CRASH(8) System Manager's Manual CRASH(8)
crash - what to do when the system crashes
This section gives at least a few clues about how to proceed if the
system crashes. It can't pretend to be complete.
Bringing it back up. If the reason for the crash is not evident (see
below for guidance on `evident') you may want to try to dump the system
if you feel up to debugging. At the moment a dump can be taken only on
magtape. With a tape mounted and ready, stop the machine, load address
44, and start. This should write a copy of all of core on the tape
with an EOF mark. Caution: Any error is taken to mean the end of core
has been reached. This means that you must be sure the ring is in, the
tape is ready, and the tape is clean and new. If the dump fails, you
can try again, but some of the registers will be lost. See below for
what to do with the tape.
In restarting after a crash, always bring up the system single-user.
This is accomplished by following the directions in boot(8) as modified
for your particular installation; a single-user system is indicated by
having a particular value in the switches (173030 unless you've changed
init) as the system starts executing. When it is running, perform a
dcheck and icheck(1) on all file systems which could have been in use
at the time of the crash. If any serious file system problems are
found, they should be repaired. When you are satisfied with the health
of your disks, check and set the date if necessary, then come up multi-
user. This is most easily accomplished by changing the single-user
value in the switches to something else, then logging out by typing an
To even boot UNIX at all, three files (and the directories leading to
them) must be intact. First, the initialization program /etc/init must
be present and executable. If it is not, the CPU will loop in user
mode at location 6. For init to work correctly, /dev/tty8 and /bin/sh
must be present. If either does not exist, the symptom is best
described as thrashing. Init will go into a fork/exec loop trying to
create a Shell with proper standard input and output.
If you cannot get the system to boot, a runnable system must be
obtained from a backup medium. The root file system may then be doc-
tored as a mounted file system as described below. If there are any
problems with the root file system, it is probably prudent to go to a
backup system to avoid working on a mounted file system.
Repairing disks. The first rule to keep in mind is that an addled disk
should be treated gently; it shouldn't be mounted unless necessary, and
if it is very valuable yet in quite bad shape, perhaps it should be
dumped before trying surgery on it. This is an area where experience
and informed courage count for much.
The problems reported by icheck typically fall into two kinds. There
can be problems with the free list: duplicates in the free list, or
free blocks also in files. These can be cured easily with an icheck
-s. If the same block appears in more than one file or if a file con-
tains bad blocks, the files should be deleted, and the free list recon-
structed. The best way to delete such a file is to use clri(1), then
remove its directory entries. If any of the affected files is really
precious, you can try to copy it to another device first.
Dcheck may report files which have more directory entries than links.
Such situations are potentially dangerous; clri discusses a special
case of the problem. All the directory entries for the file should be
removed. If on the other hand there are more links than directory
entries, there is no danger of spreading infection, but merely some
disk space that is lost for use. It is sufficient to copy the file (if
it has any entries and is useful) then use clri on its inode and remove
any directory entries that do exist.
Finally, there may be inodes reported by dcheck that have 0 links and 0
entries. These occur on the root device when the system is stopped
with pipes open, and on other file systems when the system stops with
files that have been deleted while still open. A clri will free the
inode, and an icheck -s will recover any missing blocks.
Why did it crash? UNIX types a message on the console typewriter when
it voluntarily crashes. Here is the current list of such messages,
with enough information to provide a hope at least of the remedy. The
message has the form `panic: ...', possibly accompanied by other infor-
mation. Left unstated in all cases is the possibility that hardware or
software error produced the message in some unexpected way.
The getblk routine was called with a nonexistent major device as
argument. Definitely hardware or software error.
Null device table entry for the major device used as argument to
getblk. Definitely hardware or software error.
An I/O error reading the super-block for the root file system dur-
out of inodes
A mounted file system has no more i-nodes when creating a file.
Sorry, the device isn't available; the icheck should tell you.
A device has disappeared from the mounted-device table. Defi-
nitely hardware or software error.
Like `no fs', but produced elsewhere.
The in-core inode table is full. Try increasing NINODE in
param.h. Shouldn't be a panic, just a user error.
During initialization, neither the line nor programmable clock was
found to exist.
An unrecoverable I/O error during a swap. Really shouldn't be a
panic, but it is hard to fix.
unlink - iget
The directory containing a file being deleted can't be found.
Hardware or software.
out of swap space
A program needs to be swapped out, and there is no more swap
space. It has to be increased. This really shouldn't be a panic,
but there is no easy fix.
out of text
A pure procedure program is being executed, and the table for such
things is full. This shouldn't be a panic.
An unexpected trap has occurred within the system. This is accom-
panied by three numbers: a `ka6', which is the contents of the
segmentation register for the area in which the system's stack is
kept; `aps', which is the location where the hardware stored the
program status word during the trap; and a `trap type' which
encodes which trap occurred. The trap types are:
0 bus error
1 illegal instruction
4 power fail
6 recursive system call (TRAP instruction)
7 11/70 cache parity, or programmed interrupt
10 floating point trap
11 segmentation violation
In some of these cases it is possible for octal 20 to be added into the
trap type; this indicates that the processor was in user mode when the
trap occurred. If you wish to examine the stack after such a trap,
either dump the system, or use the console switches to examine core;
the required address mapping is described below.
Interpreting dumps. All file system problems should be taken care of
before attempting to look at dumps. The dump should be read into the
file /usr/sys/core; cp(1) will do. At this point, you should execute
ps -alxk and who to print the process table and the users who were on
at the time of the crash. You should dump ( od(1)) the first 30 bytes
of /usr/sys/core. Starting at location 4, the registers R0, R1, R2,
R3, R4, R5, SP and KDSA6 (KISA6 for 11/40s) are stored. If the dump
had to be restarted, R0 will not be correct. Next, take the value of
KA6 (location 022(8) in the dump) multiplied by 0100(8) and dump
01000(8) bytes starting from there. This is the per-process data asso-
ciated with the process running at the time of the crash. Relabel the
addresses 140000 to 141776. R5 is C's frame or display pointer.
Stored at (R5) is the old R5 pointing to the previous stack frame. At
(R5)+2 is the saved PC of the calling procedure. Trace this calling
chain until you obtain an R5 value of 141756, which is where the user's
R5 is stored. If the chain is broken, you have to look for a plausible
R5, PC pair and continue from there. Each PC should be looked up in
the system's name list using adb(1) and its `:' command, to get a
reverse calling order. In most cases this procedure will give an idea
of what is wrong. A more complete discussion of system debugging is
clri(1), icheck(1), dcheck(1), boot(8)