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[Chris J Arges <1377130@xxxxxxxxxxxxxxxxxx>]

I could not easily reproduce this with a VM according to your instructions. Are there other factors that are necessary to hit this issue?
When the machine hangs do you have any message printed to the console showing the backtrace?

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https://bugs.launchpad.net/bugs/1377130

Title:
  Crash at creation of bcache if caching size > backing size

Status in “bcache-tools” package in Ubuntu:
  New
Status in “linux” package in Ubuntu:
  Incomplete

Bug description:
  Hi,

  I found a bug that crashes the machine if you create a bcache where
  the caching device size is bigger than backing device.

  This case may happen if you create a bcache with a plan to add a
  bigger disk/partition/raid later to it.

  To reproduce it follow this steps:

  start a vm with utopic-amd64-desktop iso, it must have 2 disks, one of
  16G and on of 8G

    apt-get update && apt-get install bcache-tools
    /dev/sda is 16G caching device
    /dev/sdb is 8G backing device

    make-bcache --writeback --discard -C /dev/sda -B /dev/sdb

  now the machine is hanged!

  if you reboot and reinstall bcache-tools
  the bcache device has been created

    ************************************
    bcache-super-show -f /dev/sda:
    ==============================
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			45C96B38A79275A7 [match]
    sb.version		0 [cache device]

    dev.label		(empty)
    dev.uuid		155ab65e-e74a-4ca8-91da-467ef696db63
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.cache.first_sector	1024
    dev.cache.cache_sectors	33553408
    dev.cache.total_sectors	33554432
    dev.cache.ordered	no
    dev.cache.discard	yes
    dev.cache.pos		0
    dev.cache.replacement	0 [lru]

    cset.uuid             bcaa2595-bbc8-43a7-8c44-5d0aca0f3fc8

    *****************************************************
    bcache-super-show /dev/sdb:
    =============================================
    sb.magic		ok
    sb.first_sector		8 [match]
    sb.csum			F37A1BB08A7F5DE7 [match]
    sb.version		1 [backing device]

    dev.label		(empty)
    dev.uuid		341f1c83-1797-4157-aa9e-524107db5606
    dev.sectors_per_block	1
    dev.sectors_per_bucket	1024
    dev.data.first_sector	16
    dev.data.cache_mode	1 [writeback]
    dev.data.cache_state	0 [detached]

    cset.uuid		bcaa2595-bbc8-43a7-8c44-5d0aca0f3fc8
    *****************************************************

  you can format it:
    mkfs.ext4 /dev/bcache0
    Rejet des blocs de périphérique : complété
    Creating filesystem with 2097150 4k blocks and 524288 inodes
    Filesystem UUID: b99a09de-3a92-4a44-82cd-29f75f8bf0c9
    Superblocs de secours stockés sur les blocs :
     32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632

    Allocation des tables de groupe : complété
    Écriture des tables d'i-noeuds : complété
    Création du journal (32768 blocs) : complété
    Écriture des superblocs et de l'information de comptabilité du système de
    fichiers :

  restarting the format gave me an other bug (X.org crashing):
    free from cli:
          total       used       free     shared    buffers     cached
    Mem:       1017168     814040     203128     195568      91264     513064
    -/+ buffers/cache:     209712     807456
    Swap:            0          0          0

    tail /var/log/syslog:
    [  568.016481] [ 4983]     0  4983     3332      121      11        0             0 mdadm
    [  568.016483] [ 5259]     0  5259     6703      300      19        0             0 mkfs.ext4
    [  568.016484] Out of memory: Kill process 1947 (Xorg) score 38 or sacrifice child
    [  568.016485] Killed process 1947 (Xorg) total-vm:313160kB, anon-rss:35976kB, file-rss:4232kB
    [  568.707107] systemd-logind[1396]: Failed to start unit user@112.service: Unknown unit: user@112.service
    [  568.707113] systemd-logind[1396]: Failed to start user service: Unknown unit: user@112.service
    [  568.709876] systemd-logind[1396]: New session c8 of user lightdm.
    [  568.709890] systemd-logind[1396]: Linked /tmp/.X11-unix/X0 to /run/user/112/X11-display.

  You can unregister the bcache device,but to re-use all disks, you'll need a reboot (cf bug 1377142):
    [  830.811150] bcache: cached_dev_detach_finish() Caching disabled for sdb
    [  830.949898] bcache: cache_set_free() Cache set bcaa2595-bbc8-43a7-8c44-5d0aca0f3fc8 unregistered

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