At this point I don't know enough to reproduce this.
What does /proc/mounts look like before you start dosemu?
bash-4.2$ cat /proc/mounts
rootfs / rootfs rw 0 0
10.10.0.1:/nfs/root/psion_14.1 / nfs
rw,relatime,vers=3,rsize=4096,wsize=4096,namlen=255,hard,nolock,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountproto=tcp,local_lock=all,addr=10.10.0.1
0 0
devtmpfs /dev devtmpfs rw,relatime,size=1031016k,nr_inodes=220978,mode=755 0 0
proc /proc proc rw,relatime 0 0
sysfs /sys sysfs rw,relatime 0 0
tmpfs /run tmpfs rw,relatime,mode=755 0 0
devpts /dev/pts devpts rw,relatime,gid=5,mode=620 0 0
cgroup /sys/fs/cgroup cgroup rw,relatime,cpu 0 0
/dev/shm /tmp tmpfs rw,relatime,size=524288k 0 0
/dev/shm /dev/shm tmpfs rw,relatime,size=524288k 0 0
nfs:/nfs/usr/slackware-14.1/usr /usr nfs
ro,relatime,vers=3,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountport=38337,mountproto=udp,local_lock=none,addr=10.10.0.1
0 0
nfs:/nfs/home /home nfs
rw,relatime,vers=3,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountport=38337,mountproto=udp,local_lock=none,addr=10.10.0.1
0 0
nfs:/nfs/mp3 /mp3 nfs
rw,relatime,vers=3,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountport=38337,mountproto=udp,local_lock=none,addr=10.10.0.1
0 0
nfs:/nfs/src /usr/src nfs
rw,relatime,vers=3,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountport=38337,mountproto=udp,local_lock=none,addr=10.10.0.1
0 0
nfs:/nfs/video /video nfs
rw,relatime,vers=3,rsize=262144,wsize=262144,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.10.0.1,mountvers=3,mountport=38337,mountproto=udp,local_lock=none,addr=10.10.0.1
0 0
I now know why I do not see any file in /usr after running dosemu. The whole
/usr mount disappears in /proc/mounts. When I remount it I have a usable laptop
again. Running dosemu a second time does not remove the mount again. In the mean
time I have seen /usr disappear after running other programs like xterm and
firefox. But until now never after remouting it.