Description of problem:

Noticed a node reboot due to some page_fault
[171884.552080] BUG: unable to handle kernel NULL pointer dereference at (null)


Action:
Delete the block (used for about 2 days for some IO)


Observation  from userspace:
---------------------------
[root@gprfc076 mnt]# ps -aux | grep target
root       923  0.0  0.0      0     0 ?        S<   Jun27   0:00 [target_completi]
root      7167  0.0  0.0 117300  1448 ?        S    03:30   0:00 sh -c targetcli /backstores/user:glfs delete block && targetcli /iscsi delete iqn.2016-12.org.gluster-block:d81024d7-21a6-4a8c-ac11-06fe56fee9d6 && targetcli / saveconfig > /dev/null
root      7168  0.0  0.0 258008 15840 ?        D    03:30   0:00 /usr/bin/python /usr/bin/targetcli /backstores/user:glfs delete block
root      7191  0.0  0.0 255072 14652 pts/6    D+   03:31   0:00 /usr/bin/python /usr/bin/targetcli ls
root      7289  0.0  0.0 114712   972 pts/9    S+   03:36   0:00 grep --color=auto target

You can notice that targetcli delete command went into "uninterruptible sleep", the targetcli command hung, I have also noticed tcmu-runner segfault at this time, unfortunately abrtd was not running in that machine so, could not get a core dump of it, sorry about this.


tcmu-runner logs:
---------------
2017-06-28 12:10:58.584 5965 [DEBUG] main:808 : handler path: /usr/lib64/tcmu-runner
2017-06-28 12:10:58.656 5965 [DEBUG] load_our_module:524 : Module 'target_core_user' is already loaded
2017-06-28 12:10:58.670 5965 [DEBUG] main:821 : 1 runner handlers found
2017-06-28 12:10:59.976 5965 [DEBUG] dbus_bus_acquired:437 : bus org.kernel.TCMUService1 acquired
2017-06-28 12:10:59.977 5965 [DEBUG] dbus_name_acquired:453 : name org.kernel.TCMUService1 acquired
2017-06-29 03:30:33.444 5965 [DEBUG] handle_netlink:127 : cmd 2. Got header version 2. Supported 2.


Kernel Oops:
-----------
[...]
[171884.552188] Oops: 0000 [#1] SMP 
[171884.552207] Modules linked in: fuse loop target_core_pscsi target_core_file target_core_iblock iscsi_target_mod scsi_transport_iscsi ip6t_rpfilter ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfnetlink ebtable_nat ebtable_broute bridge stp llc ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter target_core_user target_core_mod uio dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio sb_edac edac_core intel_powerclamp coretemp intel_rapl iosf_mbi kvm_intel kvm irqbypass crc32_pclmul ghash_clmulni_intel aesni_intel lrw gf128mul glue_helper
[171884.552583]  iTCO_wdt ablk_helper cryptd ipmi_ssif iTCO_vendor_support mei_me pcspkr sg dcdbas joydev ipmi_si ipmi_devintf wmi ipmi_msghandler mei acpi_power_meter acpi_pad lpc_ich shpchp nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c sd_mod crc_t10dif crct10dif_generic mgag200 i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm drm ixgbe ahci libahci libata crct10dif_pclmul crct10dif_common crc32c_intel tg3 megaraid_sas mdio i2c_core dca ptp pps_core dm_mirror dm_region_hash dm_log dm_mod
[171884.552867] CPU: 2 PID: 7294 Comm: tcmu-runner Not tainted 3.10.0-686.el7.test.x86_64 #1
[171884.552903] Hardware name: Dell Inc. PowerEdge R620/0KCKR5, BIOS 1.3.6 09/11/2012
[171884.552936] task: ffff8810092c3f40 ti: ffff880540f0c000 task.ti: ffff880540f0c000
[171884.552968] RIP: 0010:[<ffffffffc05397c2>]  [<ffffffffc05397c2>] tcmu_vma_fault+0x72/0xf0 [target_core_user]
[171884.553014] RSP: 0000:ffff880540f0fd58  EFLAGS: 00010246
[171884.553038] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff880000000400
[171884.553069] RDX: ffff880000000250 RSI: 00003ffffffff000 RDI: 0000000000000000
[171884.553100] RBP: ffff880540f0fd68 R08: 0000000000000000 R09: ffff880540f0fde8
[171884.553131] R10: 0000000000000002 R11: 0000000000000000 R12: ffff880540f0fd80
[171884.553162] R13: ffff88101b6616c8 R14: 0000000000000000 R15: ffff88081f3fe398
[171884.553193] FS:  00007f1cab944880(0000) GS:ffff88081fa40000(0000) knlGS:0000000000000000
[171884.553228] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[171884.553254] CR2: 0000000000000000 CR3: 0000000544b96000 CR4: 00000000000407e0
[171884.553285] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[171884.553315] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[171884.553346] Stack:
[171884.553357]  0000000000000000 ffff880540f0fde8 ffff880540f0fdc8 ffffffff811ad122
[171884.554683]  0000000000000000 ffff8810000000a8 0000000000000000 00007f1c8e6ef000
[171884.556000]  0000000000000000 0000000000000000 ffff88081f3fe398 0000000026551ff4
[171884.557312] Call Trace:
[171884.558616]  [<ffffffff811ad122>] __do_fault+0x52/0xe0
[171884.559912]  [<ffffffff811ad5cb>] do_read_fault.isra.44+0x4b/0x130
[171884.561204]  [<ffffffff811b1ed1>] handle_mm_fault+0x691/0x1010
[171884.562484]  [<ffffffff811b8c9e>] ? do_mmap_pgoff+0x31e/0x3e0
[171884.563743]  [<ffffffff816aef74>] __do_page_fault+0x154/0x450
[171884.564984]  [<ffffffff816af2a5>] do_page_fault+0x35/0x90
[171884.566218]  [<ffffffff816ab4c8>] page_fault+0x28/0x30
[171884.567428] Code: d7 48 63 d2 48 8d 04 52 48 c1 e7 0c 48 c1 e0 04 48 01 c6 48 03 be 80 10 00 00 83 be 90 10 00 00 02 74 36 e8 31 6c c8 c0 48 89 c3 <48> 8b 03 f6 c4 80 75 59 f0 ff 43 1c 48 8b 03 a9 00 00 00 80 74 
[171884.569986] RIP  [<ffffffffc05397c2>] tcmu_vma_fault+0x72/0xf0 [target_core_user]
[171884.571233]  RSP <ffff880540f0fd58>
[171884.572457] CR2: 0000000000000000

How reproducible:
Have hit only once. Chances being very rare.