Issue

• System crashed with following call traces on console:

  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<ffffffffa0030a42>] write_callback+0x22/0x110 [dm_mirror]
  PGD 41feea067 PUD 600a1f067 PMD 0 
  Oops: 0000 [#1] SMP 
  [...]
  CPU: 0 PID: 58486 Comm: kworker/0:0 Not tainted 3.10.0-514.21.1.el7.x86_64 #1
  Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 04/14/2014
  Workqueue: kmirrord do_mirror [dm_mirror]
  task: ffff880a8de6bec0 ti: ffff88019c508000 task.ti: ffff88019c508000
  RIP: 0010:[<ffffffffa0030a42>]  [<ffffffffa0030a42>] write_callback+0x22/0x110 [dm_mirror]
  [...]
  Stack:
   ffff88027fc69f80 ffff88027fc69f80 ffff8802d7eb4888 ffffffffa0030a20
   ffff88027fc69c80 ffff88019c50bb70 ffffffffa000a969 0000000000000000
   ffff8805d7eac900 ffff88019c50bd08 ffff88019c50bc28 ffffffffa000acb4
  Call Trace:
   [<ffffffffa0030a20>] ? recovery_complete+0x150/0x150 [dm_mirror]
   [<ffffffffa000a969>] dec_count+0x49/0x50 [dm_mod]
   [<ffffffffa000acb4>] dispatch_io+0x2e4/0x3a0 [dm_mod]
   [<ffffffffa000a740>] ? vm_next_page+0x30/0x30 [dm_mod]
   [<ffffffffa000a7a0>] ? km_get_page+0x60/0x60 [dm_mod]
   [<ffffffffa0030a20>] ? recovery_complete+0x150/0x150 [dm_mirror]
   [<ffffffffa000ae7e>] dm_io+0x10e/0x2e0 [dm_mod]
   [<ffffffff810c52b4>] ? try_to_wake_up+0x174/0x340
   [<ffffffffa000a740>] ? vm_next_page+0x30/0x30 [dm_mod]
   [<ffffffffa000a7a0>] ? km_get_page+0x60/0x60 [dm_mod]
   [<ffffffffa0031174>] do_writes+0x304/0x650 [dm_mirror]
   [<ffffffffa0031004>] ? do_writes+0x194/0x650 [dm_mirror]
   [<ffffffffa0030a20>] ? recovery_complete+0x150/0x150 [dm_mirror]
   [<ffffffffa00322df>] do_mirror+0x10f/0x230 [dm_mirror]
   [<ffffffff810a845b>] process_one_work+0x17b/0x470
   [<ffffffff810a9296>] worker_thread+0x126/0x410
   [<ffffffff810a9170>] ? rescuer_thread+0x460/0x460
   [<ffffffff810b0a4f>] kthread+0xcf/0xe0
   [<ffffffff810b0980>] ? kthread_create_on_node+0x140/0x140
   [<ffffffff81697418>] ret_from_fork+0x58/0x90
   [<ffffffff810b0980>] ? kthread_create_on_node+0x140/0x140
  Code: e1 e9 6d ff ff ff 0f 1f 00 66 66 66 66 90 55 48 89 e5 41 56 41 55 41 54 53 48 89 f3 48 83 ec 08 48 8b 46 08 48 85 ff 48 89 7d d8 <4c> 8b 20 48 c7 46 08 00 00 00 00 75 17 31 f6 48 89 df e8 f7 89 
  RIP  [<ffffffffa0030a42>] write_callback+0x22/0x110 [dm_mirror]
   RSP <ffff88019c50bb20>
  CR2: 0000000000000000