Kernel Extensions and Device Support Programming Concepts

Memory Overlay Detection System (MODS)

Some of the most difficult types of problems to debug are what are generally called "memory overlays." Memory overlays include the following:

• Writing to memory that is owned by another program or routine
• Writing past the end (or before the beginning) of declared variables or arrays
• Writing past the end (or before the beginning) of dynamically allocated memory
• Writing to or reading from freed memory
• Freeing memory twice
• Calling memory allocation routines with incorrect parameters or under incorrect conditions.

In the kernel environment (including the kernel, kernel extensions, and device drivers), memory overlay problems have been especially difficult to debug because tools for finding them have not been available. Starting with AIX 4.2.1, however, the Memory Overlay Detection System (MODS) helps detect memory overlay problems in the kernel, kernel extensions, and device drivers.

bosdebug command

The bosdebug command turns the MODS facility on and off. Only the root user can run the bosdebug command.

To turn on the base MODS support, type:

bosdebug -M

For a description of all the available options, type:

bosdebug -? 

Once you have run bosdebug with the options you want, run the bosboot -a command, then shut down and reboot your system (using the shutdown -r command). If you need to make any changes to your bosdebug settings, you must run bosboot -a and shutdown -r again.

When to use the MODS feature

This feature is useful in the following circumstances:

• When developing your own kernel extensions or device drivers and you want to test them thoroughly.
• When asked to turn this feature on by IBM technical support service to help in further diagnosing a problem that you are experiencing.

How MODS works

The primary goal of the MODS feature is to produce a dump file that accurately identifies the problem.

MODS works by turning on additional checking to help detect the conditions listed above. When any of these conditions is detected, your system crashes immediately and produces a dump file that points directly at the offending code. (In previous versions, a system dump might point to unrelated code that happened to be running later when the invalid situation was finally detected.)

If your system crashes while the MODS is turned on, then MODS has most likely done its job.

The xmalloc subcommand provides details on exactly what memory address (if any) was involved in the situation, and displays mini-tracebacks for the allocation or free records of this memory.

Similarly, the netm command displays allocation and free records for memory allocated using the net_malloc kernel service (for example, mbufs, mclusters, etc.).

You can use these commands, as well as standard crash techniques, to determine exactly what went wrong.

MODS limitations

There are limitations to the Memory Overlay Detection System. Although it significantly improves your chances, MODS cannot detect all memory overlays. Also, turning MODS on has a small negative impact on overall system performance and causes somewhat more memory to be used in the kernel and the network memory heaps. If your system is running at full CPU utilization, or if you are already near the maximums for kernel memory usage, turning on the MODS may cause performance degradation and/or system hangs.

Practical experience with the MODS, however, suggests that the great majority of customers will be able to use it with minimal impact to their systems.

MODS benefits

You will see these benefits from using the MODS:

• You can more easily test and debug your own kernel extensions and devicedrivers.
• Difficult problems that once required multiple attempts to recreate and debug them will generally require many fewer such attempts.