btrfs: make btrfs_read_disk_super return struct btrfs_disk_super

Instead of returning both the page and the super block structure, make
btrfs_read_disk_super just return a pointer to struct btrfs_disk_super.
As a result the function signature is simplified. Also,
read_cache_page_gfp can never return NULL so check its return value only
for IS_ERR.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -1251,49 +1251,48 @@ void btrfs_release_disk_super(struct btrfs_super_block *super)
 	put_page(page);
 }
 
-static int btrfs_read_disk_super(struct block_device *bdev, u64 bytenr,
-				 struct page **page,
-				 struct btrfs_super_block **disk_super)
+static struct btrfs_super_block *btrfs_read_disk_super(struct block_device *bdev,
+						       u64 bytenr)
 {
+	struct btrfs_super_block *disk_super;
+	struct page *page;
 	void *p;
 	pgoff_t index;
 
 	/* make sure our super fits in the device */
 	if (bytenr + PAGE_SIZE >= i_size_read(bdev->bd_inode))
-		return 1;
+		return ERR_PTR(-EINVAL);
 
 	/* make sure our super fits in the page */
-	if (sizeof(**disk_super) > PAGE_SIZE)
-		return 1;
+	if (sizeof(*disk_super) > PAGE_SIZE)
+		return ERR_PTR(-EINVAL);
 
 	/* make sure our super doesn't straddle pages on disk */
 	index = bytenr >> PAGE_SHIFT;
-	if ((bytenr + sizeof(**disk_super) - 1) >> PAGE_SHIFT != index)
-		return 1;
+	if ((bytenr + sizeof(*disk_super) - 1) >> PAGE_SHIFT != index)
+		return ERR_PTR(-EINVAL);
 
 	/* pull in the page with our super */
-	*page = read_cache_page_gfp(bdev->bd_inode->i_mapping,
-				   index, GFP_KERNEL);
+	page = read_cache_page_gfp(bdev->bd_inode->i_mapping, index, GFP_KERNEL);
 
-	if (IS_ERR(*page))
-		return 1;
+	if (IS_ERR(page))
+		return ERR_CAST(page);
 
-	p = page_address(*page);
+	p = page_address(page);
 
 	/* align our pointer to the offset of the super block */
-	*disk_super = p + offset_in_page(bytenr);
+	disk_super = p + offset_in_page(bytenr);
 
-	if (btrfs_super_bytenr(*disk_super) != bytenr ||
-	    btrfs_super_magic(*disk_super) != BTRFS_MAGIC) {
+	if (btrfs_super_bytenr(disk_super) != bytenr ||
+	    btrfs_super_magic(disk_super) != BTRFS_MAGIC) {
 		btrfs_release_disk_super(p);
-		return 1;
+		return ERR_PTR(-EINVAL);
 	}
 
-	if ((*disk_super)->label[0] &&
-		(*disk_super)->label[BTRFS_LABEL_SIZE - 1])
-		(*disk_super)->label[BTRFS_LABEL_SIZE - 1] = '\0';
+	if (disk_super->label[0] && disk_super->label[BTRFS_LABEL_SIZE - 1])
+		disk_super->label[BTRFS_LABEL_SIZE - 1] = 0;
 
-	return 0;
+	return disk_super;
 }
 
 int btrfs_forget_devices(const char *path)
@@ -1319,7 +1318,6 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, fmode_t flags,
 	bool new_device_added = false;
 	struct btrfs_device *device = NULL;
 	struct block_device *bdev;
-	struct page *page;
 	u64 bytenr;
 
 	lockdep_assert_held(&uuid_mutex);
@@ -1337,8 +1335,9 @@ struct btrfs_device *btrfs_scan_one_device(const char *path, fmode_t flags,
 	if (IS_ERR(bdev))
 		return ERR_CAST(bdev);
 
-	if (btrfs_read_disk_super(bdev, bytenr, &page, &disk_super)) {
-		device = ERR_PTR(-EINVAL);
+	disk_super = btrfs_read_disk_super(bdev, bytenr);
+	if (IS_ERR(disk_super)) {
+		device = ERR_CAST(disk_super);
 		goto error_bdev_put;
 	}