[GitHub/mt8127/android_kernel_alcatel_ttab.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 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.
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
	19	thread scans the memory every 10 minutes (by default) and prints any new
	20	unreferenced objects found. To trigger an intermediate scan and display
	21	all the possible memory leaks:
	22
	23	# mount -t debugfs nodev /sys/kernel/debug/
	24	# cat /sys/kernel/debug/kmemleak
	25
	26	Note that the orphan objects are listed in the order they were allocated
	27	and one object at the beginning of the list may cause other subsequent
	28	objects to be reported as orphan.
	29
	30	Memory scanning parameters can be modified at run-time by writing to the
	31	/sys/kernel/debug/kmemleak file. The following parameters are supported:
	32
	33	off - disable kmemleak (irreversible)
	34	stack=on - enable the task stacks scanning
	35	stack=off - disable the tasks stacks scanning
	36	scan=on - start the automatic memory scanning thread
	37	scan=off - stop the automatic memory scanning thread
	38	scan=<secs> - set the automatic memory scanning period in seconds (0
	39	to disable it)
	40
	41	Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
	42	the kernel command line.
	43
	44	Basic Algorithm
	45	---------------
	46
	47	The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
	48	friends are traced and the pointers, together with additional
	49	information like size and stack trace, are stored in a prio search tree.
	50	The corresponding freeing function calls are tracked and the pointers
	51	removed from the kmemleak data structures.
	52
	53	An allocated block of memory is considered orphan if no pointer to its
	54	start address or to any location inside the block can be found by
	55	scanning the memory (including saved registers). This means that there
	56	might be no way for the kernel to pass the address of the allocated
	57	block to a freeing function and therefore the block is considered a
	58	memory leak.
	59
	60	The scanning algorithm steps:
	61
	62	1. mark all objects as white (remaining white objects will later be
	63	considered orphan)
	64	2. scan the memory starting with the data section and stacks, checking
65	the values against the addresses stored in the prio search tree. If
66	a pointer to a white object is found, the object is added to the
67	gray list
68	3. scan the gray objects for matching addresses (some white objects
69	can become gray and added at the end of the gray list) until the
70	gray set is finished
71	4. the remaining white objects are considered orphan and reported via
72	/sys/kernel/debug/kmemleak
73
74	Some allocated memory blocks have pointers stored in the kernel's
75	internal data structures and they cannot be detected as orphans. To
76	avoid this, kmemleak can also store the number of values pointing to an
77	address inside the block address range that need to be found so that the
78	block is not considered a leak. One example is __vmalloc().
79
80	Kmemleak API
81	------------
82
83	See the include/linux/kmemleak.h header for the functions prototype.
84
85	kmemleak_init - initialize kmemleak
86	kmemleak_alloc - notify of a memory block allocation
87	kmemleak_free - notify of a memory block freeing
88	kmemleak_not_leak - mark an object as not a leak
89	kmemleak_ignore - do not scan or report an object as leak
90	kmemleak_scan_area - add scan areas inside a memory block
91	kmemleak_no_scan - do not scan a memory block
92	kmemleak_erase - erase an old value in a pointer variable
93	kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
94	kmemleak_free_recursive - as kmemleak_free but checks the recursiveness
95
96	Dealing with false positives/negatives
97	--------------------------------------
98
99	The false negatives are real memory leaks (orphan objects) but not
100	reported by kmemleak because values found during the memory scanning
101	point to such objects. To reduce the number of false negatives, kmemleak
102	provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
103	kmemleak_erase functions (see above). The task stacks also increase the
104	amount of false negatives and their scanning is not enabled by default.
105
106	The false positives are objects wrongly reported as being memory leaks
107	(orphan). For objects known not to be leaks, kmemleak provides the
108	kmemleak_not_leak function. The kmemleak_ignore could also be used if
109	the memory block is known not to contain other pointers and it will no
110	longer be scanned.
111
112	Some of the reported leaks are only transient, especially on SMP
113	systems, because of pointers temporarily stored in CPU registers or
114	stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
115	the minimum age of an object to be reported as a memory leak.
116
117	Limitations and Drawbacks
118	-------------------------
119
120	The main drawback is the reduced performance of memory allocation and
121	freeing. To avoid other penalties, the memory scanning is only performed
122	when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
123	intended for debugging purposes where the performance might not be the
124	most important requirement.
125
126	To keep the algorithm simple, kmemleak scans for values pointing to any
127	address inside a block's address range. This may lead to an increased
128	number of false negatives. However, it is likely that a real memory leak
129	will eventually become visible.
130
131	Another source of false negatives is the data stored in non-pointer
132	values. In a future version, kmemleak could only scan the pointer
133	members in the allocated structures. This feature would solve many of
134	the false negative cases described above.
135
136	The tool can report false positives. These are cases where an allocated
137	block doesn't need to be freed (some cases in the init_call functions),
138	the pointer is calculated by other methods than the usual container_of
139	macro or the pointer is stored in a location not scanned by kmemleak.
140
141	Page allocations and ioremap are not tracked. Only the ARM and x86
142	architectures are currently supported.