Comment 4 for bug 1787281

Test on Trusty

Before:

$ uname -a
Linux bionic 3.13.0-155-generic #205-Ubuntu SMP Fri Aug 10 15:53:26 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
$ echo 9 | sudo tee /proc/sys/kernel/printk
9
$ sudo losetup --find --show --partscan rlv_grkgld.1mb
<hung>
[ 270.506420] partition (null) (3 pp's found) is not contiguous
[ 270.510221] partition (null) (1 pp's found) is not contiguous
[ 270.513952] partition (null) (68 pp's found) is not contiguous
...
[ 270.593589] partition (null) (3 pp's found) is not contiguous
[ 270.595603] partition (null) (2 pp's found) is not contiguous
[ 270.597428] BUG: unable to handle kernel paging request at 0000000000001000
[ 270.599525] IP: [<ffffffff81379d4d>] strnlen+0xd/0x40
[ 270.601404] PGD 0
[ 270.601404] Oops: 0000 [#1] SMP
[ 270.601404] Modules linked in: squashfs isofs nls_iso8859_1 kvm_intel kvm serio_raw sch_fq_codel iscsi
_tcp libiscsi_tcp libiscsi scsi_transport_iscsi ip_tables x_tables autofs4 btrfs libcrc32c raid10 raid456
 async_memcpy async_raid6_recov async_pq async_xor async_tx xor raid6_pq raid1 raid0 multipath linear psm
ouse floppy
[ 270.601404] CPU: 1 PID: 972 Comm: losetup Not tainted 3.13.0-155-generic #205-Ubuntu
[ 270.601404] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[ 270.601404] task: ffff88003998e000 ti: ffff88003b1b6000 task.ti: ffff88003b1b6000
[ 270.601404] RIP: 0010:[<ffffffff81379d4d>] [<ffffffff81379d4d>] strnlen+0xd/0x40
[ 270.601404] RSP: 0018:ffff88003b1b7888 EFLAGS: 00010086
[ 270.601404] RAX: ffffffff81a674a1 RBX: ffffffff81ecbdec RCX: fffffffffffffffe
[ 270.601404] RDX: 0000000000001000 RSI: ffffffffffffffff RDI: 0000000000001000
[ 270.601404] RBP: ffff88003b1b7888 R08: 000000000000ffff R09: 000000000000ffff
[ 270.601404] R10: ffffffff813e27f0 R11: ffff88003b1b773e R12: 0000000000001000
[ 270.601404] R13: ffffffff81ecc1c0 R14: 00000000ffffffff R15: 0000000000000000
[ 270.601404] FS: 00007fbcbba18740(0000) GS:ffff88003ee80000(0000) knlGS:0000000000000000
[ 270.601404] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 270.601404] CR2: 0000000000001000 CR3: 000000003a9fe000 CR4: 0000000000000670
[ 270.601404] Stack:
[ 270.601404] ffff88003b1b78c0 ffffffff8137c0ab ffffffff81ecbdec ffffffff81ecc1c0
[ 270.601404] ffff88003b1b79c0 ffffffff81a939c6 ffffffff81a939c6 ffff88003b1b7928
[ 270.601404] ffffffff8137d521 0000000000000086 ffff88003b1b773e 000000000000000c
[ 270.601404] Call Trace:
[ 270.601404] [<ffffffff8137c0ab>] string.isra.5+0x3b/0xf0
[ 270.601404] [<ffffffff8137d521>] vsnprintf+0x1c1/0x610
[ 270.601404] [<ffffffff8137d97d>] vscnprintf+0xd/0x30
[ 270.601404] [<ffffffff810c4b91>] vprintk_emit+0x111/0x530
[ 270.601404] [<ffffffff8173313a>] printk+0x67/0x69
[ 270.601404] [<ffffffff8135a683>] aix_partition+0x613/0x620
[ 270.601404] [<ffffffff813768de>] ? radix_tree_lookup_slot+0xe/0x10
[ 270.601404] [<ffffffff8135e0a0>] msdos_partition+0x870/0x890
[ 270.601404] [<ffffffff81158581>] ? read_cache_page+0x21/0x30
[ 270.601404] [<ffffffff8135826d>] ? read_dev_sector+0x2d/0x90
[ 270.601404] [<ffffffff8137da39>] ? snprintf+0x39/0x40
[ 270.601404] [<ffffffff8135d830>] ? parse_solaris_x86+0x230/0x230
[ 270.601404] [<ffffffff81358eaa>] check_partition+0x10a/0x240
[ 270.601404] [<ffffffff81358ad7>] rescan_partitions+0xb7/0x2c0
[ 270.601404] [<ffffffff813540ef>] blkdev_ioctl+0xef/0x7d0
[ 270.601404] [<ffffffff8173cfa9>] ? schedule_timeout+0x279/0x310
[ 270.601404] [<ffffffff812016f3>] ioctl_by_bdev+0x33/0x40
[ 270.601404] [<ffffffff814c765a>] loop_set_status+0x39a/0x3b0
[ 270.601404] [<ffffffff814c7930>] loop_set_status64+0x50/0x70
[ 270.601404] [<ffffffff814c9938>] lo_ioctl+0x1e8/0x730
[ 270.601404] [<ffffffff8135421f>] blkdev_ioctl+0x21f/0x7d0
[ 270.601404] [<ffffffff8174aa3c>] ? system_call_after_swapgs+0x156/0x170
[ 270.601404] [<ffffffff812016b1>] block_ioctl+0x41/0x50
[ 270.601404] [<ffffffff811da953>] do_vfs_ioctl+0x2e3/0x4d0
[ 270.601404] [<ffffffff8174a9fd>] ? system_call_after_swapgs+0x117/0x170
[ 270.601404] [<ffffffff8174a9f6>] ? system_call_after_swapgs+0x110/0x170
[ 270.601404] [<ffffffff8174a9ef>] ? system_call_after_swapgs+0x109/0x170
[ 270.601404] [<ffffffff8174a9e8>] ? system_call_after_swapgs+0x102/0x170
[ 270.601404] [<ffffffff8174a9e1>] ? system_call_after_swapgs+0xfb/0x170
[ 270.601404] [<ffffffff8174a9da>] ? system_call_after_swapgs+0xf4/0x170
[ 270.601404] [<ffffffff8174a9d3>] ? system_call_after_swapgs+0xed/0x170
[ 270.601404] [<ffffffff8174a9cc>] ? system_call_after_swapgs+0xe6/0x170
[ 270.601404] [<ffffffff811dabc1>] SyS_ioctl+0x81/0xa0
[ 270.601404] [<ffffffff8174a99b>] ? system_call_after_swapgs+0xb5/0x170
[ 270.601404] [<ffffffff8174aa70>] system_call_fastpath+0x1a/0x1f
[ 270.601404] Code: c0 01 80 38 00 75 f7 48 29 f8 5d c3 31 c0 5d c3 66 66 66 66 66 2e 0f 1f 84 00 00 00
00 00 55 48 85 f6 48 8d 4e ff 48 89 e5 74 2a <80> 3f 00 74 25 48 89 f8 31 d2 eb 10 0f 1f 80 00 00 00 00 48 83
[ 270.601404] RIP [<ffffffff81379d4d>] strnlen+0xd/0x40
[ 270.601404] RSP <ffff88003b1b7888>
[ 270.601404] CR2: 0000000000001000
[ 270.732715] ---[ end trace c22abe83af8ab594 ]---

After:

$ uname -a
Linux bionic 3.13.0-155-generic #205+sf181954.1 SMP Wed Aug 15 17:05:33 -03 2018 x86_64 x86_64 x86_64 GNU/Linux
$ echo 9 | sudo tee /proc/sys/kernel/printk
9
$ sudo losetup --find --show --partscan rlv_grkgld.1mb
/dev/loop0