Re: [PATCH v8 14/14] mm: thp: always enable mTHP support

[Usama Arif <usama.arif@linux.dev>]

On Thu, 17 Sep 2026 21:45:35 -0400 Luiz Capitulino <luizcap@redhat.com> 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 a06025b87e7c..a2d6de3ea988 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -189,6 +189,15 @@ 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;
> @@ -196,7 +205,7 @@ 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. */
> @@ -979,7 +988,7 @@ 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);
> @@ -1001,6 +1010,15 @@ 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);
> @@ -1091,9 +1109,6 @@ static int __init hugepage_init(void)
>  	int err;
>  	struct kobject *hugepage_kobj;
>  
> -	if (!pgtable_has_pmd_leaves())
> -		return -EINVAL;
> -

Removing this guard lets start_stop_khugepaged() run on a system
without PMD leaves. All mTHP orders default to never, but the global always
or madvise flag still makes hugepage_enabled() return true.

That starts an idle khugepaged thread and can unnecessarily raise
min_free_kbytes. Could hugepage_enabled() instead test whether
an enabled order remains after masking PMD_ORDER when
pgtable_has_pmd_leaves() is false?

>  	/*
>  	 * hugepages can't be allocated by the buddy allocator
>  	 */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index bc2de3a7c1ea..8c0f7e3efeeb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2046,11 +2046,14 @@ 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);
>  	/*
> @@ -2058,7 +2061,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>  	 * sysfs configs.
>  	 */
>  	if (!vma || !vma_is_anon_shmem(vma) || shmem_huge_force)
> -		return global_orders;
> +		return global_orders & ~disabled_orders;
>  
>  	/*
>  	 * Following the 'deny' semantics of the top level, force the huge
> @@ -2072,7 +2075,7 @@ 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);
> @@ -2083,6 +2086,7 @@ 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;
>  }
>  
> @@ -5630,7 +5634,7 @@ 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;
> -- 
> 2.55.0
> 
>