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

[Andrew Morton]

On Tue,  9 Jun 2026 22:37:59 +0800 Xueyuan Chen <xueyuan.chen21@xxxxxxxxx> 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@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/CAHbLzkrXXe7r3n3jXgDKtwZhRqj=jDx9E6dLOULohnhBguvi9A@xxxxxxxxxxxxxx/
> 
> ...
>
> --- 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?