mm/hugetlb: use folio->lru int demote_free_hugetlb_folios()

We are demoting hugetlb folios to smaller hugetlb folios; let's avoid
messing with pages where avoidable and handle it more similar to
__split_huge_page_tail().

Link: https://lkml.kernel.org/r/20250113131611.2554758-7-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: davidhildenbrand

mm/hugetlb.c

@@ -3826,13 +3826,15 @@ static long demote_free_hugetlb_folios(struct hstate *src, struct hstate *dst,
 
 		for (i = 0; i < pages_per_huge_page(src); i += pages_per_huge_page(dst)) {
 			struct page *page = folio_page(folio, i);
+			/* Careful: see __split_huge_page_tail() */
+			struct folio *new_folio = (struct folio *)page;
 
-			page->mapping = NULL;
 			clear_compound_head(page);
 			prep_compound_page(page, dst->order);
 
-			init_new_hugetlb_folio(dst, page_folio(page));
-			list_add(&page->lru, &dst_list);
+			new_folio->mapping = NULL;
+			init_new_hugetlb_folio(dst, new_folio);
+			list_add(&new_folio->lru, &dst_list);
 		}
 	}