ext4: add a function which adds a new group descriptors to a fs

This patch adds a function named ext4_add_new_descs() which adds one
or more new group descriptors to a fs and whose code is copied from
ext4_group_add().

The function will be used by new resize implementation.
Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/resize.c

@@ -735,6 +735,52 @@ static void update_backups(struct super_block *sb,
 	}
 }
 
+/*
+ * ext4_add_new_descs() adds @count group descriptor of groups
+ * starting at @group
+ *
+ * @handle: journal handle
+ * @sb: super block
+ * @group: the group no. of the first group desc to be added
+ * @resize_inode: the resize inode
+ * @count: number of group descriptors to be added
+ */
+static int ext4_add_new_descs(handle_t *handle, struct super_block *sb,
+			      ext4_group_t group, struct inode *resize_inode,
+			      ext4_group_t count)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	struct ext4_super_block *es = sbi->s_es;
+	struct buffer_head *gdb_bh;
+	int i, gdb_off, gdb_num, err = 0;
+
+	for (i = 0; i < count; i++, group++) {
+		int reserved_gdb = ext4_bg_has_super(sb, group) ?
+			le16_to_cpu(es->s_reserved_gdt_blocks) : 0;
+
+		gdb_off = group % EXT4_DESC_PER_BLOCK(sb);
+		gdb_num = group / EXT4_DESC_PER_BLOCK(sb);
+
+		/*
+		 * We will only either add reserved group blocks to a backup group
+		 * or remove reserved blocks for the first group in a new group block.
+		 * Doing both would be mean more complex code, and sane people don't
+		 * use non-sparse filesystems anymore.  This is already checked above.
+		 */
+		if (gdb_off) {
+			gdb_bh = sbi->s_group_desc[gdb_num];
+			err = ext4_journal_get_write_access(handle, gdb_bh);
+
+			if (!err && reserved_gdb && ext4_bg_num_gdb(sb, group))
+				err = reserve_backup_gdb(handle, resize_inode, group);
+		} else
+			err = add_new_gdb(handle, resize_inode, group);
+		if (err)
+			break;
+	}
+	return err;
+}
+
 /* Add group descriptor data to an existing or new group descriptor block.
  * Ensure we handle all possible error conditions _before_ we start modifying
  * the filesystem, because we cannot abort the transaction and not have it