panic ffs_truncate3 (maybe fuse being evil)

[Konstantin Belousov]

On Sun, Jan 17, 2016 at 06:35:43PM +1100, Bruce Evans wrote:
> Setting IO_UNIT for all file types in vn_write() is not so good.  Too
> many file types that can't support it go through vn_write().  It should
> be an error for them, but is just ignored.  Some file types like pipes
> could reasonably try to support it if it is set.  E.g., {PIPE_BUF}
> gives the normal limit of for atomic writes on pipes, bug it would
> be reasonable to use IO_UNIT to specify trying to do larger atomic
> writes and failing if this is not possible.

Pipes and most other special things, and even cdevs, do not perform
io throught {vn,VOP}_{read,write}.  The vnode bypass is common and
usually that things io is routed through the file ops.