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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 6/1/26 8:02 PM, Baolin Wang wrote:
> 
> 
> On 5/29/26 10:55 PM, 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;
> 
> Seems we should also filter the PMD-sized order in shmem_getattr() if 
> pgtable_has_pmd_leaves() returns false, like I mentioned in the previous 
> link[1].
> 
> [1] https://lore.kernel.org/linux-mm/6591a74c-7ef9-4614-9ae9- 
> cb2fbed86ebf@linux.alibaba.com/

You already did this in patch 7. So LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>