Re: [PATCH v1 34/36] kfence: drop nth_page() usage

[Lorenzo Stoakes]

On Thu, Aug 28, 2025 at 12:01:38AM +0200, David Hildenbrand wrote:
> We want to get rid of nth_page(), and kfence init code is the last user.
>
> Unfortunately, we might actually walk a PFN range where the pages are
> not contiguous, because we might be allocating an area from memblock
> that could span memory sections in problematic kernel configs (SPARSEMEM
> without SPARSEMEM_VMEMMAP).

Sad.

>
> We could check whether the page range is contiguous
> using page_range_contiguous() and failing kfence init, or making kfence
> incompatible these problemtic kernel configs.

Sounds iffy though.

>
> Let's keep it simple and simply use pfn_to_page() by iterating PFNs.

Yes.

>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>

Stared at this and can't see anything wrong, so - LGTM and:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  mm/kfence/core.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 0ed3be100963a..727c20c94ac59 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -594,15 +594,14 @@ static void rcu_guarded_free(struct rcu_head *h)
>   */
>  static unsigned long kfence_init_pool(void)
>  {
> -	unsigned long addr;
> -	struct page *pages;
> +	unsigned long addr, start_pfn;
>  	int i;
>
>  	if (!arch_kfence_init_pool())
>  		return (unsigned long)__kfence_pool;
>
>  	addr = (unsigned long)__kfence_pool;
> -	pages = virt_to_page(__kfence_pool);
> +	start_pfn = PHYS_PFN(virt_to_phys(__kfence_pool));
>
>  	/*
>  	 * Set up object pages: they must have PGTY_slab set to avoid freeing
> @@ -613,11 +612,12 @@ static unsigned long kfence_init_pool(void)
>  	 * enters __slab_free() slow-path.
>  	 */
>  	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> -		struct slab *slab = page_slab(nth_page(pages, i));
> +		struct slab *slab;
>
>  		if (!i || (i % 2))
>  			continue;
>
> +		slab = page_slab(pfn_to_page(start_pfn + i));
>  		__folio_set_slab(slab_folio(slab));
>  #ifdef CONFIG_MEMCG
>  		slab->obj_exts = (unsigned long)&kfence_metadata_init[i / 2 - 1].obj_exts |
> @@ -665,10 +665,12 @@ static unsigned long kfence_init_pool(void)
>
>  reset_slab:
>  	for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> -		struct slab *slab = page_slab(nth_page(pages, i));
> +		struct slab *slab;
>
>  		if (!i || (i % 2))
>  			continue;
> +
> +		slab = page_slab(pfn_to_page(start_pfn + i));
>  #ifdef CONFIG_MEMCG
>  		slab->obj_exts = 0;
>  #endif
> --
> 2.50.1
>