Jann Horn [off-list ref] wrote:

I don't really know. I guess it depends on how it's being used? If
someone decides to e.g. make a file browser that installs watches for
a bunch of mountpoints for some fancy sidebar showing the device
mounts on the system, or something like that, that probably shouldn't
inhibit unmounting... I don't know if that's a realistic use case.

In such a use case, I would envision the browser putting a watch on "/".  A
watch sees all events in the subtree rooted at that point and you must apply a
filter that filters them out if you're not interested (filter on
WATCH_INFO_IN_SUBTREE using info_filter and info_mask).

David