[Bug 1424734] Re: Incorrect multipath device names in /etc/fstab

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Title:
  Incorrect multipath device names in /etc/fstab

Status in partman-target package in Ubuntu:
  New

Bug description:
  Problem:
  -------

  The multipath installation inserts incorrect device names on
  /etc/fstab.

  The multipath partitions have different device names during the installation 
  and on the installed system (specifically, the disk-partition separator).

  During installation: /dev/mapper/mpathXpY
  On installed system: /dev/mapper/mpathX-partY

  
  Impact:
  ------

  The multipath partitions in /etc/fstab are not mountable/usable -- any partition
  that is not specified by UUID (at least swap).

  The system only boots because the root partition is specified by UUID, thus
  not using the entry in /etc/fstab. 
  The swap partition is not made effective as swap.

  This is the /etc/fstab from installation:

  	~ # grep mapper /target/etc/fstab
  	/dev/mapper/mpath0p2 /               ext4    errors=remount-ro 0       1
  	/dev/mapper/mpath0p3 none            swap    sw              0       0

  And this is the environment in the installed system.

  	$ grep mapper /etc/fstab
  	/dev/mapper/mpath0p2 /               ext4    errors=remount-ro 0       1
  	/dev/mapper/mpath0p3 none            swap    sw              0       0

  	$ ls -1 /dev/mapper
  	control
  	mpath0
  	mpath0-part1
  	mpath0-part2
  	mpath0-part3

  	$ mount | grep ' / '
  	/dev/mapper/mpath0-part2 on / type ext4 (rw,errors=remount-ro)

  	$ grep root=UUID /proc/cmdline 
  	BOOT_IMAGE=/boot/vmlinux-3.18.0-13-generic root=UUID=35c634b0-f667-48a8-84f0-a571aa96d103 ro

  	$ cat /proc/swaps 
  	Filename                                Type            Size    Used    Priority
  	$

  
  Cause:
  -----

  The multipath block devices are created/named by different programs on
  installation and installed system, and each one uses a different disk-partition separator.

  During installation: libparted (via parted-server)
  On installed system: kpartx (via udev rules)

  On libparted, The 'p' separator is hard-coded in these functions:
  [libparted/arch/linux.c]

  	_dm_add_partition():
  		if (asprintf (&vol_name, "%sp%d", dev_name, part->num) == -1)

  	_device_get_part_path():
  		snprintf (result, result_len, "%sp%d", devpath, num);
   
  On kpartx udev rules, the '-part' separator is used:
  [/lib/udev/rules.d/95-kpartx.rules]

  	ENV{DM_STATE}=="ACTIVE", ENV{DM_UUID}=="mpath-*", \
  		RUN+="/sbin/kpartx -a -p -part /dev/$name"


  
  Solution proposal:
  -----------------

  Ideally, one side or another should be 'fixed' (either libparted or
  kpartx rules).

  I wrote this contained work-around (patch attached) to a partman's finish.d/ trigger,
  standing between each side, as I'm not sure of the fallout of either change.

  It intercepts and changes the multipath device names inserted in
  /etc/fstab.

  It patches '/lib/partman/finish.d/40fstab_hd_entries' because that file already
  has a 'case' including '/dev/mapper', so it seemed an appropriate place. 
  The change is /in/ the addfstab() function, because the specified device must 
  actually exist until it's called, as 'mapdevfs $dev' is an argument.

  
  With it applied, this is the new /etc/fstab during installation:

  	~ # grep mapper /target/etc/fstab 
  	/dev/mapper/mpath0-part2 /               ext4    errors=remount-ro 0       1
  	/dev/mapper/mpath0-part3 none            swap    sw              0       0

  
  And on the installed system, the swap is available/functional now:

  	$ grep mapper /etc/fstab
  	/dev/mapper/mpath0-part2 /               ext4    errors=remount-ro 0       1
  	/dev/mapper/mpath0-part3 none            swap    sw              0       0

  	$ mount | grep ' / '
  	/dev/mapper/mpath0-part2 on / type ext4 (rw,errors=remount-ro)

  	$ cat /proc/swaps 
  	Filename                                Type            Size    Used    Priority
  	/dev/mapper/mpath0-part3                partition       5492672 0       -1

  
  Steps to reproduce:
  ------------------

  You can use qemu/kvm if you don't have multipath hardware handy, just specify
  2 drives with the same serial number.  This is an example for ppc64el, easily
  changeable for amd64 (change qemu-system & -M)

  $ qemu-img create -f qcow2 vivid-multipath.qcow2 128g

  $ qemu-system-ppc64 -M pseries \
  	-enable-kvm \
  	-nographic -nodefaults \
  	-monitor stdio -serial pty \
  	-m 2G \
  	-net nic -net user \
  	-drive file=vivid-multipath.qcow2,serial=abcdef \
  	-drive file=vivid-multipath.qcow2,serial=abcdef \
  	-kernel netboot/vmlinux \
  	-initrd netboot/initrd.gz \
  	-append 'disk-detect/multipath/enable=true grub-installer/multipath=true'

  Note: to boot the installed system, remove the -kernel/-initrd/-append options.
        if the system takes long to boot in the initrd, use the 'debug' option
        in the kernel command line (current work-around for another problem).

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