kmemleak: Add documentation on the memory leak detector

			Configure the RouterBoard 532 series on-chip

			Ethernet adapter MAC address.

	kmemleak=	[KNL] Boot-time kmemleak enable/disable

			Valid arguments: on, off

			Default: on

	kstack=N	[X86] Print N words from the kernel stack

			in oops dumps.

Kernel Memory Leak Detector

===========================

Introduction

------------

Kmemleak provides a way of detecting possible kernel memory leaks in a

way similar to a tracing garbage collector

(http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),

with the difference that the orphan objects are not freed but only

reported via /sys/kernel/debug/kmemleak. A similar method is used by the

Valgrind tool (memcheck --leak-check) to detect the memory leaks in

user-space applications.

Usage

-----

CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel

thread scans the memory every 10 minutes (by default) and prints any new

unreferenced objects found. To trigger an intermediate scan and display

all the possible memory leaks:

  # mount -t debugfs nodev /sys/kernel/debug/

  # cat /sys/kernel/debug/kmemleak

Note that the orphan objects are listed in the order they were allocated

and one object at the beginning of the list may cause other subsequent

objects to be reported as orphan.

Memory scanning parameters can be modified at run-time by writing to the

/sys/kernel/debug/kmemleak file. The following parameters are supported:

  off		- disable kmemleak (irreversible)

  stack=on	- enable the task stacks scanning

  stack=off	- disable the tasks stacks scanning

  scan=on	- start the automatic memory scanning thread

  scan=off	- stop the automatic memory scanning thread

  scan=<secs>	- set the automatic memory scanning period in seconds (0

		  to disable it)

Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on

the kernel command line.

Basic Algorithm

---------------

The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and

friends are traced and the pointers, together with additional

information like size and stack trace, are stored in a prio search tree.

The corresponding freeing function calls are tracked and the pointers

removed from the kmemleak data structures.

An allocated block of memory is considered orphan if no pointer to its

start address or to any location inside the block can be found by

scanning the memory (including saved registers). This means that there

might be no way for the kernel to pass the address of the allocated

block to a freeing function and therefore the block is considered a

memory leak.

The scanning algorithm steps:

  1. mark all objects as white (remaining white objects will later be

     considered orphan)

  2. scan the memory starting with the data section and stacks, checking

     the values against the addresses stored in the prio search tree. If

     a pointer to a white object is found, the object is added to the

     gray list

  3. scan the gray objects for matching addresses (some white objects

     can become gray and added at the end of the gray list) until the

     gray set is finished

  4. the remaining white objects are considered orphan and reported via

     /sys/kernel/debug/kmemleak

Some allocated memory blocks have pointers stored in the kernel's

internal data structures and they cannot be detected as orphans. To

avoid this, kmemleak can also store the number of values pointing to an

address inside the block address range that need to be found so that the

block is not considered a leak. One example is __vmalloc().

Kmemleak API

------------

See the include/linux/kmemleak.h header for the functions prototype.

kmemleak_init		 - initialize kmemleak

kmemleak_alloc		 - notify of a memory block allocation

kmemleak_free		 - notify of a memory block freeing

kmemleak_not_leak	 - mark an object as not a leak

kmemleak_ignore		 - do not scan or report an object as leak

kmemleak_scan_area	 - add scan areas inside a memory block

kmemleak_no_scan	 - do not scan a memory block

kmemleak_erase		 - erase an old value in a pointer variable

kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness

kmemleak_free_recursive	 - as kmemleak_free but checks the recursiveness

Dealing with false positives/negatives

--------------------------------------

The false negatives are real memory leaks (orphan objects) but not

reported by kmemleak because values found during the memory scanning

point to such objects. To reduce the number of false negatives, kmemleak

provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and

kmemleak_erase functions (see above). The task stacks also increase the

amount of false negatives and their scanning is not enabled by default.

The false positives are objects wrongly reported as being memory leaks

(orphan). For objects known not to be leaks, kmemleak provides the

kmemleak_not_leak function. The kmemleak_ignore could also be used if

the memory block is known not to contain other pointers and it will no

longer be scanned.

Some of the reported leaks are only transient, especially on SMP

systems, because of pointers temporarily stored in CPU registers or

stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing

the minimum age of an object to be reported as a memory leak.

Limitations and Drawbacks

-------------------------

The main drawback is the reduced performance of memory allocation and

freeing. To avoid other penalties, the memory scanning is only performed

when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is

intended for debugging purposes where the performance might not be the

most important requirement.

To keep the algorithm simple, kmemleak scans for values pointing to any

address inside a block's address range. This may lead to an increased

number of false negatives. However, it is likely that a real memory leak

will eventually become visible.

Another source of false negatives is the data stored in non-pointer

values. In a future version, kmemleak could only scan the pointer

members in the allocated structures. This feature would solve many of

the false negative cases described above.

The tool can report false positives. These are cases where an allocated

block doesn't need to be freed (some cases in the init_call functions),

the pointer is calculated by other methods than the usual container_of

macro or the pointer is stored in a location not scanned by kmemleak.

Page allocations and ioremap are not tracked. Only the ARM and x86

architectures are currently supported.