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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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	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	Basic Algorithm
66	---------------
67	
68	The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
69	friends are traced and the pointers, together with additional
70	information like size and stack trace, are stored in a rbtree.
71	The corresponding freeing function calls are tracked and the pointers
72	removed from the kmemleak data structures.
73	
74	An allocated block of memory is considered orphan if no pointer to its
75	start address or to any location inside the block can be found by
76	scanning the memory (including saved registers). This means that there
77	might be no way for the kernel to pass the address of the allocated
78	block to a freeing function and therefore the block is considered a
79	memory leak.
80	
81	The scanning algorithm steps:
82	
83	  1. mark all objects as white (remaining white objects will later be
84	     considered orphan)
85	  2. scan the memory starting with the data section and stacks, checking
86	     the values against the addresses stored in the rbtree. If
87	     a pointer to a white object is found, the object is added to the
88	     gray list
89	  3. scan the gray objects for matching addresses (some white objects
90	     can become gray and added at the end of the gray list) until the
91	     gray set is finished
92	  4. the remaining white objects are considered orphan and reported via
93	     /sys/kernel/debug/kmemleak
94	
95	Some allocated memory blocks have pointers stored in the kernel's
96	internal data structures and they cannot be detected as orphans. To
97	avoid this, kmemleak can also store the number of values pointing to an
98	address inside the block address range that need to be found so that the
99	block is not considered a leak. One example is __vmalloc().
100	
101	Testing specific sections with kmemleak
102	---------------------------------------
103	
104	Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
105	quite extensive. This can also be the case if you have very buggy code
106	when doing development. To work around these situations you can use the
107	'clear' command to clear all reported unreferenced objects from the
108	/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
109	you can find new unreferenced objects; this should help with testing
110	specific sections of code.
111	
112	To test a critical section on demand with a clean kmemleak do:
113	
114	  # echo clear > /sys/kernel/debug/kmemleak
115	  ... test your kernel or modules ...
116	  # echo scan > /sys/kernel/debug/kmemleak
117	
118	Then as usual to get your report with:
119	
120	  # cat /sys/kernel/debug/kmemleak
121	
122	Freeing kmemleak internal objects
123	---------------------------------
124	
125	To allow access to previosuly found memory leaks after kmemleak has been
126	disabled by the user or due to an fatal error, internal kmemleak objects
127	won't be freed when kmemleak is disabled, and those objects may occupy
128	a large part of physical memory.
129	
130	In this situation, you may reclaim memory with:
131	
132	  # echo clear > /sys/kernel/debug/kmemleak
133	
134	Kmemleak API
135	------------
136	
137	See the include/linux/kmemleak.h header for the functions prototype.
138	
139	kmemleak_init		 - initialize kmemleak
140	kmemleak_alloc		 - notify of a memory block allocation
141	kmemleak_alloc_percpu	 - notify of a percpu memory block allocation
142	kmemleak_free		 - notify of a memory block freeing
143	kmemleak_free_part	 - notify of a partial memory block freeing
144	kmemleak_free_percpu	 - notify of a percpu memory block freeing
145	kmemleak_not_leak	 - mark an object as not a leak
146	kmemleak_ignore		 - do not scan or report an object as leak
147	kmemleak_scan_area	 - add scan areas inside a memory block
148	kmemleak_no_scan	 - do not scan a memory block
149	kmemleak_erase		 - erase an old value in a pointer variable
150	kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
151	kmemleak_free_recursive	 - as kmemleak_free but checks the recursiveness
152	
153	Dealing with false positives/negatives
154	--------------------------------------
155	
156	The false negatives are real memory leaks (orphan objects) but not
157	reported by kmemleak because values found during the memory scanning
158	point to such objects. To reduce the number of false negatives, kmemleak
159	provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
160	kmemleak_erase functions (see above). The task stacks also increase the
161	amount of false negatives and their scanning is not enabled by default.
162	
163	The false positives are objects wrongly reported as being memory leaks
164	(orphan). For objects known not to be leaks, kmemleak provides the
165	kmemleak_not_leak function. The kmemleak_ignore could also be used if
166	the memory block is known not to contain other pointers and it will no
167	longer be scanned.
168	
169	Some of the reported leaks are only transient, especially on SMP
170	systems, because of pointers temporarily stored in CPU registers or
171	stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
172	the minimum age of an object to be reported as a memory leak.
173	
174	Limitations and Drawbacks
175	-------------------------
176	
177	The main drawback is the reduced performance of memory allocation and
178	freeing. To avoid other penalties, the memory scanning is only performed
179	when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
180	intended for debugging purposes where the performance might not be the
181	most important requirement.
182	
183	To keep the algorithm simple, kmemleak scans for values pointing to any
184	address inside a block's address range. This may lead to an increased
185	number of false negatives. However, it is likely that a real memory leak
186	will eventually become visible.
187	
188	Another source of false negatives is the data stored in non-pointer
189	values. In a future version, kmemleak could only scan the pointer
190	members in the allocated structures. This feature would solve many of
191	the false negative cases described above.
192	
193	The tool can report false positives. These are cases where an allocated
194	block doesn't need to be freed (some cases in the init_call functions),
195	the pointer is calculated by other methods than the usual container_of
196	macro or the pointer is stored in a location not scanned by kmemleak.
197	
198	Page allocations and ioremap are not tracked.
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