panic ffs_truncate3 (maybe fuse being evil)

[Konstantin Belousov]

On Thu, Jan 14, 2016 at 10:14:18PM -0500, Rick Macklem wrote:
> Kostik wrote:
> > What is the exact value of lblkno ?
> > 
> It was -1 for the crash I looked at. (Since all are the same backtrace and
> happen for the same test sequence, I suspect they all are.)
> 
> > There are two kinds of buffers with negative lblk which may exist on
> > the UFS vnode queues.  One is indirect block, and another is the metadata
> > block.  Metadata lblks are -1 and -2, indir blocks numbers are calculated
> > by formula you can see in calculation of indir_lbn[] in ffs_truncate().
> > 
> Thanks. I knew indirect blocks were -ve block #s, but I didn't know about
> the metadata ones.
s/metadata/ext attributes data/

Sorry.

> So, if I've understood you correctly, a -1 b_lblkno is an extattr one and
> the assert should allow it and not panic?
Yes.

> 
> I can change the assert, although it will end up about 6 lines of code;-)
> If you think I should do this, I will do so and test it.
It is under INVARIANTS anyway.  I do see a value in checking that truncation
is consistent.  We did have many bugs there, which usually caused user data
loss.  Some caution is worth it, IMO.