kernel bug when mounting a qemu-nbd backed image

[Ryan Harper]

1. % lsb_release -rd
Description: Ubuntu 15.10
Release: 15.10

2.
% apt-cache policy linux-image-generic
linux-image-generic:
  Installed: (none)
  Candidate: 4.2.0.30.33
  Version table:
     4.2.0.30.33 0
        500 http://us.archive.ubuntu.com/ubuntu/ wily-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ wily-security/main amd64 Packages
     4.2.0.16.18 0
        500 http://us.archive.ubuntu.com/ubuntu/ wily/main amd64 Packages

(kriek) ~ % apt-cache policy qemu-utils
qemu-utils:
  Installed: 1:2.3+dfsg-5ubuntu9.2
  Candidate: 1:2.3+dfsg-5ubuntu9.2
  Version table:
 *** 1:2.3+dfsg-5ubuntu9.2 0
        500 http://us.archive.ubuntu.com/ubuntu/ wily-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ wily-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1:2.3+dfsg-5ubuntu9 0
        500 http://us.archive.ubuntu.com/ubuntu/ wily/main amd64 Packages

3. Configure an NBD device with a qemu image and then mount it. The base image is a wily amd64 cloud-image in raw format.
Created the qcow2 device with: qemu-img create -f qcow2 -b /path/to/cloud-image.raw boot.img 2G
Connect to nbd: sudo qemu-nbd -c /dev/nbd0 boot.img
Mount it: sudo mount /dev/nbd0 /mnt
Mount succeeds and I can examine the image.

% qemu-img info --backing-chain boot.img
image: boot.img
file format: qcow2
virtual size: 2.0G (2147483648 bytes)
disk size: 196K
cluster_size: 65536
backing file: /srv/images/wily/amd64/20160218/vmtest.root-image
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

image: /srv/images/wily/amd64/20160218/vmtest.root-image
file format: raw
virtual size: 1.4G (1468006400 bytes)
disk size: 1.4G

4. mount command hung and kernel emitted BUG().
In reproducing this may be relevant.

a. Configure NBD with boot.img as in (3)
b. while still mounted rm boot.img
c. disconnect: qemu-nbd -d /dev/nbd0
d. unmount: umount /mnt

Then repeat (3).

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: linux-image-4.2.0-19-generic 4.2.0-19.23
ProcVersionSignature: Ubuntu 4.2.0-19.23-generic 4.2.6
Uname: Linux 4.2.0-19-generic x86_64
AlsaVersion: Advanced Linux Sound Architecture Driver Version k4.2.0-19-generic.
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay'
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/by-path', '/dev/snd/hwC0D3', '/dev/snd/pcmC0D8p', '/dev/snd/pcmC0D7p', '/dev/snd/pcmC0D3p', '/dev/snd/controlC0', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
Card0.Amixer.info: Error: [Errno 2] No such file or directory: 'amixer'
Card0.Amixer.values: Error: [Errno 2] No such file or directory: 'amixer'
Date: Thu Mar 3 09:18:14 2016
HibernationDevice: RESUME=/dev/mapper/kriek--vg-swap_1
InstallationDate: Installed on 2015-05-10 (297 days ago)
InstallationMedia: Ubuntu-Server 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.3)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.2.0-19-generic root=/dev/mapper/hostname--vg-root ro nomdmonddf nomdmonisw
RelatedPackageVersions:
 linux-restricted-modules-4.2.0-19-generic N/A
 linux-backports-modules-4.2.0-19-generic N/A
 linux-firmware 1.149.3
RfKill: Error: [Errno 2] No such file or directory: 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 11/05/2014
dmi.bios.vendor: Intel Corp.
dmi.bios.version: RKPPT10H.86A.0037.2014.1105.1813
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: D53427RKE
dmi.board.vendor: Intel Corporation
dmi.board.version: G87971-406
dmi.chassis.type: 3
dmi.modalias: dmi:bvnIntelCorp.:bvrRKPPT10H.86A.0037.2014.1105.1813:bd11/05/2014:svn:pn:pvr:rvnIntelCorporation:rnD53427RKE:rvrG87971-406:cvn:ct3:cvr:

[Brad Figg (brad-figg)]

This change was made by a bot.