[Bug 208691] "panic: ffs_valloc: dup alloc" as soon as UFS root partition is mounted

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=208691

--- Comment #2 from Martin Guy <martinwguy at gmail.com> ---
Thanks Kirk, that made the system boot again. Unfortunately the interesting
parts of the fsck output scrolled off the screen before I could copy them, so I
don't know what the filesystem defect was that provoked this panic.

However, if the same thing happens to a distant server with only a console, it
would panic while mounting / to go into single-user mode too, making the system
unrecoverable.

Peter H wrote to say:
> the first (journaled) fsck just replays the journal; it does not
> check the file system. That is why journaling fsck is so fast.

So another way of seeing the problem is that, if "fsck -p" fails, it tells the
user to run "fsck -y" which says "yes" to fsck's initial "USE JOURNAL?"
question and this results in a faster check that doesn't fix the
kernel-panicking inconsistency. It needs -fy to get round the "USE JOURNAL?"
option.

Another solution could be to always do a full check without the journal when a
fast filesystem is not clean, instead of "-p"? I don't see a combination of
fsck flags to do that. -fyC looks like the closest but needs changing to make
-C override -f and skip the check if the filesystem is clean.

Or turning write-behind off by default until a better fix is found?

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