Re: [PATCH v2 10/11] mm: thp: always enable mTHP support

[Luiz Capitulino <luizcap@redhat.com>]

On 2026-02-10 04:56, Baolin Wang wrote:
> 
> 
> On 2/10/26 6:14 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 | 11 +++++++----
>>   mm/shmem.c       |  4 +++-
>>   2 files changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 1e5ea2e47f79..882331592928 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) | BIT(PUD_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. */
>> @@ -806,6 +809,9 @@ 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));
> 
> I think you should also handle the 'huge_anon_orders_inherit' setting in this function if pgtable_has_pmd_leaves() returns false. Shmem as well.
> 
> if (!anon_orders_configured)
>      huge_anon_orders_inherit = BIT(PMD_ORDER);

Good catch. So, would you agree that should set it to BIT(PMD_ORDER - 1)
in this case?

> 
>> +
>>       order = highest_order(orders);
>>       while (orders) {
>>           thpsize = thpsize_create(order, *hugepage_kobj);
>> @@ -905,9 +911,6 @@ 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 1c98e84667a4..cb325d1e2d1e 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1827,7 +1827,7 @@ 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,
>> @@ -1935,6 +1935,8 @@ 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));
> 
> Moving this check into shmem_allowable_huge_orders() would be more appropriate.

Will do.

Thanks a lot for the very fast review.

> 
>>       if (orders > 0) {
>>           suitable_orders = shmem_suitable_orders(inode, vmf,
>