Comment 0 for bug 1933074

I believe, I found a bug in ext4 in recent kernel versions.
I stumbled across this while I was trying to restore a backup to a new VM.

How to reproduce this bug:

1. Use a virtual/physical machine with "Ubuntu 18.04.5 LTS" and kernel version 4.15.0-144-generic.
2. add a secondary disk to hold the test files.
3. prepare and mount the filesystem with enabled 'large_dir' flag:
mkfs.ext4 -m0 /dev/sdb1;
tune2fs -O large_dir /dev/sdb1;
mkdir /mnt/storage;
mount /dev/sdb1 /mnt/storage;
4. change to directory and create approx. 16 mio files
cd /mnt/storage;
i=0;
while (( $i < 20000000 )); do
  i=$(( $i + 1 ));
  (( $i % 1000 == 0 )) && echo $i;
  touch file_$i.dat || break;
done

Expected behaviour:
- 20 mio files shoud be created without error

What happened instead:
- The loop aborts with an error message:
# 16263100
# touch: cannot touch 'file_16263173.dat': Structure needs cleaning
- dmesg gives a little more details:
# [Mon Jun 21 03:15:18 2021] EXT4-fs error (device sdb): dx_probe:855: inode #2: block 146221: comm touch: directory leaf block found instead of index block

Additional notes:
- This occurs on kernel version 4.15.0-144-generic
- Not sure, but I believe one test was run on 4.15.0-143-generic and failed too.
- Did not check against 4.15.0-142-generic
- On 4.15.0-141-generic, the problem does not exist. Behaviour is as expected.