Re: EXT4-fs error (device dm-42): ext4_mb_generate_buddy:741: group 1904, 32254 clusters in bitmap, 32258 in gd

[Ted Ts'o <tytso@mit.edu>]

I think this patch should fix things.  Could you give it a try (after
re-enabling uninit_bg and then running e2fsck on the file systems)?

Many thanks,

						- Ted

commit b6b218940d47f20fc14e4d203930c575be0c65bf
Author: Theodore Ts'o <tytso@mit.edu>
Date:   Thu Jun 7 00:26:56 2012 -0400

    ext4: fix the free blocks calculation for ext3 file systems w/ uninit_bg
    
    Ext3 filesystems that are converted to use as many ext4 file systems
    as possible will enable uninit_bg to speed up e2fsck times.  File
    systems with an native ext3 layout of inode tables and block
    allocation bitmaps (as opposed to ext4's flex_bg layout), ext4 would
    sometimes incorrectly calculate the number of free blocks in an
    uninitialized block group.  As a result, when first allocating a block
    in block group marked uninitalized, ext4 would incorrectly report that
    the file system was corrupt:
    
    EXT4-fs error (device dm-42): ext4_mb_generate_buddy:741: group 1699, 32254 clusters in bitmap, 32258 in gd
    
    Fix the calculation in ext4_num_overhead_clusters() to avoid this
    problem.
    
    Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
index 99b6324..cee7812 100644
--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -90,8 +90,8 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
 	 * unusual file system layouts.
 	 */
 	if (ext4_block_in_group(sb, ext4_block_bitmap(sb, gdp), block_group)) {
-		block_cluster = EXT4_B2C(sbi, (start -
-					       ext4_block_bitmap(sb, gdp)));
+		block_cluster = EXT4_B2C(sbi,
+					 ext4_block_bitmap(sb, gdp) - start);
 		if (block_cluster < num_clusters)
 			block_cluster = -1;
 		else if (block_cluster == num_clusters) {
@@ -102,7 +102,7 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
 
 	if (ext4_block_in_group(sb, ext4_inode_bitmap(sb, gdp), block_group)) {
 		inode_cluster = EXT4_B2C(sbi,
-					 start - ext4_inode_bitmap(sb, gdp));
+					 ext4_inode_bitmap(sb, gdp) - start);
 		if (inode_cluster < num_clusters)
 			inode_cluster = -1;
 		else if (inode_cluster == num_clusters) {
@@ -114,7 +114,7 @@ unsigned ext4_num_overhead_clusters(struct super_block *sb,
 	itbl_blk = ext4_inode_table(sb, gdp);
 	for (i = 0; i < sbi->s_itb_per_group; i++) {
 		if (ext4_block_in_group(sb, itbl_blk + i, block_group)) {
-			c = EXT4_B2C(sbi, start - itbl_blk + i);
+			c = EXT4_B2C(sbi, itbl_blk + i - start);
 			if ((c < num_clusters) || (c == inode_cluster) ||
 			    (c == block_cluster) || (c == itbl_cluster))
 				continue;