Comment 828 for bug 500069

Had this again 20 minutes ago.
Was dopying 8.7GiB of data from one directory to another directory on the same filesystem (ext4 (rw,relatime,data=ordered)) on the same disk (Western Digital WDC WD30EZRX-00D8PB0 spinning metal disk).

The KDE UI became unresponsive (Everything other than /home and user data in on a SSD), could not launch any new applications. Opening a new tab on Firefox to go to Youtube didnt load the page, and kept saying waiting for youtube.com in the status bar (network gets halted?).

dmesg shows these, are they important?

[25013.905943] INFO: task DOMCacheThread:17496 blocked for more than 120 seconds.
[25013.905945] Tainted: P OE 4.15.0-54-generic #58-Ubuntu
[25013.905947] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[25013.905949] DOMCacheThread D 0 17496 2243 0x00000000
[25013.905951] Call Trace:
[25013.905954] __schedule+0x291/0x8a0
[25013.905957] schedule+0x2c/0x80
[25013.905959] jbd2_log_wait_commit+0xb0/0x120
[25013.905962] ? wait_woken+0x80/0x80
[25013.905965] __jbd2_journal_force_commit+0x61/0xb0
[25013.905967] jbd2_journal_force_commit+0x21/0x30
[25013.905970] ext4_force_commit+0x29/0x2d
[25013.905972] ext4_sync_file+0x14a/0x3b0
[25013.905975] vfs_fsync_range+0x51/0xb0
[25013.905977] do_fsync+0x3d/0x70
[25013.905980] SyS_fsync+0x10/0x20
[25013.905982] do_syscall_64+0x73/0x130
[25013.905985] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[25013.905987] RIP: 0033:0x7fc9cb839b07
[25013.905988] RSP: 002b:00007fc9a7aeb200 EFLAGS: 00000293 ORIG_RAX: 000000000000004a
[25013.905990] RAX: ffffffffffffffda RBX: 00000000000000a0 RCX: 00007fc9cb839b07
[25013.905992] RDX: 0000000000000000 RSI: 00007fc9a7aeaff0 RDI: 00000000000000a0
[25013.905993] RBP: 0000000000000000 R08: 0000000000000000 R09: 72732f656d6f682f
[25013.905994] R10: 0000000000000000 R11: 0000000000000293 R12: 00000000000001f6
[25013.905995] R13: 00007fc97fc5d038 R14: 00007fc9a7aeb340 R15: 00007fc987523380