The gnome-system-monitor package preinstalled with Ubuntu 18.04 appears to have a problem with excluding /dev/loop* mounts from the File System tab.
Attached is a screenshot of what it shows on my system.
As you can see, there's a lot of /dev/loop* entries, and a lot of duplication of available disk space percentage.
This is a pastebin of $ mount:
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
udev on /dev type devtmpfs (rw,nosuid,relatime,size=8101140k,nr_inodes=2025285,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=1627500k,mode=755)
/dev/nvme0n1p3 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
mqueue on /dev/mqueue type mqueue (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=45,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=22759)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
sunrpc on /run/rpc_pipefs type rpc_pipefs (rw,relatime)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
configfs on /sys/kernel/config type configfs (rw,relatime)
nfsd on /proc/fs/nfsd type nfsd (rw,relatime)
/var/lib/snapd/snaps/gnome-3-26-1604_74.snap on /snap/gnome-3-26-1604/74 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/telegram-desktop_384.snap on /snap/telegram-desktop/384 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-3-26-1604_70.snap on /snap/gnome-3-26-1604/70 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-calculator_180.snap on /snap/gnome-calculator/180 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/core_6034.snap on /snap/core/6034 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-calculator_238.snap on /snap/gnome-calculator/238 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/skype_63.snap on /snap/skype/63 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-characters_103.snap on /snap/gnome-characters/103 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/core_5897.snap on /snap/core/5897 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-characters_139.snap on /snap/gnome-characters/139 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-system-monitor_57.snap on /snap/gnome-system-monitor/57 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-system-monitor_51.snap on /snap/gnome-system-monitor/51 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/telegram-desktop_291.snap on /snap/telegram-desktop/291 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/skype_66.snap on /snap/skype/66 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-logs_37.snap on /snap/gnome-logs/37 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gtk-common-themes_701.snap on /snap/gtk-common-themes/701 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-calculator_260.snap on /snap/gnome-calculator/260 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gtk-common-themes_818.snap on /snap/gtk-common-themes/818 type squashfs (ro,nodev,relatime,x-gdu.hide)
/dev/nvme0n1p4 on /home type ext4 (rw,relatime,data=ordered)
/dev/sda2 on /boot/efi type vfat (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
/var/lib/snapd/snaps/gnome-characters_124.snap on /snap/gnome-characters/124 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gnome-logs_45.snap on /snap/gnome-logs/45 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/telegram-desktop_399.snap on /snap/telegram-desktop/399 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/gtk-common-themes_808.snap on /snap/gtk-common-themes/808 type squashfs (ro,nodev,relatime,x-gdu.hide)
/var/lib/snapd/snaps/core_5742.snap on /snap/core/5742 type squashfs (ro,nodev,relatime,x-gdu.hide)
tmpfs on /run/user/121 type tmpfs (rw,nosuid,nodev,relatime,size=1627496k,mode=700,uid=121,gid=125)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=1627496k,mode=700,uid=1000,gid=1000)
gvfsd-fuse on /run/user/1000/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=1000,group_id=1000)
/dev/fuse on /run/user/1000/doc type fuse (rw,nosuid,nodev,relatime,user_id=1000,group_id=1000)
tmpfs on /run/snapd/ns type tmpfs (rw,nosuid,noexec,relatime,size=1627500k,mode=755)
nsfs on /run/snapd/ns/gnome-system-monitor.mnt type nsfs (rw)
And this is a pastebin of my current `snap` version and `$ cat /proc/self/mountinfo`:
$ snap --version
snap 2.36.2
snapd 2.36.2
series 16
ubuntu 18.04
kernel 4.15.0-42-generic
$ cat /proc/self/mountinfo
22 29 0:21 / /sys rw,nosuid,nodev,noexec,relatime shared:7 - sysfs sysfs rw
23 29 0:4 / /proc rw,nosuid,nodev,noexec,relatime shared:14 - proc proc rw
24 29 0:6 / /dev rw,nosuid,relatime shared:2 - devtmpfs udev rw,size=8101140k,nr_inodes=2025285,mode=755
25 24 0:22 / /dev/pts rw,nosuid,noexec,relatime shared:3 - devpts devpts rw,gid=5,mode=620,ptmxmode=000
26 29 0:23 / /run rw,nosuid,noexec,relatime shared:5 - tmpfs tmpfs rw,size=1627500k,mode=755
29 0 259:3 / / rw,relatime shared:1 - ext4 /dev/nvme0n1p3 rw,errors=remount-ro,data=ordered
30 22 0:7 / /sys/kernel/security rw,nosuid,nodev,noexec,relatime shared:8 - securityfs securityfs rw
31 24 0:26 / /dev/shm rw,nosuid,nodev shared:4 - tmpfs tmpfs rw
32 26 0:27 / /run/lock rw,nosuid,nodev,noexec,relatime shared:6 - tmpfs tmpfs rw,size=5120k
33 22 0:28 / /sys/fs/cgroup ro,nosuid,nodev,noexec shared:9 - tmpfs tmpfs ro,mode=755
34 33 0:29 / /sys/fs/cgroup/unified rw,nosuid,nodev,noexec,relatime shared:10 - cgroup2 cgroup rw,nsdelegate
35 33 0:30 / /sys/fs/cgroup/systemd rw,nosuid,nodev,noexec,relatime shared:11 - cgroup cgroup rw,xattr,name=systemd
36 22 0:31 / /sys/fs/pstore rw,nosuid,nodev,noexec,relatime shared:12 - pstore pstore rw
37 22 0:32 / /sys/firmware/efi/efivars rw,nosuid,nodev,noexec,relatime shared:13 - efivarfs efivarfs rw
38 33 0:33 / /sys/fs/cgroup/hugetlb rw,nosuid,nodev,noexec,relatime shared:15 - cgroup cgroup rw,hugetlb
39 33 0:34 / /sys/fs/cgroup/pids rw,nosuid,nodev,noexec,relatime shared:16 - cgroup cgroup rw,pids
40 33 0:35 / /sys/fs/cgroup/net_cls,net_prio rw,nosuid,nodev,noexec,relatime shared:17 - cgroup cgroup rw,net_cls,net_prio
41 33 0:36 / /sys/fs/cgroup/cpu,cpuacct rw,nosuid,nodev,noexec,relatime shared:18 - cgroup cgroup rw,cpu,cpuacct
42 33 0:37 / /sys/fs/cgroup/rdma rw,nosuid,nodev,noexec,relatime shared:19 - cgroup cgroup rw,rdma
43 33 0:38 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime shared:20 - cgroup cgroup rw,freezer
44 33 0:39 / /sys/fs/cgroup/perf_event rw,nosuid,nodev,noexec,relatime shared:21 - cgroup cgroup rw,perf_event
45 33 0:40 / /sys/fs/cgroup/cpuset rw,nosuid,nodev,noexec,relatime shared:22 - cgroup cgroup rw,cpuset
46 33 0:41 / /sys/fs/cgroup/devices rw,nosuid,nodev,noexec,relatime shared:23 - cgroup cgroup rw,devices
47 33 0:42 / /sys/fs/cgroup/memory rw,nosuid,nodev,noexec,relatime shared:24 - cgroup cgroup rw,memory
48 33 0:43 / /sys/fs/cgroup/blkio rw,nosuid,nodev,noexec,relatime shared:25 - cgroup cgroup rw,blkio
49 24 0:19 / /dev/mqueue rw,relatime shared:26 - mqueue mqueue rw
50 22 0:8 / /sys/kernel/debug rw,relatime shared:27 - debugfs debugfs rw
51 23 0:44 / /proc/sys/fs/binfmt_misc rw,relatime shared:28 - autofs systemd-1 rw,fd=45,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=22759
52 24 0:45 / /dev/hugepages rw,relatime shared:29 - hugetlbfs hugetlbfs rw,pagesize=2M
53 26 0:46 / /run/rpc_pipefs rw,relatime shared:30 - rpc_pipefs sunrpc rw
54 22 0:47 / /sys/fs/fuse/connections rw,relatime shared:31 - fusectl fusectl rw
55 22 0:20 / /sys/kernel/config rw,relatime shared:32 - configfs configfs rw
89 23 0:48 / /proc/fs/nfsd rw,relatime shared:33 - nfsd nfsd rw
91 29 7:0 / /snap/gnome-3-26-1604/74 ro,nodev,relatime shared:34 - squashfs /dev/loop0 ro
163 29 7:1 / /snap/telegram-desktop/384 ro,nodev,relatime shared:35 - squashfs /dev/loop1 ro
165 29 7:2 / /snap/gnome-3-26-1604/70 ro,nodev,relatime shared:36 - squashfs /dev/loop2 ro
167 29 7:4 / /snap/gnome-calculator/180 ro,nodev,relatime shared:37 - squashfs /dev/loop4 ro
93 29 7:3 / /snap/core/6034 ro,nodev,relatime shared:38 - squashfs /dev/loop3 ro
255 29 7:6 / /snap/gnome-calculator/238 ro,nodev,relatime shared:39 - squashfs /dev/loop6 ro
456 29 7:5 / /snap/skype/63 ro,nodev,relatime shared:40 - squashfs /dev/loop5 ro
458 29 7:8 / /snap/gnome-characters/103 ro,nodev,relatime shared:41 - squashfs /dev/loop8 ro
94 29 7:7 / /snap/core/5897 ro,nodev,relatime shared:42 - squashfs /dev/loop7 ro
96 29 7:9 / /snap/gnome-characters/139 ro,nodev,relatime shared:43 - squashfs /dev/loop9 ro
98 29 7:11 / /snap/gnome-system-monitor/57 ro,nodev,relatime shared:44 - squashfs /dev/loop11 ro
100 29 7:10 / /snap/gnome-system-monitor/51 ro,nodev,relatime shared:45 - squashfs /dev/loop10 ro
102 29 7:12 / /snap/telegram-desktop/291 ro,nodev,relatime shared:46 - squashfs /dev/loop12 ro
105 29 7:13 / /snap/skype/66 ro,nodev,relatime shared:47 - squashfs /dev/loop13 ro
104 29 7:14 / /snap/gnome-logs/37 ro,nodev,relatime shared:48 - squashfs /dev/loop14 ro
108 29 7:15 / /snap/gtk-common-themes/701 ro,nodev,relatime shared:49 - squashfs /dev/loop15 ro
110 29 7:17 / /snap/gnome-calculator/260 ro,nodev,relatime shared:50 - squashfs /dev/loop17 ro
112 29 7:16 / /snap/gtk-common-themes/818 ro,nodev,relatime shared:51 - squashfs /dev/loop16 ro
114 29 259:4 / /home rw,relatime shared:52 - ext4 /dev/nvme0n1p4 rw,data=ordered
115 29 8:2 / /boot/efi rw,relatime shared:53 - vfat /dev/sda2 rw,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro
118 29 7:18 / /snap/gnome-characters/124 ro,nodev,relatime shared:54 - squashfs /dev/loop18 ro
357 29 7:19 / /snap/gnome-logs/45 ro,nodev,relatime shared:55 - squashfs /dev/loop19 ro
363 29 7:20 / /snap/telegram-desktop/399 ro,nodev,relatime shared:56 - squashfs /dev/loop20 ro
184 29 7:21 / /snap/gtk-common-themes/808 ro,nodev,relatime shared:57 - squashfs /dev/loop21 ro
185 29 7:22 / /snap/core/5742 ro,nodev,relatime shared:58 - squashfs /dev/loop22 ro
683 26 0:53 / /run/user/121 rw,nosuid,nodev,relatime shared:479 - tmpfs tmpfs rw,size=1627496k,mode=700,uid=121,gid=125
541 26 0:52 / /run/user/1000 rw,nosuid,nodev,relatime shared:277 - tmpfs tmpfs rw,size=1627496k,mode=700,uid=1000,gid=1000
555 541 0:54 / /run/user/1000/gvfs rw,nosuid,nodev,relatime shared:287 - fuse.gvfsd-fuse gvfsd-fuse rw,user_id=1000,group_id=1000
783 541 0:56 / /run/user/1000/doc rw,nosuid,nodev,relatime shared:566 - fuse /dev/fuse rw,user_id=1000,group_id=1000
801 26 0:23 /snapd/ns /run/snapd/ns rw,nosuid,noexec,relatime - tmpfs tmpfs rw,size=1627500k,mode=755
1216 801 0:3 mnt:[4026532554] /run/snapd/ns/gnome-system-monitor.mnt rw - nsfs nsfs rw
How to reproduce
If your version of gnome-system-monitor is the default installed with Ubuntu 18.04, you should already have this problem. If not, try removing the current version sudo apt remove gnome-system-monitor, and installing it from the snap store sudo snap install gnome-system-monitor to have the problem show up.
It could be helpful to look into $ cat /run/mount/utab (I'm mentioning this only because it was suggested to me to mention it). Unfortunately I didn't do that before removing the snap package, so I don't have the output available.
Expected behaviour
I would expect to only see the actual partitions in my system, in my case: /, /home, and /boot/efi, since I have /home`on its own partition.
Workaround
A workaround to get rid of this problem is to remove the snap package and install it via apt sudo snap remove gnome-system-monitor && sudo apt install gnome-system-monitor. It worked for me, and after that I only had the three partitions mentioned above showing up in the File System tab.
Plus, I do have the output for $ cat /run/mount/utab this time:
$ cat /run/mount/utab
SRC=/dev/loop0 TARGET=/snap/gnome-3-26-1604/74 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop1 TARGET=/snap/telegram-desktop/384 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop2 TARGET=/snap/gnome-3-26-1604/70 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop4 TARGET=/snap/gnome-calculator/180 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop3 TARGET=/snap/core/6034 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop6 TARGET=/snap/gnome-calculator/238 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop5 TARGET=/snap/skype/63 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop8 TARGET=/snap/gnome-characters/103 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop7 TARGET=/snap/core/5897 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop9 TARGET=/snap/gnome-characters/139 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop12 TARGET=/snap/telegram-desktop/291 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop13 TARGET=/snap/skype/66 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop14 TARGET=/snap/gnome-logs/37 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop15 TARGET=/snap/gtk-common-themes/701 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop17 TARGET=/snap/gnome-calculator/260 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop16 TARGET=/snap/gtk-common-themes/818 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop18 TARGET=/snap/gnome-characters/124 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop19 TARGET=/snap/gnome-logs/45 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop20 TARGET=/snap/telegram-desktop/399 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop21 TARGET=/snap/gtk-common-themes/808 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/loop22 TARGET=/snap/core/5742 ROOT=/ OPTS=x-gdu.hide
SRC=/dev/nvme0n1p1 TARGET=/media/username/CA85-F422 ROOT=/ OPTS=uhelper=udisks2
SRC=/dev/sda4 TARGET=/media/username/26C601D7C601A85F ROOT=/ OPTS=uhelper=udisks2
Status changed to 'Confirmed' because the bug affects multiple users.