Re: [RFC PATCH v2 1/3] mm/huge_memory: make persistent huge zero folio read-only

[Lance Yang <lance.yang@linux.dev>]

On Tue, Jun 09, 2026 at 12:45:49PM -0700, Andrew Morton wrote:
>On Tue,  9 Jun 2026 22:37:59 +0800 Xueyuan Chen <xueyuan.chen21@gmail.com> wrote:
>
>> The huge zero folio is shared globally, and its contents should never
>> change after initialization. As Jann Horn pointed out[1], the kernel has
>> had bugs, including security bugs, where read-only pages were later written
>> to. If the persistent huge zero folio is read-only in the direct map, such
>> writes fault instead of silently corrupting the shared zero contents.
>> 
>> Add arch_make_pages_readonly() so mm code can request read-only direct-map
>> protection for a page range. Direct-map protection is
>> architecture-specific, so the generic weak implementation does nothing.
>> 
>> This was inspired by Jann Horn's read-only zero page work[1] and follow-up
>> discussion[2] with Yang Shi.
>> 
>> [1] https://lore.kernel.org/linux-mm/20260508-ro-zeropage-v1-1-9808abc20b49@google.com/
>> [2] https://lore.kernel.org/linux-mm/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@mail.gmail.com/
>> 
>> ...
>>
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -308,6 +308,11 @@ static unsigned long shrink_huge_zero_folio_scan(struct shrinker *shrink,
>>  	return 0;
>>  }
>>  
>> +bool __weak arch_make_pages_readonly(struct page *page, int nr_pages)
>> +{
>> +	return false;
>> +}
>> +
>>  static struct shrinker *huge_zero_folio_shrinker;
>>  
>>  #ifdef CONFIG_SYSFS
>> @@ -982,8 +987,14 @@ static int __init thp_shrinker_init(void)
>>  		 * that get_huge_zero_folio() will most likely not fail as
>>  		 * thp_shrinker_init() is invoked early on during boot.
>>  		 */
>> -		if (!get_huge_zero_folio())
>> +		if (!get_huge_zero_folio()) {
>>  			pr_warn("Allocating persistent huge zero folio failed\n");
>> +			return 0;
>> +		}
>> +
>> +		arch_make_pages_readonly(folio_page(huge_zero_folio, 0),
>> +					HPAGE_PMD_NR);
>
>Can it simply pass the folio?

Right, this came from the RFC v1 discussion[1]. David preferred a page-
range helper for possible future non-folio callers, not something folio-
only.

Of course, we could also add a folio wrapper on top of that if needed :)

[1] https://lore.kernel.org/linux-mm/929875a2-9e94-4dbc-9c98-b342ccc3f4e2@kernel.org/

Thanks, Lance