Sorry, my previous dump had a segfault in the printk just before the real segfault. Search for "NOELBK" below. Note that _submit_bh is getting a buffer_head with bh->b_bdev==NULL
[ 60.372845] nbd: registered device at major 43
[ 61.500163] EXT4-fs (nbd1): mounted filesystem with ordered data mode. Opts: (null)
[ 61.549895] block nbd1: NBD_DISCONNECT
[ 61.562890] block nbd1: Receive control failed (result -32)
[ 61.563773] block nbd1: queue cleared
[ 62.581005] block nbd1: Attempted send on closed socket
[ 62.581399] CPU: 3 PID: 2295 Comm: tail Tainted: G E 3.18.0+ #4
[ 62.581433] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[ 62.581436] ffff8800db119800 ffff88021331b9b8 ffffffff81764bdc 0000000000000001
[ 62.581440] ffff8800db9ec0c0 ffff88021331b9e8 ffffffffa04dbb2a ffff88003787c3c8
[ 62.581444] 0000000000000005 ffff8802111b9e30 ffff8800d9e85800 ffff88021331ba08
[ 62.581472] Call Trace:
[ 62.581482] [<ffffffff81764bdc>] dump_stack+0x46/0x58
[ 62.581512] [<ffffffffa04dbb2a>] do_nbd_request+0x13a/0x185 [nbd]
[ 62.581520] [<ffffffff81351df7>] __blk_run_queue+0x37/0x50
[ 62.581549] [<ffffffff813569d3>] blk_queue_bio+0x323/0x380
[ 62.581554] [<ffffffff81351c40>] generic_make_request+0xc0/0x110
[ 62.581560] [<ffffffff81351cf9>] submit_bio+0x69/0x130
[ 62.581589] [<ffffffff8134bba6>] submit_bio_wait+0x56/0x70
[ 62.581595] [<ffffffff81357f6e>] blkdev_issue_flush+0x5e/0x90
[ 62.581601] [<ffffffff81272521>] ext4_sync_fs+0xc1/0x180
[ 62.581630] [<ffffffff8120ce42>] sync_filesystem+0x82/0xb0
[ 62.581635] [<ffffffff811dee64>] generic_shutdown_super+0x34/0x100
[ 62.581639] [<ffffffff811df277>] kill_block_super+0x27/0x70
[ 62.581643] [<ffffffff811df589>] deactivate_locked_super+0x49/0x60
[ 62.581671] [<ffffffff811dfb5e>] deactivate_super+0x4e/0x70
[ 62.581675] [<ffffffff811fc3a3>] cleanup_mnt+0x43/0x90
[ 62.581680] [<ffffffff811fc442>] __cleanup_mnt+0x12/0x20
[ 62.581684] [<ffffffff8108bd34>] task_work_run+0xc4/0xe0
[ 62.581713] [<ffffffff81071729>] do_exit+0x2d9/0xa80
[ 62.581718] [<ffffffff810a49ce>] ? dequeue_task_fair+0x44e/0x660
[ 62.581723] [<ffffffff8107afdf>] ? recalc_sigpending+0x1f/0x60
[ 62.581751] [<ffffffff81071f5f>] do_group_exit+0x3f/0xa0
[ 62.581755] [<ffffffff8107dd63>] get_signal+0x1e3/0x730
[ 62.581761] [<ffffffff81012508>] do_signal+0x28/0xaa0
[ 62.581766] [<ffffffff810ae580>] ? prepare_to_wait_event+0x110/0x110
[ 62.581794] [<ffffffff811dcc9c>] ? vfs_read+0x9c/0x180
[ 62.581798] [<ffffffff81012fe9>] do_notify_resume+0x69/0xb0
[ 62.581803] [<ffffffff8176d2cf>] int_signal+0x12/0x17
[ 62.581831] blk_update_request: I/O error, dev nbd1, sector 0
[ 62.582718] NOELBK: !buffer_mapped(bh) bh=ffff8800d201d5b0 bh->b_bdev= (null)
[ 62.583290] ------------[ cut here ]------------
[ 62.583646] kernel BUG at fs/buffer.c:3013!
[ 62.583848] invalid opcode: 0000 [#1] SMP
[ 62.584555] Modules linked in: nbd(E) nf_conntrack_ipv4(E) nf_defrag_ipv4(E) xt_conntrack(E) nf_conntrack(E) ipt_REJECT(E) nf_reject_ipv4(E) xt_CHECKSUM(E) iptable_mangle(E) xt_tcpudp(E) bridge(E) stp(E) llc(E) ip6table_filter(E) ip6_tables(E) iptable_filter(E) ip_tables(E) ebtable_nat(E) ebtables(E) x_tables(E) ib_iser(E) rdma_cm(E) iw_cm(E) ib_cm(E) ib_sa(E) ib_mad(E) ib_core(E) ib_addr(E) iscsi_tcp(E) libiscsi_tcp(E) libiscsi(E) scsi_transport_iscsi(E) dm_crypt(E) openvswitch(E) gre(E) vxlan(E) ip6_udp_tunnel(E) udp_tunnel(E) libcrc32c(E) ppdev(E) dm_multipath(E) scsi_dh(E) snd_intel8x0(E) i2c_piix4(E) serio_raw(E) joydev(E) snd_ac97_codec(E) ac97_bus(E) parport_pc(E) snd_pcm(E) parport(E) mac_hid(E) snd_timer(E) snd(E) soundcore(E) nfsd(E) auth_rpcgss(E) nfs_acl(E) nfs(E) lockd(E) grace(E) sunrpc(E) fscache(E) btrfs(E) xor(E) raid6_pq(E) hid_generic(E) usbhid(E) hid(E) psmouse(E) ahci(E) libahci(E) e1000(E) video(E)
[ 62.586689] CPU: 3 PID: 2295 Comm: tail Tainted: G E 3.18.0+ #4
[ 62.586689] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[ 62.586689] task: ffff8800da7b0000 ti: ffff880213318000 task.ti: ffff880213318000
[ 62.586689] RIP: 0010:[<ffffffff81210e3a>] [<ffffffff81210e3a>] _submit_bh+0x16a/0x190
[ 62.586689] RSP: 0018:ffff88021331ba78 EFLAGS: 00010246
[ 62.586689] RAX: 0000000004000005 RBX: ffff8800d201d5b0 RCX: 0000000000000006
[ 62.586689] RDX: 0000000000000000 RSI: 0000000000000246 RDI: ffff88021fd8d310
[ 62.586689] RBP: ffff88021331ba98 R08: 0000000000000096 R09: ffff8800000b8500
[ 62.586689] R10: 000000000000021f R11: ffff88021331b75e R12: ffff880211f39000
[ 62.586689] R13: 0000000000001411 R14: 0000000000000000 R15: ffff880212a75000
[ 62.586689] FS: 00007f9fedd08740(0000) GS:ffff88021fd80000(0000) knlGS:0000000000000000
[ 62.586689] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 62.586689] CR2: 00007f877f92a250 CR3: 00000000da54d000 CR4: 00000000000006e0
[ 62.586689] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 62.586689] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 62.586689] Stack:
[ 62.586689] ffff880211f39000 ffff880211f39000 ffff8800d201d5b0 0000000000001411
[ 62.586689] ffff88021331baa8 ffffffff81210e70 ffff88021331bae8 ffffffff812add09
[ 62.586689] 0000000000013640 ffff880211f39000 ffff880212a75000 ffff880211f39024
[ 62.586689] Call Trace:
[ 62.586689] [<ffffffff81210e70>] submit_bh+0x10/0x20
[ 62.586689] [<ffffffff812add09>] jbd2_write_superblock+0x89/0x190
[ 62.586689] [<ffffffff812adeb4>] jbd2_mark_journal_empty+0x64/0xa0
[ 62.586689] [<ffffffff812ae0e1>] jbd2_journal_destroy+0x1f1/0x220
[ 62.586689] [<ffffffff810ae580>] ? prepare_to_wait_event+0x110/0x110
[ 62.586689] [<ffffffff8127bcc4>] ext4_put_super+0x64/0x350
[ 62.586689] [<ffffffff811deea6>] generic_shutdown_super+0x76/0x100
[ 62.586689] [<ffffffff811df277>] kill_block_super+0x27/0x70
[ 62.586689] [<ffffffff811df589>] deactivate_locked_super+0x49/0x60
[ 62.586689] [<ffffffff811dfb5e>] deactivate_super+0x4e/0x70
[ 62.586689] [<ffffffff811fc3a3>] cleanup_mnt+0x43/0x90
[ 62.586689] [<ffffffff811fc442>] __cleanup_mnt+0x12/0x20
[ 62.586689] [<ffffffff8108bd34>] task_work_run+0xc4/0xe0
[ 62.586689] [<ffffffff81071729>] do_exit+0x2d9/0xa80
[ 62.586689] [<ffffffff810a49ce>] ? dequeue_task_fair+0x44e/0x660
[ 62.586689] [<ffffffff8107afdf>] ? recalc_sigpending+0x1f/0x60
[ 62.586689] [<ffffffff81071f5f>] do_group_exit+0x3f/0xa0
[ 62.586689] [<ffffffff8107dd63>] get_signal+0x1e3/0x730
[ 62.586689] [<ffffffff81012508>] do_signal+0x28/0xaa0
[ 62.586689] [<ffffffff810ae580>] ? prepare_to_wait_event+0x110/0x110
[ 62.586689] [<ffffffff811dcc9c>] ? vfs_read+0x9c/0x180
[ 62.586689] [<ffffffff81012fe9>] do_notify_resume+0x69/0xb0
[ 62.586689] [<ffffffff8176d2cf>] int_signal+0x12/0x17
[ 62.586689] Code: a1 e8 bb c5 13 00 89 d8 5b 41 5c 41 5d 41 5e 5d c3 41 f6 c5 01 0f 84 0e ff ff ff f0 80 63 01 f7 e9 04 ff ff ff 0f 0b 0f 0b 0f 0b <0f> 0b 48 8b 56 30 48 c7 c7 c8 d4 aa 81 31 c0 e8 51 f8 54 00 e9
[ 62.586689] RIP [<ffffffff81210e3a>] _submit_bh+0x16a/0x190
[ 62.586689] RSP <ffff88021331ba78>
[ 62.651459] ---[ end trace 9a5bbaaa46349cc9 ]---
[ 62.652389] Fixing recursive fault but reboot is needed!