bug#34143: [coreutils 8.28] du -x is reporting a lower disk usage for /mnt when partitions are mounted

[Joseph Paul]

   Hello,

   It may not be a bug at all, but I was surprised to find out that 'du
   -x' is reporting a lower disk usage on /mnt when partitions are
   mounted.

   Here is an example with '/mnt' belonging to the same filesystem as '/'
   :

   linux$ LANG=EN ls -l /mnt
   total 12
   drwxr-xr-x 2 root root 4096 Jul  8  2018 VL1800
   drwxr-xr-x 2 root root 4096 Jul  8  2018 data
   drwxr-xr-x 3 root root 4096 Aug 19 13:24 nfs

   # I mount two partitions from other disks on /mnt

   linux$ sudo mount /mnt/VL1800
   linux$ sudo mount /mnt/data

   linux$ du -x /mnt
   4    /mnt/nfs/nas
   8    /mnt/nfs
   12    /mnt

   # I unmount the two partitions

   linux$ sudo umount /mnt/VL1800
   linux$ sudo umount /mnt/data

   linux$ du -x /mnt
   4    /mnt/data
   4    /mnt/VL1800
   4    /mnt/nfs/nas
   8    /mnt/nfs
   20    /mnt

   /mnt is now bigger.

   Is this a normal result, because even when mounted, physically, the
   directories '/mnt/VL1800' and '/mnt/data' still  exist on the '/'
   filesystem, or not ?
   Shouldn't they still occupy 4Kb of disk space each on the '/'
   filesystem when partitions are mounted ?

   Best regards,
   Joseph Paul