When the userspace daemon mount.davfs dies before the file system is unmounted, then the kernel file system fuse will hang forever and it is not possible to normally unmount. I think this is a bug in fuse and filed a report to the kernel bug tracker long ago. https://bugzilla.kernel.org/show_bug.cgi?id=115971 There is always a chance that a userspace process crashes. So the kernel should be able to handle this gracefully.
The other question is why mount.davfs crashed? - Is the problem reproducible and can you test whether it only happens when managed by systemd? - mount.davfs may crash silently when it runs out of memory. For every file (or directory) it gets to know of it will require about 200 to 300 bytes of working memory and it will not free any of it before it terminates. - When mount.davfs is stopped with signal SIGTERM or SIGHUB there should be a message in daemon.log. - You could use the debug options to get a lot of debug messages in daemon.log which might show at what action the daemon dies. These logs may get very lengthy. Option "debug kernel" and "debug cache" could be a start. Werner
