Failure in Merging Directories Located Under the Same OverlayFS Mountpoint

[gliargovas]

When attempting to merge two directories residing under the same overlayfs mountpoint within try, the operation fails, rendering the common directory unmountable within try.

Error Reproduction:

$ ./try -D sandbox1 "echo foo > file.txt"
...
$ ./try -L sandbox1 "cat file.txt"
...
try(/tmp/tmp.VwOWiakeQW): Warning: Failed mounting /users as an overlay and mergerfs or unionfs not set and could not be found, see "/tmp/tmp.B3jpnj64oq"
/tmp/tmp.LyCvWjVjEY: 7: cd: can't cd to /users/gliargko/try

Error log displays:

mount: /tmp/tmp.NSMc55yqg2/temproot/users: mount(2) system call failed: Too many levels of symbolic links.

dmesg output:

[ 2107.018994] overlayfs: fs on 'sandbox1/upperdir/users' does not support file handles, falling back to xino=off.
[ 2107.019006] overlayfs: overlapping lowerdir path

Installing mergerfs solves the issue.

• The initial mount failure is due to overlayfs's inability to handle overlapping directories. What causes these overlaps?
• It seems like mergerfs resolves this as it effectively manages overlapping paths, allowing the directories to merge seamlessly. Why isn't overlayfs enough?

[ezrizhu]

Linking in https://github.com/torvalds/linux/blob/401644852d0b2a278811de38081be23f74b5bb04/fs/overlayfs/super.c#L1226