Device layer rollup commit.
* cdevsw_add() is now required. cdevsw_add() and cdevsw_remove() may specify
a mask/match indicating the range of supported minor numbers. Multiple
cdevsw_add()'s using the same major number, but distinctly different
ranges, may be issued. All devices that failed to call cdevsw_add() before
now do.
* cdevsw_remove() now automatically marks all devices within its supported
range as being destroyed.
* vnode->v_rdev is no longer resolved when the vnode is created. Instead,
only v_udev (a newly added field) is resolved. v_rdev is resolved when
the vnode is opened and cleared on the last close.
* A great deal of code was making rather dubious assumptions with regards
to the validity of devices associated with vnodes, primarily due to
the persistence of a device structure due to being indexed by (major, minor)
instead of by (cdevsw, major, minor). In particular, if you run a program
which connects to a USB device and then you pull the USB device and plug
it back in, the vnode subsystem will continue to believe that the device
is open when, in fact, it isn't (because it was destroyed and recreated).
In particular, note that all the VFS mount procedures now check devices
via v_udev instead of v_rdev prior to calling VOP_OPEN(), since v_rdev
is NULL prior to the first open.
* The disk layer's device interaction has been rewritten. The disk layer
(i.e. the slice and disklabel management layer) no longer overloads
its data onto the device structure representing the underlying physical
disk. Instead, the disk layer uses the new cdevsw_add() functionality
to register its own cdevsw using the underlying device's major number,
and simply does NOT register the underlying device's cdevsw. No
confusion is created because the device hash is now based on
(cdevsw,major,minor) rather then (major,minor).
NOTE: This also means that underlying raw disk devices may use the entire
device minor number instead of having to reserve the bits used by the disk
layer, and also means that can we (theoretically) stack a fully
disklabel-supported 'disk' on top of any block device.
* The new reference counting scheme prevents this by associating a device
with a cdevsw and disconnecting the device from its cdevsw when the cdevsw
is removed. Additionally, all udev2dev() lookups run through the cdevsw
mask/match and only successfully find devices still associated with an
active cdevsw.
* Major work on MFS: MFS no longer shortcuts vnode and device creation. It
now creates a real vnode and a real device and implements real open and
close VOPs. Additionally, due to the disk layer changes, MFS is no longer
limited to 255 mounts. The new limit is 16 million. Since MFS creates a
real device node, mount_mfs will now create a real /dev/mfs<PID> device
that can be read from userland (e.g. so you can dump an MFS filesystem).
* BUF AND DEVICE STRATEGY changes. The struct buf contains a b_dev field.
In order to properly handle stacked devices we now require that the b_dev
field be initialized before the device strategy routine is called. This
required some additional work in various VFS implementations. To enforce
this requirement, biodone() now sets b_dev to NODEV. The new disk layer
will adjust b_dev before forwarding a request to the actual physical
device.
* A bug in the ISO CD boot sequence which resulted in a panic has been fixed.
Testing by: lots of people, but David Rhodus found the most aggregious bugs.