ext4: use journal inode to determine journal overhead

When a file system contains an internal journal that has not been
loaded, use the journal inode's i_size field to determine its
contribution to the file system's overhead.  (The journal's j_maxlen
field is normally used to determine its size, but it's unavailable when
the journal has not been loaded.)
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -3210,6 +3210,8 @@ int ext4_calculate_overhead(struct super_block *sb)
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_super_block *es = sbi->s_es;
+	struct inode *j_inode;
+	unsigned int j_blocks, j_inum = le32_to_cpu(es->s_journal_inum);
 	ext4_group_t i, ngroups = ext4_get_groups_count(sb);
 	ext4_fsblk_t overhead = 0;
 	char *buf = (char *) get_zeroed_page(GFP_NOFS);
@@ -3240,10 +3242,23 @@ int ext4_calculate_overhead(struct super_block *sb)
 			memset(buf, 0, PAGE_SIZE);
 		cond_resched();
 	}
-	/* Add the internal journal blocks as well */
+
+	/*
+	 * Add the internal journal blocks whether the journal has been
+	 * loaded or not
+	 */
 	if (sbi->s_journal && !sbi->journal_bdev)
 		overhead += EXT4_NUM_B2C(sbi, sbi->s_journal->j_maxlen);
-
+	else if (ext4_has_feature_journal(sb) && !sbi->s_journal) {
+		j_inode = ext4_get_journal_inode(sb, j_inum);
+		if (j_inode) {
+			j_blocks = j_inode->i_size >> sb->s_blocksize_bits;
+			overhead += EXT4_NUM_B2C(sbi, j_blocks);
+			iput(j_inode);
+		} else {
+			ext4_msg(sb, KERN_ERR, "can't get journal size");
+		}
+	}
 	sbi->s_overhead = overhead;
 	smp_wmb();
 	free_page((unsigned long) buf);