main/postmarketos-mkinitfs: not able to resize root partition backed by device mapper

Right now i am unsure if this affects any kinds of device mapper partitions or not. The issue described here is affecting SHIFT6mq (axolotl), which has support enabled for dynamic partitions.

Click me to show some more background information

axolotl uses dynamic partitions, which means it has a phyiscal super partition, which contains multiple logical partitions.

This is a lpdump with Android installed on Slot A and postmarketOS installed on Slot B.

axolotl:/ # lpdump                                                                                                                                                                                                                                                                                                           
Slot 0:
Metadata version: 10.0
Metadata size: 716 bytes
Metadata max size: 65536 bytes
Metadata slot count: 3
Header flags: none
Partition table:
------------------------
  Name: product_a
  Group: axolotl_dynamic_partitions_a
  Attributes: readonly
  Extents:
    0 .. 4225695 linear super 3072
------------------------
  Name: system_a
  Group: axolotl_dynamic_partitions_a
  Attributes: readonly
  Extents:
    0 .. 1852623 linear super 5288448
------------------------
  Name: system_b
  Group: axolotl_dynamic_partitions_b
  Attributes: readonly
  Extents:
    0 .. 4227071 linear super 12584448
------------------------
  Name: system_ext_a
  Group: axolotl_dynamic_partitions_a
  Attributes: readonly
  Extents:
    0 .. 1056959 linear super 4230144
------------------------
  Name: vendor_a
  Group: axolotl_dynamic_partitions_a
  Attributes: readonly
  Extents:
    0 .. 991999 linear super 7142400
------------------------
Super partition layout:
------------------------
super: 3072 .. 4228768: product_a (4225696 sectors)
super: 4230144 .. 5287104: system_ext_a (1056960 sectors)
super: 5288448 .. 7141072: system_a (1852624 sectors)
super: 7142400 .. 8134400: vendor_a (992000 sectors)
super: 12584448 .. 16811520: system_b (4227072 sectors)
------------------------
Block device table:
------------------------
  Partition name: super
  First sector: 2048
  Size: 12884901888 bytes
  Flags: none
------------------------
Group table:
------------------------
  Name: default
  Maximum size: 0 bytes
  Flags: none
------------------------
  Name: axolotl_dynamic_partitions_a
  Maximum size: 6438256640 bytes
  Flags: none
------------------------
  Name: axolotl_dynamic_partitions_b
  Maximum size: 6438256640 bytes
  Flags: none
------------------------

It is possible to resize the logical partitions, remove and create partitions using fastbootd

For example axolotl's super partition has a total size of 12884901888. To account for overhead and error correction, the total usable size is 12876513280.

As it is also an A/B device, it contains two groups, each with the size of 6438256640 (12876513280 / 2 -> 6438256640).

postmarketOS can be installed in a logical partition since dynamic partition support has landed.

However a lot of usable space will be lost without resizing. You can delete all partitions and resize / create a new one to be used by postmarketOS.

For example to delete all logical partitions (on slot B) and resize the system partition to the max usable space:

• fastboot delete-logical-partition product_b
• fastboot delete-logical-partition system_ext_b
• fastboot delete-logical-partition vendor_b
• fastboot resize-logical-partition system_b 6438256640

This is also the configuration used with this issue.

When resizing the root partition, init looks up all devices in /dev/mappper. Please refer to the code at the time of writing this issue.

When trying to get the physical partition of the mapped device, it assumes that the device exists in /dev whereas for this case with axolotl it is actually located in /dev/mapper

/ # find_root_partition
/dev/mapper/system_b2

/ # resize_root_partition
Error: Could not stat device /dev/system_b - No such file or directory.

/ # find /dev -name "*system*"
/dev/mapper/system_b2
/dev/mapper/system_b1
/dev/mapper/system_ext_a
/dev/mapper/system_b
/dev/mapper/system_a

/ # parted -s /dev/mapper/system_b print free
Model: Linux device-mapper (linear) (dm)
Disk /dev/mapper/system_b: 6438MB
Sector size (logical/physical): 4096B/4096B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system  Flags
        36.9kB  8389kB  8352kB           Free Space
 1      8389kB  256MB   247MB   primary  ext2         boot
 2      256MB   2073MB  1817MB  primary  ext4
        2073MB  6438MB  4365MB           Free Space

Bonus: manual steps to resize successfully

pmbootstrap initfs hook_add debug-shell
pmbootstrap flasher flash_kernel

telnet 172.16.42.1

source init_functions.sh

mount_boot_partition /boot
extract_initramfs_extra /boot/initramfs-extra

wait_root_partition
delete_old_install_partition

kpartx -d /dev/mapper/system_b2
parted /dev/mapper/system_b resizepart 2 100%
kpartx -afs /dev/mapper/system_b

parted /dev/mapper/system_b print free

e2fsck -f /dev/mapper/system_b2
resize2fs -f /dev/mapper/system_b2