Using snapshot fs mount does not protect against all changes on underlying layer

[amir73il]

When mounting snpahsot fs over /real_fs there may still be references to the original /real_fs mount in running processes or other namespaces etc, so the snapshot fs mount itself cannot guaranty COW on change from those references.

It should be possible to use fsnotify pre modify hooks instead of or in addition to snapshot fs mount in order to cover these cases.

[amir73il]

https://github.com/amir73il/linux/commits/ovl-watch