softdep as a mount(8) option

[Mark Johnston]

Hi,

I'd like to finally enable the use of SU (not SU+J) on some small UFS
filesystems. The fact that SU is enabled using a flag in the superblock
poses a problem for me, however: the systems containing these
filesystems may at any time be repurposed to run a kernel that supports
SU but contains bugs[*] that render it unusable. I therefore can't
persistently enable SU in these systems.

I'm wondering if it would be possible to enable SU using a mount
option rather than with a persistent flag. fsck_ffs conditionalizes some
of its logic on whether SU is configured - is this necessary for
correctness? That is, if I run fsck on an unclean filesystem that had
been mounted with SU, and fsck runs as though SU hadn't been configured,
what problems might arise?

[*] These bugs are a result of local modifications and aren't in
FreeBSD.