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

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Message-ID: <d2bc3d79-5d00-4943-80bd-01b144529177@linux.alibaba.com>
Date: Tue, 10 Feb 2026 17:56:09 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Luiz Capitulino <luizcap@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, david@...nel.org
Cc: ryan.roberts@....com, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Subject: Re: [PATCH v2 10/11] mm: thp: always enable mTHP support



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@...hat.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);

> +
>   	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.

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