Speaking about ubuntu 22.04 with the hwe kernel and "overlayroot" package
installed.
Say I've got such fstab:
/dev/sda3 / ext4 errors=remount-ro 0 1
/dev/sdb1 /nas xfs defaults 0 2
When booting I append "overlayroot=tmpfs" to the kernel cmdline in grub. It
gives me such overlay
mounts:
overlayroot / overlay
rw,relatime,lowerdir=/media/root-ro,upperdir=/media/root-rw/overlay,workdir=/media/root-rw/overlay-workdir/_,nouserxattr
0 0
/media/root-ro/nas /nas overlay
rw,relatime,lowerdir=/media/root-ro/nas,upperdir=/media/root-rw/overlay/nas,workdir=/media/root-rw/overlay-workdir/nas,nouserxattr
0 0
With that I get a warning in the kernel log:
kernel: overlayfs: upperdir is in-use as upperdir/workdir of another mount,
accessing files
from both mounts will result in undefined behavior.
The warning is caused by the fact the second overlay's upperdir
(/media/root-rw/overlay/nas) is a
subdir of the first overlay's upperdir /media/root-rw/overlay).
Interestingly, there is no such thing for workdir:
/media/root-rw/overlay-workdir/nas vs
/media/root-rw/overlay-workdir/_ [1]
Overall it likely works, but I wonder: why overlayroot uses a dedicated subdir
("_") of
/media/root-rw/overlay-workdir for root's workdir, but is fuses things together
for upperdirs? If
only the root's upperdir was /media/root-rw/overlay/_, it would be much cleaner
and without
warnings.
[1] Introduced under title "get workdir set right" in
https://git.launchpad.net/cloud-initramfs-tools/commit/?id=def357d70848ca10b51a6ce795e6e94c3e13c9d5
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