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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

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	Please check DEBUG_KMEMLEAK dependencies in lib/Kconfig.debug for supported
16	architectures.
17	
18	Usage
19	-----
20	
21	CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
22	thread scans the memory every 10 minutes (by default) and prints the
23	number of new unreferenced objects found. To display the details of all
24	the possible memory leaks:
25	
26	  # mount -t debugfs nodev /sys/kernel/debug/
27	  # cat /sys/kernel/debug/kmemleak
28	
29	To trigger an intermediate memory scan:
30	
31	  # echo scan > /sys/kernel/debug/kmemleak
32	
33	To clear the list of all current possible memory leaks:
34	
35	  # echo clear > /sys/kernel/debug/kmemleak
36	
37	New leaks will then come up upon reading /sys/kernel/debug/kmemleak
38	again.
39	
40	Note that the orphan objects are listed in the order they were allocated
41	and one object at the beginning of the list may cause other subsequent
42	objects to be reported as orphan.
43	
44	Memory scanning parameters can be modified at run-time by writing to the
45	/sys/kernel/debug/kmemleak file. The following parameters are supported:
46	
47	  off		- disable kmemleak (irreversible)
48	  stack=on	- enable the task stacks scanning (default)
49	  stack=off	- disable the tasks stacks scanning
50	  scan=on	- start the automatic memory scanning thread (default)
51	  scan=off	- stop the automatic memory scanning thread
52	  scan=<secs>	- set the automatic memory scanning period in seconds
53			  (default 600, 0 to stop the automatic scanning)
54	  scan		- trigger a memory scan
55	  clear		- clear list of current memory leak suspects, done by
56			  marking all current reported unreferenced objects grey
57	  dump=<addr>	- dump information about the object found at <addr>
58	
59	Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
60	the kernel command line.
61	
62	Memory may be allocated or freed before kmemleak is initialised and
63	these actions are stored in an early log buffer. The size of this buffer
64	is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
65	
66	Basic Algorithm
67	---------------
68	
69	The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
70	friends are traced and the pointers, together with additional
71	information like size and stack trace, are stored in a prio search tree.
72	The corresponding freeing function calls are tracked and the pointers
73	removed from the kmemleak data structures.
74	
75	An allocated block of memory is considered orphan if no pointer to its
76	start address or to any location inside the block can be found by
77	scanning the memory (including saved registers). This means that there
78	might be no way for the kernel to pass the address of the allocated
79	block to a freeing function and therefore the block is considered a
80	memory leak.
81	
82	The scanning algorithm steps:
83	
84	  1. mark all objects as white (remaining white objects will later be
85	     considered orphan)
86	  2. scan the memory starting with the data section and stacks, checking
87	     the values against the addresses stored in the prio search tree. If
88	     a pointer to a white object is found, the object is added to the
89	     gray list
90	  3. scan the gray objects for matching addresses (some white objects
91	     can become gray and added at the end of the gray list) until the
92	     gray set is finished
93	  4. the remaining white objects are considered orphan and reported via
94	     /sys/kernel/debug/kmemleak
95	
96	Some allocated memory blocks have pointers stored in the kernel's
97	internal data structures and they cannot be detected as orphans. To
98	avoid this, kmemleak can also store the number of values pointing to an
99	address inside the block address range that need to be found so that the
100	block is not considered a leak. One example is __vmalloc().
101	
102	Testing specific sections with kmemleak
103	---------------------------------------
104	
105	Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
106	quite extensive. This can also be the case if you have very buggy code
107	when doing development. To work around these situations you can use the
108	'clear' command to clear all reported unreferenced objects from the
109	/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
110	you can find new unreferenced objects; this should help with testing
111	specific sections of code.
112	
113	To test a critical section on demand with a clean kmemleak do:
114	
115	  # echo clear > /sys/kernel/debug/kmemleak
116	  ... test your kernel or modules ...
117	  # echo scan > /sys/kernel/debug/kmemleak
118	
119	Then as usual to get your report with:
120	
121	  # cat /sys/kernel/debug/kmemleak
122	
123	Kmemleak API
124	------------
125	
126	See the include/linux/kmemleak.h header for the functions prototype.
127	
128	kmemleak_init		 - initialize kmemleak
129	kmemleak_alloc		 - notify of a memory block allocation
130	kmemleak_alloc_percpu	 - notify of a percpu memory block allocation
131	kmemleak_free		 - notify of a memory block freeing
132	kmemleak_free_part	 - notify of a partial memory block freeing
133	kmemleak_free_percpu	 - notify of a percpu memory block freeing
134	kmemleak_not_leak	 - mark an object as not a leak
135	kmemleak_ignore		 - do not scan or report an object as leak
136	kmemleak_scan_area	 - add scan areas inside a memory block
137	kmemleak_no_scan	 - do not scan a memory block
138	kmemleak_erase		 - erase an old value in a pointer variable
139	kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
140	kmemleak_free_recursive	 - as kmemleak_free but checks the recursiveness
141	
142	Dealing with false positives/negatives
143	--------------------------------------
144	
145	The false negatives are real memory leaks (orphan objects) but not
146	reported by kmemleak because values found during the memory scanning
147	point to such objects. To reduce the number of false negatives, kmemleak
148	provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
149	kmemleak_erase functions (see above). The task stacks also increase the
150	amount of false negatives and their scanning is not enabled by default.
151	
152	The false positives are objects wrongly reported as being memory leaks
153	(orphan). For objects known not to be leaks, kmemleak provides the
154	kmemleak_not_leak function. The kmemleak_ignore could also be used if
155	the memory block is known not to contain other pointers and it will no
156	longer be scanned.
157	
158	Some of the reported leaks are only transient, especially on SMP
159	systems, because of pointers temporarily stored in CPU registers or
160	stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
161	the minimum age of an object to be reported as a memory leak.
162	
163	Limitations and Drawbacks
164	-------------------------
165	
166	The main drawback is the reduced performance of memory allocation and
167	freeing. To avoid other penalties, the memory scanning is only performed
168	when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
169	intended for debugging purposes where the performance might not be the
170	most important requirement.
171	
172	To keep the algorithm simple, kmemleak scans for values pointing to any
173	address inside a block's address range. This may lead to an increased
174	number of false negatives. However, it is likely that a real memory leak
175	will eventually become visible.
176	
177	Another source of false negatives is the data stored in non-pointer
178	values. In a future version, kmemleak could only scan the pointer
179	members in the allocated structures. This feature would solve many of
180	the false negative cases described above.
181	
182	The tool can report false positives. These are cases where an allocated
183	block doesn't need to be freed (some cases in the init_call functions),
184	the pointer is calculated by other methods than the usual container_of
185	macro or the pointer is stored in a location not scanned by kmemleak.
186	
187	Page allocations and ioremap are not tracked.
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