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

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Message-ID: <79c6f081-f670-458e-9916-0f8ab501057a@redhat.com>
Date: Mon, 17 Nov 2025 14:14:32 -0500
From: Luiz Capitulino <luizcap@...hat.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>,
 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 2025-11-17 12:47, David Hildenbrand (Red Hat) wrote:
> 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.

OK, I'll do it.