The mismatched mount mess

Incidentally, the worst part of cross-namespace sharing of mounts is not just the fs-level mount options being ignored rather than having mount(2) fail on their mismatch. The real nastiness comes from mount -o remount done in one place and affecting every mount of that sucker. The mount-level options (nosuid among them) are not an issue - they won't do anything to other mounts. Per-fs ones bloody well will. And no, we really can't make each option per-mount - I dare you to try and handle the things like -o errors=panic or -o errors=remount-ro on per-mountpoint basis; good luck propagating the "originating" mount towards each ext4_error() in there. Especially fun when it comes to errors on e.g. writeback. Or -o data=... for the same ext4...