title: loop device setup (losetup, mount, etc) fails in a container immediately after host reboot
body: <p>I'm trying to populate a disk image in a container environment (podman) on Centos 8.  I had originally run into issues with accessing the loop device from the container until finding on SO and other sources that I needed to run podman as root and with the <code>--privileged</code> option.</p>
  <p>While this did solve my problem in general, I noticed that after rebooting my host, my first attempt to setup a loop device in the container would fail (<code>failed to set up loop device: No such file or directory</code>), but after exiting and relaunching the container it would succeed (/dev/loop0).  If for some reason I needed to set up a second loop device (/dev/loop1) in the container (after having gotten a first one working), it too would fail until I exited and relaunched the container.</p>
  <p>Experimenting a bit further, I found I could avoid the errors entirely if I ran <code>losetup --find --show &lt;file created with dd&gt;</code> enough times to attach the maximum number of loop devices I would need, then detached all of those with <code>losetup -D</code>, I could avoid the loop device errors in the container entirely.</p>
  <p>I suspect I'm missing something obvious about what losetup does on the host which it is apparently not able to do entirely within a container, or maybe this is more specifically a Centos+podman+losetup issue.  Any insight as to what is going on and why I have to preattach/detach the loop devices after a reboot to avoid problems inside my container?</p>
  <p>Steps to reproduce on a Centos 8 system (after having attached/detached once following a reboot):</p>
  <pre><code>$ dd if=/dev/zero of=file bs=1024k count=10
  10+0 records in
  10+0 records out
  10485760 bytes (10 MB, 10 MiB) copied, 0.00826706 s, 1.3 GB/s
  $ cp file 1.img
  $ cp file 2.img
  $ cp file 3.img
  $ cp file 4.img
  $ sudo podman run -it --privileged --rm -v .:/images centos:8 bash
  
  [root@2da5317bde3e /]# cd images
  [root@2da5317bde3e images]# ls
  1.img  2.img  3.img  4.img  file
  [root@2da5317bde3e images]# losetup --find --show 1.img
  /dev/loop0
  [root@2da5317bde3e images]# losetup --find --show 2.img
  losetup: 2.img: failed to set up loop device: No such file or directory
  [root@2da5317bde3e images]# losetup -D
  [root@2da5317bde3e images]# exit
  
  exit
  $ sudo podman run -it --privileged --rm -v .:/images centos:8 bash
  
  [root@f9e41a21aea4 /]# cd images
  [root@f9e41a21aea4 images]# losetup --find --show 1.img
  /dev/loop0
  [root@f9e41a21aea4 images]# losetup --find --show 2.img
  /dev/loop1
  [root@f9e41a21aea4 images]# losetup --find --show 3.img
  losetup: 3.img: failed to set up loop device: No such file or directory
  [root@f9e41a21aea4 /]# losetup -D
  [root@f9e41a21aea4 images]# exit
  
  exit
  $ sudo podman run -it --privileged --rm -v .:/images centos:8 bash
  
  [root@c93cb71b838a /]# cd images
  [root@c93cb71b838a images]# losetup --find --show 1.img
  /dev/loop0
  [root@c93cb71b838a images]# losetup --find --show 2.img
  /dev/loop1
  [root@c93cb71b838a images]# losetup --find --show 3.img
  /dev/loop2
  [root@c93cb71b838a images]# losetup --find --show 4.img
  losetup: 4.img: failed to set up loop device: No such file or directory
  </code></pre>