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[Kick In <pierre-andre.morey@xxxxxxxxxxxxx>]

** Also affects: bcache-tools (Ubuntu)
   Importance: Undecided
       Status: New

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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 --write-back --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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