Re: [PATCH v4 9/9] mm: thp: always enable mTHP support

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/1/26 21:18, 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 | 23 ++++++++++++++++++-----
>  mm/shmem.c       | 14 +++++++++-----
>  2 files changed, 27 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 32254febe097..c1765c8e3dc6 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -126,6 +126,14 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	else
>  		supported_orders = THP_ORDERS_ALL_FILE_DEFAULT;
>  
> +	if (!pgtable_has_pmd_leaves()) {
> +		/*
> +		 * The CPU doesn't support PMD-sized pages, assume it
> +		 * doesn't support PUD-sized pages either.
> +		 */

I'd say here "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 +141,7 @@ 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 +856,7 @@ 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 +878,14 @@ 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()) {
> +		/*
> +		 * The CPU doesn't support PMD-sized pages, assume it
> +		 * doesn't support PUD-sized pages either.
> +		 */
> +		orders &= ~(BIT(PMD_ORDER) | BIT(PUD_ORDER));
> +	}
> +
>  	order = highest_order(orders);
>  	while (orders) {
>  		thpsize = thpsize_create(order, *hugepage_kobj);
> @@ -969,9 +985,6 @@ 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 a48f034830cd..23893c2bc2dd 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1840,16 +1840,19 @@ 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, filter_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())
> +		filter_orders = BIT(PMD_ORDER) | BIT(PUD_ORDER);

Would "disabled_orders" or "unavailable_orders" be more appropriate?

There is no need to disable PUD-orders, as shmem does not support PUD-orders
(unlike DAX). So you can keep it simpler here.

> +
>  	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 & ~filter_orders;
>  
>  	/*
>  	 * Following the 'deny' semantics of the top level, force the huge
> @@ -1863,7 +1866,7 @@ 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) & ~filter_orders;
>  
>  	/* Allow mTHP that will be fully within i_size. */
>  	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
> @@ -1874,6 +1877,7 @@ 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;
>  }
>  
> @@ -5457,7 +5461,7 @@ 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);

Do we really want to change that? We can just leave the defaults as is, no?

-- 
Cheers,

David