[PATCH] ext4: Find the group descriptors on a 1k-block bigalloc,meta_bg filesystem

["Darrick J. Wong" <darrick.wong@oracle.com>]

On a filesystem with a 1k block size, the group descriptors live in
block 2, not block 1.  If the filesystem has bigalloc,meta_bg set,
however, the calculation of the group descriptor table location does
not take this into account and returns the wrong block number.  Fix
the calculation to return the correct value for this case.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/ext4/super.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 7f52a54..d968c64 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2420,6 +2420,16 @@ static ext4_fsblk_t descriptor_loc(struct super_block *sb,
 	if (ext4_bg_has_super(sb, bg))
 		has_super = 1;
 
+	/*
+	 * If we have a meta_bg fs with 1k blocks, group 0's GDT is at
+	 * block 2, not 1.  If s_first_data_block == 0 (bigalloc is enabled
+	 * on modern mke2fs or blksize > 1k on older mke2fs) then we must
+	 * compensate.
+	 */
+	if (sb->s_blocksize == 1024 && nr == 0 &&
+	    le32_to_cpu(EXT4_SB(sb)->s_es->s_first_data_block) == 0)
+		has_super++;
+
 	return (has_super + ext4_group_first_block_no(sb, bg));
 }