kfence: drop nth_page() usage

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.

Link: https://lkml.kernel.org/r/20250901150359.867252-36-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
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: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 0ed3be100963a8ae62a82c63580d73e130d2c148..727c20c94ac5964f0ad8a13e47d525be6122015c 100644 (file)
--- 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