Scenario 1 - lovely diagnostic
$ df -h /dev/sda1
df: cannot access '/dev/sda1': over-mounted by another device
$ mount | grep boot/efi
/dev/sda6 on /boot type ext4 (rw,relatime,seclabel,data=ordered)
/dev/sda1 on /boot/efi type vfat
(rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=winnt,errors=remount-ro)
none on /boot/efi type tmpfs (rw,relatime,seclabel)
Scenario 2 - breaks down
$ df -h |grep /dev/sda1
$ df -ah |grep /dev/sda1
/dev/sda1 - - - - /boot/efi
above is consistent (same output as in scenario 1), but below is bad:
$ df -h /dev/sda1 # or same output from `/boot/efi`
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 3.9G 0 3.9G 0% /boot/efi
Case 2 situation is as follows:
$ mount | grep boot
/dev/sda6 on /boot type ext4 (rw,relatime,seclabel,data=ordered)
/dev/sda1 on /boot/efi type vfat
(rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=ascii,shortname=winnt,errors=remount-ro)
none on /boot type tmpfs (rw,relatime,seclabel)
i.e. the tmpfs on /boot is the most recent mount, masking the /boot/efi
mount (and I created a dummy `/boot/efi` directory in the tmpfs).
The practical use-case is that people get very confused when their
distribution sets up pam_ecryptfs. If `df` was more reliable about
detecting overmounts, then it would provide better diagnostics for when
pam_ecryptfs overmounts a filesystem which was mounted by fstab.
https://unix.stackexchange.com/questions/370043/mounting-a-subdirectory-of-home-onto-an-own-partition-confuses-disk-usage-report/395630?noredirect=1#comment706009_395630
