linux-kernel - Re: [RFC 09/10] mm: thp: always enable mTHP support

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Message-ID: <dac20466-adac-4e47-8f50-87f4774fd57b@kernel.org>
Date: Mon, 17 Nov 2025 18:47:13 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Luiz Capitulino <luizcap@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: ryan.roberts@....com, akpm@...ux-foundation.org,
 lorenzo.stoakes@...cle.com
Subject: Re: [RFC 09/10] mm: thp: always enable mTHP support

On 06.11.25 22:28, 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.
> 
> Signed-off-by: Luiz Capitulino <luizcap@...hat.com>
> ---
>   mm/huge_memory.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 0f016ea7082d..4117833c53ef 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);
> +

Won't this leave PUD_ORDER set?

Ideally, later we'd also have a

if (!pgtable_has_pud_leaves())
	supported_orders &= ~BIT(PUD_ORDER);


For now you could simply clear PUD_ORDER for now as well.

No PMD leaves -> No PUD leaves, so clear them in all cases to not leave 
PUD_ORDER set.

-- 
Cheers

David