[PATCH v2 35/37] kfence: drop nth_page() usage

David Hildenbrand
2025-09-01T17:09:14Z

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).

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.

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

Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
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