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/[e-mail]
Signed-off-by: David Hildenbrand <[e-mail]>
Reviewed-by: Marco Elver <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/kfence/core.c

···
  */
 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
···
 	 * 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 |
···
 
 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