VM panics with blocked tasks due to slow or stalled I/O

Issue

• System rebooted due to blocked tasks
• Frequent reboot (unexpected). Trending show system rebooted once every 2-3 days, usually at night around 12 - 3 am (GMT+0800). No abnormalities found from log. No hardware errors. No application errors. System is in cloud environment.

• VM guest panicked with the following:

  INFO: task kjournald:911 blocked for more than 120 seconds.
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  kjournald     D ffff81000901d7a0     0   911    279           936   873 (L-TLB)
  ffff81083de83cf0 0000000000000046 ffff81083fafc080 ffff81083e0cc540
  0000000dd7e412dc 000000000000000a ffff81083fafc080 ffff81011cfc8080
  000000115554a5db 00000000000020cd ffff81083fafc268 000000033e6daa38
  Call Trace:
  [<ffffffff8006f1f5>] do_gettimeofday+0x40/0x90
  [<ffffffff800154ce>] sync_buffer+0x0/0x3f
  [<ffffffff800647ea>] io_schedule+0x3f/0x67
  [<ffffffff80015509>] sync_buffer+0x3b/0x3f
  [<ffffffff80064a16>] __wait_on_bit+0x40/0x6e
  [<ffffffff800154ce>] sync_buffer+0x0/0x3f
  [<ffffffff80064ab0>] out_of_line_wait_on_bit+0x6c/0x78
  [<ffffffff800a1bd2>] wake_bit_function+0x0/0x23
  [<ffffffff8003ddd5>] lock_timer_base+0x1b/0x3c
  [<ffffffff8004b6b6>] try_to_del_timer_sync+0x7f/0x88
  [<ffffffff800a1ba4>] autoremove_wake_function+0x0/0x2e
  [<ffffffff800a198c>] keventd_create_kthread+0x0/0xc4
  [<ffffffff800a198c>] keventd_create_kthread+0x0/0xc4
  [<ffffffff80032bdc>] kthread+0xfe/0x132
  [<ffffffff8005efb1>] child_rip+0xa/0x11
  [<ffffffff800a198c>] keventd_create_kthread+0x0/0xc4
  [<ffffffff80032ade>] kthread+0x0/0x132
  [<ffffffff8005efa7>] child_rip+0x0/0x11
  Kernel panic - not syncing: hung_task: blocked tasks