panic: ffs_snapshot_mount: already on list

[manu%netbsd.org@localhost (Emmanuel Dreyfus)]

This morning, a busy NetBSD-8.0/amd64 server greeted me with an
unpleasant panic on reboot: ffs_snapshot_mount: xxx already on list

I booted in single user mode and ran fsck. After cleaning up things, the
filesystem would mount read-only, but mounting read-write, with or
without -o log, would raise the same panic.

There is no mount option to disable snapshots, hence I tried to build a
custom kernel with options FFS_NO_SNAPSHOT to run without snapshot
support and get this filesystem to mount. No luck, even with so called
FFS_NO_SNAPSHOT, it would still crash in snapshot-related
ffs_snapshot_mount.

I patched ffs_snapshot_mount to #ifndef FFS_NO_SNAPSHOT, the whole
function, rebuilt a kernel and booted on that. With that, I was able to
mount read-only. I then removed the obsolete snapshot backend files from
the filesystem and rebooted on GENERIC. No panic anymore, it only
complains at mount time:
ffs_snapshot_mount: vget failed 2
ffs_snapshot_mount: vget failed 2
ffs_snapshot_mount: vget failed 2

Beside the problem that  FFS_NO_SNAPSHOT does not really disable
snapshot code, I think we should have a nosnapshot or nofss mount option
to handle such a scenario. Anyone has opinion on this?

-- 
Emmanuel Dreyfus
http://hcpnet.free.fr/pubz
manu%netbsd.org@localhost