After switching to a newer kernel (4.9.110) I am not able to mount my
ext4-filesystem read-write. fsck does not find any errors or anything to
adjust. After spending hours of reading and learning I found the reason
in these lines:
From kernel fs/ext4/super.c, introduced in 2018-06-14:
ext4_fsblk_t last_bg_block = sb_block + ext4_bg_num_gdb(sb, 0) + 1;
block_bitmap = ext4_block_bitmap(sb, gdp);
// ... snip ...
// For every Group i:
if (block_bitmap >= sb_block + 1 &&
block_bitmap <= last_bg_block) {
ext4_msg(sb, KERN_ERR, "ext4_check_descriptors: "
"Block bitmap for group %u overlaps "
"block group descriptors", i);
if (!(sb->s_flags & MS_RDONLY))
return 0;
}
}
Consequently my error message is "ext4_check_descriptors: Block bitmap
for group 1 overlaps block group descriptors".
But I do not understand why this happens (Hey, yesterday at that time I
didn't know how ext4 works) nor how to solve this issue. See this
excerpt of dumpe2fs to understand my confusion:
Filesystem features: has_journal ext_attr dir_index filetype
meta_bg extent 64bit flex_bg sparse_super large_file huge_file dir_nlink
extra_isize metadata_csum
Block size: 4096
Fragment size: 4096
Group descriptor size: 64
Blocks per group: 32768
First meta block group: 1217
Flex block group size: 16
Group 0: (Blocks 0-32767) csum 0xb704 [ITABLE_ZEROED]
Primary superblock at 0, Group descriptors at 1-1217
Block bitmap at 9441 (+9441), csum 0xe5bcfef3
Inode bitmap at 1233 (+1233), csum 0x8f9b318c
Inode table at 1249-1760 (+1249)
794 free blocks, 8179 free inodes, 2 directories, 8177 unused inodes
Free blocks: 9446, 31975-32767
Free inodes: 12-13, 16-8192
Group 1: (Blocks 32768-65535) csum 0x8719 [INODE_UNINIT, ITABLE_ZEROED]
Backup superblock at 32768, Group descriptors at 32769-33985
Block bitmap at 1218 (bg #0 + 1218), csum 0x671f0005
Inode bitmap at 1234 (bg #0 + 1234), csum 0x00000000
Inode table at 1761-2272 (bg #0 + 1761)
799 free blocks, 8192 free inodes, 0 directories, 8192 unused inodes
Free blocks: 34806-34807, 34815, 42178-42207, 42289-42367,
42463-42495, 45601-45667, 57956-57983, 58202-58367, 59951-60003, 63490,
63498-63503, 63512-63519, 63552-63615, 63739-63999
Free inodes: 8193-16384
From my understanding - please correct me - block_bitmap should be 1218
(because group i=1 was mentioned). sb_block+1=0+1 should be 1; so
block_bitmap>=sb_block+1 is true. But shouldn't last_bg_block be 1217
(or 1 as 64 groups fit into one block?) or less? Which value does
last_bg_block represent?
Whats wrong here? By the way: The fs grew a lot in the past years.
Any hint on whats going on here and how I can come back to a read-write
system?
Thanks in advance, also for reading!
