在cloudcone给ubuntu升级内核，完成后重启进入不了系统，在vnc看到 kernel panic：
unable to mount root fs on unknown-block

通过救援模式，死马当活马医我还拉回来了并且切换到了最新的内核，核心命令就是进入救援 rescue 模式后，其实rescue是另外一个linux系统，跟原系统不一样。
fdisk -l 查看原来系统盘的设备，如/dev/vdb1

挂载并chroot，这样就相当于使用了原系统的环境。
$ sudo mount /dev/vdb1 /mnt
$ sudo mount --bind /dev /mnt/dev
$ sudo mount --bind /dev/pts /mnt/dev/pts
$ sudo mount --bind /proc /mnt/proc
$ sudo mount --bind /sys /mnt/sys
$ sudo chroot /mnt

然后执行 update-initramfs -u -k 5.15.0-91-generic
5.15.0-91-generic 为/boot下的看到的内核文件之一，是我新升级的内核。

有警告时忽略。
最后执行： update-grub
参考：
https://www.geekswarrior.com/2019/07/solved-how-to-fix-kernel-panic-on-linux.html、
但是我后来发现，可能在重新前执行一下
dpkg --configure -a
就可以了，因为这条命令自动更新了很多启动相关的信息。
See system logs and 'systemctl status snapd.mounts-pre.target' for details.
Could not execute systemctl: at /usr/bin/deb-systemd-invoke line 142.
Setting up ubuntu-server-minimal (1.481.1) ...
Setting up linux-image-5.15.0-91-generic (5.15.0-91.101) ...
I: /boot/vmlinuz.old is now a symlink to vmlinuz-5.15.0-46-generic
I: /boot/initrd.img.old is now a symlink to initrd.img-5.15.0-46-generic
I: /boot/vmlinuz is now a symlink to vmlinuz-5.15.0-91-generic
I: /boot/initrd.img is now a symlink to initrd.img-5.15.0-91-generic
Setting up linux-modules-extra-5.15.0-91-generic (5.15.0-91.101) ...
Setting up linux-image-generic (5.15.0.91.88) ...
Setting up linux-modules-5.15.0-91-generic (5.15.0-91.101) ...
Setting up linux-generic (5.15.0.91.88) ...
Processing triggers for libc-bin (2.35-0ubuntu3.5) ...
Processing triggers for rsyslog (8.2112.0-2ubuntu2.2) ...
Processing triggers for man-db (2.10.2-1) ...
Processing triggers for plymouth-theme-ubuntu-text (0.9.5+git20211018-1ubuntu3) ...
update-initramfs: deferring update (trigger activated)
Processing triggers for install-info (6.8-4build1) ...
Processing triggers for initramfs-tools (0.140ubuntu13.4) ...
update-initramfs: Generating /boot/initrd.img-5.15.0-91-generic
I: The initramfs will attempt to resume from /dev/vda2
I: (UUID=2eafbaba-93ab-4336-8fcc-ba5e0a86aec4)
I: Set the RESUME variable to override this.
Processing triggers for linux-image-5.15.0-91-generic (5.15.0-91.101) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.15.0-91-generic
I: The initramfs will attempt to resume from /dev/vda2
I: (UUID=2eafbaba-93ab-4336-8fcc-ba5e0a86aec4)
I: Set the RESUME variable to override this.
/etc/kernel/postinst.d/zz-update-grub:
Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.15.0-91-generic
Found initrd image: /boot/initrd.img-5.15.0-91-generic
Found linux image: /boot/vmlinuz-5.15.0-46-generic
Found initrd image: /boot/initrd.img-5.15.0-46-generic
Found linux image: /boot/vmlinuz-5.15.0-43-generic
Found initrd image: /boot/initrd.img-5.15.0-43-generic
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
done