Support for including nested mountpoints in overlay?

[Nicholas Hubbard <nicholashubbard@posteo.net>]

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Hello everybody,

I have just started working with overlayfs, and ran into a problem. Specifically
I wanted to overlay the root of my filesystem with a command like the following:

# mount -t overlay overlay -o lowerdir=/,upperdir=./tmp-upper,workdir=./tmp-work ./merged

However, I noticed that my nested btrfs subvolumes and boot partition were not
included in ./merged. I quickly learned though that you could have multiple
lowerdirs. So next I tried the following command (I have a nested btrfs subvolume
at /home, and my boot partition mounted at /boot):

# mount -t overlay overlay -o lowerdir=/:/home:/boot,upperdir=./tmp-upper,workdir=./tmp-work ./merged

I was expecting that now I would have (for example) the following directories:

./merged/home/$USER
./merged/boot/grub

However I instead had:

./merged/$USER
./merged/grub

Which shows that all the lowerdirs are placed right at the root of the mergedir.

So I have two questions.

1. Is there a (easy) way say "I want to include all nested mountpoints into the
   overlay in their same directories"?
2. If there is not a (easy) way to do this, do you think it would be both feasible
   and useful to add such a feature?

If the answer to question 2 is "yes", then I would be happy to take a try at this :)

-- 
Nicholas B. Hubbard
Keys: https://github.com/NicholasBHubbard/public-keys
Key ID: 508022AE06C2C446D8072447C700A066BB25F148

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