Is it possible to atomically change mount device in Linux?

Question

Is it possible to change what device is mounted to a mount point atomically, so at no moment of time mount point can be observed empty? Here is what I am trying to do:

$ mkdir /point
$ mkdir /device1 /device2
$ echo 1 > /device1/text
$ echo 2 > /device2/text
$ mount --bind /device1 /point
$ # do some kind of magic to get /device2 mounted at /point and /device1 unmounted

Technically, I can ignore previous mount and mount new device on top, but kernel will keep keep record of both now, so it feels like "mount leak".

Philippe · Answer 1 · 2021-10-11T10:56:31.607

0

I have run following successfully [ubuntu "20.04.3 LTS (Focal Fossa)"]:

/tmp# cat test.sh
#!/bin/bash

set -e
mkdir -p /tmp/{point,device1,device2}
echo 1 > /tmp/device1/text
echo 2 > /tmp/device2/text
mount --bind /tmp/device1 /tmp/point
cat /tmp/point/text
mount --bind /tmp/device2 /tmp/point
cat /tmp/point/text
umount /tmp/device1
echo "Finished successfully"

/tmp# bash test.sh
1
2
Finished successfully
/tmp#

edited Oct 11 '21 at 10:56

answered Oct 10 '21 at 15:16

Philippe

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`umount: /tmp/device1/: not mounted.`. /device1 is not mount point. – KAction Oct 10 '21 at 17:18
@KAction See my update – Philippe Oct 10 '21 at 18:10
```+; doas bash test.sh 1 2 umount: /tmp/device1: not mounted. Finished successfully +; doas bash test.sh +; which umount /bin/umount +; umount --version umount from util-linux 2.37.2 (libmount 2.37.2: btrfs, namespaces, assert, debug) – KAction Oct 11 '21 at 10:33
Since your script does not use "set -e", I can't be sure that unmounting is successful on your machine either. – KAction Oct 11 '21 at 10:34
@KAction Added `set -e`. Same success. – Philippe Oct 11 '21 at 10:57
Can you show `cat /tmp/point/text` after umount? – KAction Oct 11 '21 at 11:58

Is it possible to atomically change mount device in Linux?

1 Answers1