linux-kernel - Changes to underlying filesystems in overlayfs

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Date:	Sat, 07 Nov 2015 17:28:54 +0300
From:	Arseny Sher <sher-ars@...dex.ru>
To:	linux-kernel@...r.kernel.org
Subject: Changes to underlying filesystems in overlayfs

Hi all,

I have some questions regarding making changes to underlying filesystems in overlayfs. From the documentation:
"Changes to the underlying filesystems while part of a mounted overlay filesystem are not allowed.  If the underlying filesystem is changed, the behavior of the overlay is undefined, though it will not result in a crash or deadlock."

Could you tell me, is it safe to
  - freeze the merged mountpoint, remounting it as ro to prevent any external changes
  - directly make changes to lowerdir or upperdir
  - immediate reboot

I hope the rebooting will clean the cache and thus manipulating underlying filesystems in this way can be considered safe. Am I right? 

There is a project overlayroot developed for mounting / as an overlay filesystem:
http://blog.dustinkirkland.com/2012/08/introducing-overlayroot-overlayfs.html
It provides utility overlayroot-chroot which lets to modify lowerdir partition while overlayfs on / is mounted.  It does this by adding proc, run, sys mounts inside lowerdir, than remounting it as rw and finally chrootting into it. It is mainly used for modyfing overlayroot config, so the next action after exiting chroot is usually reboot. The utility even doesn't freezes / while doing this, as far as I know. Is this allowed?

P.S. I know that this mail list is for unionfs'es development, not for users. Unfortunately, I couldn't find more appropriate place for such questions. I'd be happy if someone would tell me the right place.

-- 
Arseny Sher
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