[net-next-2.6.git] / Documentation / kmemleak.txt

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 the
number of new unreferenced objects found. To display the details of all
the possible memory leaks:

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

To trigger an intermediate memory scan:

  # echo scan > /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 (default)
  stack=off	- disable the tasks stacks scanning
  scan=on	- start the automatic memory scanning thread (default)
  scan=off	- stop the automatic memory scanning thread
  scan=<secs>	- set the automatic memory scanning period in seconds
		  (default 600, 0 to stop the automatic scanning)
  scan		- trigger a memory scan

Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
the kernel command line.

Memory may be allocated or freed before kmemleak is initialised and
these actions are stored in an early log buffer. The size of this buffer
is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.

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.
Commit	Line	Data
04f70336 CM	1	Kernel Memory Leak Detector
	2	===========================
	3
	4	Introduction
	5	------------
	6
	7	Kmemleak provides a way of detecting possible kernel memory leaks in a
	8	way similar to a tracing garbage collector
	9	(http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),
	10	with the difference that the orphan objects are not freed but only
	11	reported via /sys/kernel/debug/kmemleak. A similar method is used by the
	12	Valgrind tool (memcheck --leak-check) to detect the memory leaks in
	13	user-space applications.
	14
	15	Usage
	16	-----
	17
	18	CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
bab4a34a	19	thread scans the memory every 10 minutes (by default) and prints the
4698c1f2 CM	20	number of new unreferenced objects found. To display the details of all
4698c1f2 CM	21	the possible memory leaks:
04f70336 CM	22
	23	# mount -t debugfs nodev /sys/kernel/debug/
	24	# cat /sys/kernel/debug/kmemleak
	25
4698c1f2 CM	26	To trigger an intermediate memory scan:
	27
	28	# echo scan > /sys/kernel/debug/kmemleak
	29
04f70336 CM	30	Note that the orphan objects are listed in the order they were allocated
	31	and one object at the beginning of the list may cause other subsequent
	32	objects to be reported as orphan.
	33
	34	Memory scanning parameters can be modified at run-time by writing to the
	35	/sys/kernel/debug/kmemleak file. The following parameters are supported:
	36
	37	off - disable kmemleak (irreversible)
e0a2a160	38	stack=on - enable the task stacks scanning (default)
04f70336	39	stack=off - disable the tasks stacks scanning
e0a2a160	40	scan=on - start the automatic memory scanning thread (default)
04f70336	41	scan=off - stop the automatic memory scanning thread
e0a2a160 CM	42	scan=<secs> - set the automatic memory scanning period in seconds
e0a2a160 CM	43	(default 600, 0 to stop the automatic scanning)
4698c1f2	44	scan - trigger a memory scan
04f70336 CM	45
	46	Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
	47	the kernel command line.
	48
a9d9058a CM	49	Memory may be allocated or freed before kmemleak is initialised and
	50	these actions are stored in an early log buffer. The size of this buffer
	51	is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
	52
04f70336 CM	53	Basic Algorithm
	54	---------------
	55
	56	The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
	57	friends are traced and the pointers, together with additional
	58	information like size and stack trace, are stored in a prio search tree.
	59	The corresponding freeing function calls are tracked and the pointers
	60	removed from the kmemleak data structures.
	61
	62	An allocated block of memory is considered orphan if no pointer to its
	63	start address or to any location inside the block can be found by
	64	scanning the memory (including saved registers). This means that there
	65	might be no way for the kernel to pass the address of the allocated
	66	block to a freeing function and therefore the block is considered a
	67	memory leak.
	68
	69	The scanning algorithm steps:
	70
	71	1. mark all objects as white (remaining white objects will later be
	72	considered orphan)
	73	2. scan the memory starting with the data section and stacks, checking
	74	the values against the addresses stored in the prio search tree. If
	75	a pointer to a white object is found, the object is added to the
	76	gray list
	77	3. scan the gray objects for matching addresses (some white objects
	78	can become gray and added at the end of the gray list) until the
	79	gray set is finished
	80	4. the remaining white objects are considered orphan and reported via
	81	/sys/kernel/debug/kmemleak
	82
	83	Some allocated memory blocks have pointers stored in the kernel's
	84	internal data structures and they cannot be detected as orphans. To
	85	avoid this, kmemleak can also store the number of values pointing to an
	86	address inside the block address range that need to be found so that the
	87	block is not considered a leak. One example is __vmalloc().
	88
	89	Kmemleak API
	90	------------
	91
	92	See the include/linux/kmemleak.h header for the functions prototype.
	93
	94	kmemleak_init - initialize kmemleak
	95	kmemleak_alloc - notify of a memory block allocation
	96	kmemleak_free - notify of a memory block freeing
	97	kmemleak_not_leak - mark an object as not a leak
	98	kmemleak_ignore - do not scan or report an object as leak
	99	kmemleak_scan_area - add scan areas inside a memory block
	100	kmemleak_no_scan - do not scan a memory block
	101	kmemleak_erase - erase an old value in a pointer variable
	102	kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
	103	kmemleak_free_recursive - as kmemleak_free but checks the recursiveness
	104
	105	Dealing with false positives/negatives
	106	--------------------------------------
	107
	108	The false negatives are real memory leaks (orphan objects) but not
	109	reported by kmemleak because values found during the memory scanning
	110	point to such objects. To reduce the number of false negatives, kmemleak
	111	provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
	112	kmemleak_erase functions (see above). The task stacks also increase the
	113	amount of false negatives and their scanning is not enabled by default.
	114
	115	The false positives are objects wrongly reported as being memory leaks
	116	(orphan). For objects known not to be leaks, kmemleak provides the
117	kmemleak_not_leak function. The kmemleak_ignore could also be used if
118	the memory block is known not to contain other pointers and it will no
119	longer be scanned.
120
121	Some of the reported leaks are only transient, especially on SMP
122	systems, because of pointers temporarily stored in CPU registers or
123	stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
124	the minimum age of an object to be reported as a memory leak.
125
126	Limitations and Drawbacks
127	-------------------------
128
129	The main drawback is the reduced performance of memory allocation and
130	freeing. To avoid other penalties, the memory scanning is only performed
131	when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
132	intended for debugging purposes where the performance might not be the
133	most important requirement.
134
135	To keep the algorithm simple, kmemleak scans for values pointing to any
136	address inside a block's address range. This may lead to an increased
137	number of false negatives. However, it is likely that a real memory leak
138	will eventually become visible.
139
140	Another source of false negatives is the data stored in non-pointer
141	values. In a future version, kmemleak could only scan the pointer
142	members in the allocated structures. This feature would solve many of
143	the false negative cases described above.
144
145	The tool can report false positives. These are cases where an allocated
146	block doesn't need to be freed (some cases in the init_call functions),
147	the pointer is calculated by other methods than the usual container_of
148	macro or the pointer is stored in a location not scanned by kmemleak.
149
150	Page allocations and ioremap are not tracked. Only the ARM and x86
151	architectures are currently supported.