How to resize an encrypted SWAP partition (LVM)

Just because I am afraid this page might some day get deleted, I am copy pasting here the answer, which was very useful to me, when I needed to increase the size of the SWAP partition of my freshly installed Xubuntu 22.04, after I selected the "encrypted LVM partition" option in the install wizard. I am adding some commands and missing bits of information too.

Before increasing the size, I needed to decrease the size of its neighboring root volume, as the sum of both LVM volumes was equal to the capacity of my disk. Here I add another 7G to my SWAP partition.

First, check what your LVM disk currently looks like, running lsblk:

lsblk
# └─sda6                  8:6    0 464,6G  0 part
#   └─sda6_crypt        253:0    0 464,5G  0 crypt
#     ├─vgubuntu-root   253:1    0 463,6G  0 lvm   /
#     └─vgubuntu-swap_1 253:2    0   980M  0 lvm   [SWAP]

For me, sda6 was nvme0n1p3.

Now, boot your Linux from a USB stick and open a terminal:

sudo su
# `sudo`    => Execute a command as another user.
# `sudo su [user]` => Run a command with substitute user, default is root.

# Encrypted device should NOT be unlocked
lsblk # => list block devices
    # └─sda6 => no `crypt`/`lvm``

# Unlock encrypted device
cryptsetup open /dev/sda6 crypt # Enter passphrase
    # `cryptsetup` => Manage dm-crypt + LUKS encrypted volumes.
    # `cryptsetup open <device> <name>` => Opens encrypted lv as <name>

# Get logical volume identifiers
lsblk
    # └─sda6                  8:6    0 464,6G  0 part
    #   └─sda6_crypt        253:0    0 464,5G  0 crypt
    #     ├─vgubuntu-root   253:1    0 463,6G  0 lvm   /
    #     └─vgubuntu-swap_1 253:2    0   980M  0 lvm   [SWAP]

# Shrink logical root volume AND filesystem
lvresize --verbose --resizefs -L -7G /dev/mapper/vgubuntu-root
    # `lvresize` <volume> => resize a logical volume
    #   --verbose  => Give more info.
    #   --resizefs => Resize filesystem AND LV with fsadm(8).
    #   -L         => Specifies the new size of the LV,
    #                 +/- add/subtracts to/from current size, g|G is GiB.

# Check filesystem of logical root volume for errors
e2fsck -f /dev/mapper/vgubuntu-root
    # `e2fsck`<fs-path> => Check a Linux ext2/ext3/ext4 file system
    #   -f => Force checking even if the file system seems clean.

# Increase swapsize
lvresize --verbose -L +7G /dev/mapper/vgubuntu-swap_1
# Format to make it usable
mkswap /dev/mapper/vgubuntu-swap_1

Now, deactive the volume group: vgchange -a n or vgchange -a n sda6_crypt.

And finally: cryptsetup close crypt; reboot

After rebooting, check the sizes using lsblk, swapon -s and free -h. That's it!