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Based on kernel version 4.1. Page generated on 2015-06-28 12:13 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	Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390, metag and tile.
15	
16	Usage
17	-----
18	
19	CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
20	thread scans the memory every 10 minutes (by default) and prints the
21	number of new unreferenced objects found. To display the details of all
22	the possible memory leaks:
23	
24	  # mount -t debugfs nodev /sys/kernel/debug/
25	  # cat /sys/kernel/debug/kmemleak
26	
27	To trigger an intermediate memory scan:
28	
29	  # echo scan > /sys/kernel/debug/kmemleak
30	
31	To clear the list of all current possible memory leaks:
32	
33	  # echo clear > /sys/kernel/debug/kmemleak
34	
35	New leaks will then come up upon reading /sys/kernel/debug/kmemleak
36	again.
37	
38	Note that the orphan objects are listed in the order they were allocated
39	and one object at the beginning of the list may cause other subsequent
40	objects to be reported as orphan.
41	
42	Memory scanning parameters can be modified at run-time by writing to the
43	/sys/kernel/debug/kmemleak file. The following parameters are supported:
44	
45	  off		- disable kmemleak (irreversible)
46	  stack=on	- enable the task stacks scanning (default)
47	  stack=off	- disable the tasks stacks scanning
48	  scan=on	- start the automatic memory scanning thread (default)
49	  scan=off	- stop the automatic memory scanning thread
50	  scan=<secs>	- set the automatic memory scanning period in seconds
51			  (default 600, 0 to stop the automatic scanning)
52	  scan		- trigger a memory scan
53	  clear		- clear list of current memory leak suspects, done by
54			  marking all current reported unreferenced objects grey,
55			  or free all kmemleak objects if kmemleak has been disabled.
56	  dump=<addr>	- dump information about the object found at <addr>
57	
58	Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
59	the kernel command line.
60	
61	Memory may be allocated or freed before kmemleak is initialised and
62	these actions are stored in an early log buffer. The size of this buffer
63	is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
64	
65	If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
66	disabled by default. Passing "kmemleak=on" on the kernel command
67	line enables the function. 
68	
69	Basic Algorithm
70	---------------
71	
72	The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
73	friends are traced and the pointers, together with additional
74	information like size and stack trace, are stored in a rbtree.
75	The corresponding freeing function calls are tracked and the pointers
76	removed from the kmemleak data structures.
77	
78	An allocated block of memory is considered orphan if no pointer to its
79	start address or to any location inside the block can be found by
80	scanning the memory (including saved registers). This means that there
81	might be no way for the kernel to pass the address of the allocated
82	block to a freeing function and therefore the block is considered a
83	memory leak.
84	
85	The scanning algorithm steps:
86	
87	  1. mark all objects as white (remaining white objects will later be
88	     considered orphan)
89	  2. scan the memory starting with the data section and stacks, checking
90	     the values against the addresses stored in the rbtree. If
91	     a pointer to a white object is found, the object is added to the
92	     gray list
93	  3. scan the gray objects for matching addresses (some white objects
94	     can become gray and added at the end of the gray list) until the
95	     gray set is finished
96	  4. the remaining white objects are considered orphan and reported via
97	     /sys/kernel/debug/kmemleak
98	
99	Some allocated memory blocks have pointers stored in the kernel's
100	internal data structures and they cannot be detected as orphans. To
101	avoid this, kmemleak can also store the number of values pointing to an
102	address inside the block address range that need to be found so that the
103	block is not considered a leak. One example is __vmalloc().
104	
105	Testing specific sections with kmemleak
106	---------------------------------------
107	
108	Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
109	quite extensive. This can also be the case if you have very buggy code
110	when doing development. To work around these situations you can use the
111	'clear' command to clear all reported unreferenced objects from the
112	/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
113	you can find new unreferenced objects; this should help with testing
114	specific sections of code.
115	
116	To test a critical section on demand with a clean kmemleak do:
117	
118	  # echo clear > /sys/kernel/debug/kmemleak
119	  ... test your kernel or modules ...
120	  # echo scan > /sys/kernel/debug/kmemleak
121	
122	Then as usual to get your report with:
123	
124	  # cat /sys/kernel/debug/kmemleak
125	
126	Freeing kmemleak internal objects
127	---------------------------------
128	
129	To allow access to previously found memory leaks after kmemleak has been
130	disabled by the user or due to an fatal error, internal kmemleak objects
131	won't be freed when kmemleak is disabled, and those objects may occupy
132	a large part of physical memory.
133	
134	In this situation, you may reclaim memory with:
135	
136	  # echo clear > /sys/kernel/debug/kmemleak
137	
138	Kmemleak API
139	------------
140	
141	See the include/linux/kmemleak.h header for the functions prototype.
142	
143	kmemleak_init		 - initialize kmemleak
144	kmemleak_alloc		 - notify of a memory block allocation
145	kmemleak_alloc_percpu	 - notify of a percpu memory block allocation
146	kmemleak_free		 - notify of a memory block freeing
147	kmemleak_free_part	 - notify of a partial memory block freeing
148	kmemleak_free_percpu	 - notify of a percpu memory block freeing
149	kmemleak_update_trace	 - update object allocation stack trace
150	kmemleak_not_leak	 - mark an object as not a leak
151	kmemleak_ignore		 - do not scan or report an object as leak
152	kmemleak_scan_area	 - add scan areas inside a memory block
153	kmemleak_no_scan	 - do not scan a memory block
154	kmemleak_erase		 - erase an old value in a pointer variable
155	kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
156	kmemleak_free_recursive	 - as kmemleak_free but checks the recursiveness
157	
158	Dealing with false positives/negatives
159	--------------------------------------
160	
161	The false negatives are real memory leaks (orphan objects) but not
162	reported by kmemleak because values found during the memory scanning
163	point to such objects. To reduce the number of false negatives, kmemleak
164	provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
165	kmemleak_erase functions (see above). The task stacks also increase the
166	amount of false negatives and their scanning is not enabled by default.
167	
168	The false positives are objects wrongly reported as being memory leaks
169	(orphan). For objects known not to be leaks, kmemleak provides the
170	kmemleak_not_leak function. The kmemleak_ignore could also be used if
171	the memory block is known not to contain other pointers and it will no
172	longer be scanned.
173	
174	Some of the reported leaks are only transient, especially on SMP
175	systems, because of pointers temporarily stored in CPU registers or
176	stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
177	the minimum age of an object to be reported as a memory leak.
178	
179	Limitations and Drawbacks
180	-------------------------
181	
182	The main drawback is the reduced performance of memory allocation and
183	freeing. To avoid other penalties, the memory scanning is only performed
184	when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
185	intended for debugging purposes where the performance might not be the
186	most important requirement.
187	
188	To keep the algorithm simple, kmemleak scans for values pointing to any
189	address inside a block's address range. This may lead to an increased
190	number of false negatives. However, it is likely that a real memory leak
191	will eventually become visible.
192	
193	Another source of false negatives is the data stored in non-pointer
194	values. In a future version, kmemleak could only scan the pointer
195	members in the allocated structures. This feature would solve many of
196	the false negative cases described above.
197	
198	The tool can report false positives. These are cases where an allocated
199	block doesn't need to be freed (some cases in the init_call functions),
200	the pointer is calculated by other methods than the usual container_of
201	macro or the pointer is stored in a location not scanned by kmemleak.
202	
203	Page allocations and ioremap are not tracked.
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