PUD_ORDER)) : 0;
>          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;
> @@ -1846,7 +1847,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>                                                    shmem_huge_force, vma, vm_flags);
>          /* Tmpfs huge pages allocation */
>          if (!vma || !vma_is_anon_shmem(vma))
> -               return global_orders;
> +               return global_orders & ~filter_orders;
> 
>          /*
>           * Following the 'deny' semantics of the top level, force the huge
> @@ -1871,6 +1872,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>          if (global_orders > 0)
>                  mask |= READ_ONCE(huge_shmem_orders_inherit);
> 
> +       mask &= ~filter_orders;
>          return THP_ORDERS_ALL_FILE_DEFAULT & mask;
>   }
> 
> 
> Additionally, we also need a pgtable_has_pmd_leaves() check before setting huge_shmem_orders_inherit as well.
> 
> @@ -5428,7 +5430,7 @@ 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;
>