The following are known ext4 issues:

• System hangs when processing corrupted orphaned inode list

  If the orphaned inode list is corrupted, the inode might be processed repeatedly, resulting in a system hang. For example, if the orphaned inode list contains a reference to the bootloader inode, ext4_iget(), returns a bad inode, it can result in a processing loop that can hang the system. (Bug ID 24433290)

• System hangs on unmount after an append to a file with negative i_size

  While it is invalid for a file system to load an inode with a negative i_size, it is possible to create a file like this and append to it. However, doing so causes an integer overflow in the routine's underlying writeback, resulting in the kernel locking up. (Bug ID 25565527)