mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()

We can now safely iterate over all pages in a folio, so no need for the
pfn_to_page().

Also, as we already force the refcount in __init_single_page() to 1
through init_page_count(), we can just set the refcount to 0 and avoid
page_ref_freeze() + VM_BUG_ON.  Likely, in the future, we would just want
to tell __init_single_page() to which value to initialize the refcount.

Further, adjust the comments to highlight that we are dealing with an
open-coded prep_compound_page() variant, and add another comment
explaining why we really need the __init_single_page() only on the tail
pages.

Note that the current code was likely problematic, but we never ran into
it: prep_compound_tail() would have been called with an offset that might
exceed a memory section, and prep_compound_tail() would have simply added
that offset to the page pointer -- which would not have done the right
thing on sparsemem without vmemmap.

Link: https://lkml.kernel.org/r/20250901150359.867252-14-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -3237,17 +3237,18 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
 {
 	enum zone_type zone = zone_idx(folio_zone(folio));
 	int nid = folio_nid(folio);
+	struct page *page = folio_page(folio, start_page_number);
 	unsigned long head_pfn = folio_pfn(folio);
 	unsigned long pfn, end_pfn = head_pfn + end_page_number;
-	int ret;
-
-	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
-		struct page *page = pfn_to_page(pfn);
 
+	/*
+	 * As we marked all tail pages with memblock_reserved_mark_noinit(),
+	 * we must initialize them ourselves here.
+	 */
+	for (pfn = head_pfn + start_page_number; pfn < end_pfn; page++, pfn++) {
 		__init_single_page(page, pfn, zone, nid);
 		prep_compound_tail((struct page *)folio, pfn - head_pfn);
-		ret = page_ref_freeze(page, 1);
-		VM_BUG_ON(!ret);
+		set_page_count(page, 0);
 	}
 }
 
@@ -3257,12 +3258,15 @@ static void __init hugetlb_folio_init_vmemmap(struct folio *folio,
 {
 	int ret;
 
-	/* Prepare folio head */
+	/*
+	 * This is an open-coded prep_compound_page() whereby we avoid
+	 * walking pages twice by initializing/preparing+freezing them in the
+	 * same go.
+	 */
 	__folio_clear_reserved(folio);
 	__folio_set_head(folio);
 	ret = folio_ref_freeze(folio, 1);
 	VM_BUG_ON(!ret);
-	/* Initialize the necessary tail struct pages */
 	hugetlb_folio_init_tail_vmemmap(folio, 1, nr_pages);
 	prep_compound_head((struct page *)folio, huge_page_order(h));
 }