swap: convert swapon() to use a folio

Retrieve a folio from the page cache rather than a page.  Saves a couple
of conversions between page & folio.

Link: https://lkml.kernel.org/r/20240826202138.3804238-1-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/swapfile.c

@@ -3357,7 +3357,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	unsigned char *swap_map = NULL;
 	unsigned long *zeromap = NULL;
 	struct swap_cluster_info *cluster_info = NULL;
-	struct page *page = NULL;
+	struct folio *folio = NULL;
 	struct inode *inode = NULL;
 	bool inced_nr_rotate_swap = false;
 
@@ -3415,12 +3415,12 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		error = -EINVAL;
 		goto bad_swap_unlock_inode;
 	}
-	page = read_mapping_page(mapping, 0, swap_file);
-	if (IS_ERR(page)) {
-		error = PTR_ERR(page);
+	folio = read_mapping_folio(mapping, 0, swap_file);
+	if (IS_ERR(folio)) {
+		error = PTR_ERR(folio);
 		goto bad_swap_unlock_inode;
 	}
-	swap_header = kmap(page);
+	swap_header = kmap_local_folio(folio, 0);
 
 	maxpages = read_swap_header(si, swap_header, inode);
 	if (unlikely(!maxpages)) {
@@ -3574,10 +3574,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 	if (swap_file)
 		filp_close(swap_file, NULL);
 out:
-	if (page && !IS_ERR(page)) {
-		kunmap(page);
-		put_page(page);
-	}
+	if (!IS_ERR_OR_NULL(folio))
+		folio_release_kmap(folio, swap_header);
 	if (name)
 		putname(name);
 	if (inode)