mm: use a folio in copy_present_pte()

We still have to keep the page around because we need to know which page
in the folio we're copying, but we can replace five implict calls to
compound_head() with one.

Link: https://lkml.kernel.org/r/20230116191813.2145215-6-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memory.c

@@ -906,25 +906,28 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 	unsigned long vm_flags = src_vma->vm_flags;
 	pte_t pte = *src_pte;
 	struct page *page;
+	struct folio *folio;
 
 	page = vm_normal_page(src_vma, addr, pte);
-	if (page && PageAnon(page)) {
+	if (page)
+		folio = page_folio(page);
+	if (page && folio_test_anon(folio)) {
 		/*
 		 * If this page may have been pinned by the parent process,
 		 * copy the page immediately for the child so that we'll always
 		 * guarantee the pinned page won't be randomly replaced in the
 		 * future.
 		 */
-		get_page(page);
+		folio_get(folio);
 		if (unlikely(page_try_dup_anon_rmap(page, false, src_vma))) {
-			/* Page maybe pinned, we have to copy. */
-			put_page(page);
+			/* Page may be pinned, we have to copy. */
+			folio_put(folio);
 			return copy_present_page(dst_vma, src_vma, dst_pte, src_pte,
 						 addr, rss, prealloc, page);
 		}
 		rss[MM_ANONPAGES]++;
 	} else if (page) {
-		get_page(page);
+		folio_get(folio);
 		page_dup_file_rmap(page, false);
 		rss[mm_counter_file(page)]++;
 	}
@@ -937,7 +940,7 @@ copy_present_pte(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
 		ptep_set_wrprotect(src_mm, addr, src_pte);
 		pte = pte_wrprotect(pte);
 	}
-	VM_BUG_ON(page && PageAnon(page) && PageAnonExclusive(page));
+	VM_BUG_ON(page && folio_test_anon(folio) && PageAnonExclusive(page));
 
 	/*
 	 * If it's a shared mapping, mark it clean in