device vnodes, and structural confusion

[David Holland <dholland-tech%netbsd.org@localhost>]

As has come up many times in the past, we have a long-standing
problem, or family of problems, where in the course of operating on a
particular file system we end up not calling the file system's own
vnode or fs operations, but the root file system's operations. This
causes varying degrees of lossage depending on exactly what happens.

One of the ways this happens came up today while working on PR 47937.
The vnode structure has both a v_mount field and a v_specmountpoint
field(*); both are pointers to a struct mount (that is, a filesystem)
and it's very easy for people to use the wrong one, or not be sure
which is which.

(*) v_specmountpoint is not actually a field, it's a fake field
provided by a macro.

In the short term, to reduce the confusion I would like to make the
following changes:

   1. Rename v_mount, which is the filesystem the vnode is on (almost
   always / for device vnodes), to "v_myfs".

   2. Rather than renaming v_specmountpoint, which is the filesystem
   mounted on a device vnode, kill it off entirely, and replace it
   with an inline (or noninline) function "specvn_getmountedfs()".
   Make this function assert if called on a non-VBLK vnode instead of
   silently producing trash.

This is going to be a fairly large massedit, but I think it's
worthwhile for clarity even if it doesn't turn up any mistakes.
And I expect it to turn up at least a couple howlers. :-/

Also, while doing the edit, where feasible stop retrieving the fs
structure from the vnode structure (or other structures) by going
through the device-mounted-on. There should in general be no reason to
do this at all; a filesystem's own structures should all have adequate
pointers to one another without going through the device.

In the long run, I think we should rearrange the way device vnodes are
handled, so that the device vnodes seen and handled belong entirely to
specfs and the vnodes for the on-disk inodes are stubs hidden behind
the specfs vnodes. This would make it much more obvious when the wrong
fs's ops get called; also it would have the generally beneficial
effect of allowing a lot of rote cut and paste filesystem stuff for
supporting device nodes to be flushed.

More on that when my ideas solidify a bit.

-- 
David A. Holland
dholland%netbsd.org@localhost