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CRASH(8V)																 CRASH(8V)

NAME

       crash - what happens when the system crashes

DESCRIPTION

       This section explains what happens when the system crashes and (very briefly) how to analyze crash dumps.

       When the system crashes voluntarily it prints a message of the form

	      panic: why i gave up the ghost

       on  the	console,  takes  a  dump  on  a mass storage peripheral, and then invokes an automatic reboot procedure as described in reboot(8).
       Unless some unexpected inconsistency is encountered in the state of the file systems due to hardware or software failure, the  system  will
       then  resume multi-user operations.  If the automatic file system check fails, the file systems should be checked and repaired with fsck(8)
       before continuing.

       The system has a large number of internal consistency checks; if one of these fails, then it will panic with a very short message  indicat-
       ing  which  one failed.	In many instances, this will be the name of the routine which detected the error, or a two-word description of the
       inconsistency.  A full understanding of most panic messages requires perusal of the source code for the system.

       The most common cause of system failures is hardware failure, which can reflect itself in different ways.  Here are the messages which  are
       most likely, with some hints as to causes.  Left unstated in all cases is the possibility that hardware or software error produced the mes-
       sage in some unexpected way.

       iinit  This cryptic panic message results from a failure to mount the root filesystem  during  the  bootstrap  process.	 Either  the  root
	      filesystem  has  been corrupted, or the system is attempting to use the wrong device as root filesystem.	Usually, an alternate copy
	      of the system binary or an alternate root filesystem can be used to bring up the system to investigate.

       Can't exec /etc/init
	      This is not a panic message, as reboots are likely to be futile.	Late in the bootstrap procedure, the system was unable	to  locate
	      and  execute  the  initialization  process, init(8).  The root filesystem is incorrect or has been corrupted, or the mode or type of
	      /etc/init forbids execution.

       hard IO err in swap
	      The system encountered an error trying to write to the swap device or an error in reading critical information from  a  disk  drive.
	      The offending disk should be fixed if it is broken or unreliable.

       timeout table overflow
	      This  really  shouldn't  be a panic, but until the data structure involved is made to be extensible, running out of entries causes a
	      crash.  If this happens, make the timeout table bigger.  (NCALL in param.c)

       trap type %o
	      An unexpected trap has occurred within the system; the trap types are:

       0    bus error
       1    illegal instruction trap
       2    BPT/trace trap
       3    IOT
       4    power fail trap (if autoreboot fails)
       5    EMT
       6    recursive system call (TRAP instruction)
       7    programmed interrupt request
       11   protection fault (segmentation violation)
       12   parity trap

       In some of these cases it is possible for octal 020 to be added into the trap type; this indicates that the processor was in user mode when
       the trap occurred.

       In  addition  to the trap type, the system will have printed out three (or four) other numbers: ka6, which is the contents of the segmenta-
       tion 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; pc, which was the system's program counter when it faulted (already incremented to the next word); __ovno, the over-
       lay number of the currently loaded kernel overlay when the trap occurred.

       The favorite trap types in system crashes are trap types 0 and 11, indicating a wild reference.	The code is the  referenced  address,  and
       the pc at the time of the fault is printed.  These problems tend to be easy to track down if they are kernel bugs since the processor stops
       cold, but random flakiness seems to cause this sometimes.  The debugger can be used to locate the instruction and subroutine  corresponding
       to  the PC value.  If that is insufficient to suggest the nature of the problem, more detailed examination of the system status at the time
       of the trap usually can produce an explanation.

       init died
	      The system initialization process has exited.  This is bad news, as no new users will then be able to log in.  Rebooting is the only
	      fix, so the system just does it right away.

       out of mbufs: map full
	      The  network has exhausted its private page map for network buffers.  This usually indicates that buffers are being lost, and rather
	      than allow the system to slowly degrade, it reboots immediately.	The map may be made larger if necessary.

       out of swap space
	      This really shouldn't be panics but there's no other satisfactory solution.  The size of the swap area must be increased.  The  sys-
	      tem attempts to avoid running out of swap by refusing to start new processes when short of swap space (resulting in ``No more proce-
	      ses'' messages from the shell).

       &remap_area > SEG5
       _end > SEG5
	      The kernel detected at boot time that an unacceptable portion of its data space extended into the region controlled  by  KDSA5.	In
	      the  case of the first message, the size of the kernel's data segment (excluding the file, proc, and text tables) must be decreased.
	      In the latter case, there are two possibilities: if &remap_area is not greater than 0120000, the kernel must be  recompiled  without
	      defining the option NOKA5.  Otherwise, as above, the size of the kernel's data segment must be decreased.

       That  completes the list of panic types you are likely to see.  There are many other panic messages which are less likely to occur; most of
       them detect logical inconsistencies within the kernel and thus ``cannot happen'' unless some part of the kernel has been modified.

       If the system stops or hangs without a panic, it is possible to stop it and take a dump of memory before rebooting.  A panic can be  forced
       from  the  console,  which will allow a dump, automatic reboot and file system check.  This is accomplished by halting the CPU, putting the
       processor in kernel mode, loading the PC with 40, and continuing without a reset (use continue, not start).  To put the processor in kernel
       mode,  make  sure  the  two high bits in the processor status word are zero.  (you'll need to consult the procesor handbook describing your
       processor to determine how to access the PC and PS ...)	The message ``panic:  forced from console'' should print, and the automatic reboot
       will start.

       If this fails a dump of memory can be made on magtape: mount a tape (with write ring!), halt the CPU, load address 044, and perform a start
       (which does a reset).  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.  After this completes, halt again and reboot.

       After rebooting, or after an automatic file system check fails, check and fix the file systems with fsck.  If the system will not reboot, a
       runnable  system must be obtained from a backup medium after verifying that the hardware is functioning normally.  A damaged root file sys-
       tem should be patched while running with an alternate root if possible.

       When the system crashes if crash dumping was enabled it writes (or at least attempts to write) an image of memory into the back end of  the
       dump device, usually the same as the primary swap area.	After the system is rebooted, the program savecore(8) runs and preserves a copy of
       this core image and the current system in a specified directory for later perusal.  See savecore(8) for details.  A  magtape  dump  can	be
       read onto disk with dd(1).

       To  analyze  a  dump  you should begin by running adb(1) with the -k flag on the system load image and core dump.  If the core image is the
       result of a panic, the panic message is printed.  Normally the command ``$c'' or ``$C'' will provide a stack trace from the  point  of  the
       crash  and  this  will provide a clue as to what went wrong.  ps(1) and pstat(8)canalsobeused to print the process table at the time of the
       crash via: ps -alxk and pstat -p.  If the mapping or the stack frame are incorrect, the following magic locations may  be  examined  in	an
       attempt	to find out what went wrong.  The registers R0, R1, R2, R3, R4, R5, SP, and KDSA6 (or KISA6 for machines without separate instruc-
       tion and data) are saved at location 04.  If the core dump was taken on disk, these values also appear at 0300.	The value of KDSA6 (KISA6)
       multiplied  by  0100(8) gives the address of the user structure and kernel stack for the running process.  Relabel these addresses 0140000
       through 0142000.  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 to an R5 value of 0141756 (0141754 for overlaid kernels), which is where the
       user's R5 is stored.  If the chain is broken, look for a plausible R5, PC pair and continue from there.	In most cases this procedure  will
       give an idea of what is wrong.

       A more complete discussion of system debugging is impossible here.  See, however, ``Using ADB to Debug the UNIX Kernel''.

SEE ALSO

       adb(1), ps(1), pstat(1), boot(8), fsck(8), reboot(8), savecore(8)
       PDP-11 Processor Handbook for various processors for more information about PDP-11 memory management and general architecture.
       Using ADB to Debug the UNIX Kernel

3rd Berkeley Distribution					   July 11, 1987							 CRASH(8V)