cryptsetup refuses to luksFormat?

[Josef Wolf]

Hello,

I want to encrypt the whole disk (including boot) on ubuntu-2204 as described on
https://discourse.maas.io/t/deploying-servers-with-full-disk-encryption-luks2/3286

So I enter following commands:

   # mount -o remount,ro /boot
   # install -m0600 /dev/null /tmp/boot.tar
   # tar -C /boot --acls --xattrs --one-file-system -cf /tmp/boot.tar .
   # umount /boot/efi
   # umount /boot
   # cryptsetup -y luksFormat --type luks1 /dev/nvme0n1p3
   WARNING: Device /dev/nvme0n1p3 already contains a 'ext4' superblock signature.
   
   WARNING!
   ========
   This will overwrite dataon /dev/nvme0n1p3 irrevocably.
    
   Are you sure? (Type 'yes' in capital letters); YES
   Device /dev/nvme0n1p3 is in use. Cannot proceed with format operation.

Uh, not what I expected. So i start to investigate:

   # grep /dev/nvme0n1p3 /proc/mounts
   # lsof | grep /dev/nvme0n1p3
   # lsof /dev/nvme0n1p3
   # fuser /dev/nvme0n1p3

So:
- umount succeeded
- /proc/mounts, lsof, fuser don's show any sign that the partition is in use

BUT:

    strace cryptsetup luksFormat --type luks1 /dev/nvme0n1p3 2>&1 | \
      egrep '(nvme0n1p3|close)'

shows

    openat(AT_FDCWD, "/dev/nvme0n1p3", O_RDONLY|ODIRECT) = 3
    close(3)
    openat(AT_FDCWD, "/dev/nvme0n1p3", O_RDONLY|O_NONBLOCK|OCLOEXEC) = 3
    close(3)
    openat(AT_FDCWD, "/dev/nvme0n1p3", O_RDWR|OEXCL) = -1 EBUSY (Device or resource busy)

So there must be something holdig a hand on the partition.

Any idea what that might be?

-- 
Josef Wolf
jw at raven.inka.de