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1Kernel Memory Leak Detector
2===========================
3
4Introduction
5------------
6
7Kmemleak provides a way of detecting possible kernel memory leaks in a
8way similar to a tracing garbage collector
ae13c65b 9(https://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),
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10with the difference that the orphan objects are not freed but only
11reported via /sys/kernel/debug/kmemleak. A similar method is used by the
12Valgrind tool (memcheck --leak-check) to detect the memory leaks in
13user-space applications.
4762c984 14Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390, metag and tile.
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15
16Usage
17-----
18
19CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
bab4a34a 20thread scans the memory every 10 minutes (by default) and prints the
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21number of new unreferenced objects found. To display the details of all
22the possible memory leaks:
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23
24 # mount -t debugfs nodev /sys/kernel/debug/
25 # cat /sys/kernel/debug/kmemleak
26
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27To trigger an intermediate memory scan:
28
29 # echo scan > /sys/kernel/debug/kmemleak
30
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31To clear the list of all current possible memory leaks:
32
33 # echo clear > /sys/kernel/debug/kmemleak
34
35New leaks will then come up upon reading /sys/kernel/debug/kmemleak
36again.
37
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38Note that the orphan objects are listed in the order they were allocated
39and one object at the beginning of the list may cause other subsequent
40objects to be reported as orphan.
41
42Memory 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)
e0a2a160 46 stack=on - enable the task stacks scanning (default)
04f70336 47 stack=off - disable the tasks stacks scanning
e0a2a160 48 scan=on - start the automatic memory scanning thread (default)
04f70336 49 scan=off - stop the automatic memory scanning thread
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50 scan=<secs> - set the automatic memory scanning period in seconds
51 (default 600, 0 to stop the automatic scanning)
4698c1f2 52 scan - trigger a memory scan
30b37101 53 clear - clear list of current memory leak suspects, done by
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54 marking all current reported unreferenced objects grey,
55 or free all kmemleak objects if kmemleak has been disabled.
189d84ed 56 dump=<addr> - dump information about the object found at <addr>
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57
58Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
59the kernel command line.
60
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61Memory may be allocated or freed before kmemleak is initialised and
62these actions are stored in an early log buffer. The size of this buffer
63is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
64
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65If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
66disabled by default. Passing "kmemleak=on" on the kernel command
67line enables the function.
68
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69Basic Algorithm
70---------------
71
72The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
73friends are traced and the pointers, together with additional
4762c984 74information like size and stack trace, are stored in a rbtree.
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75The corresponding freeing function calls are tracked and the pointers
76removed from the kmemleak data structures.
77
78An allocated block of memory is considered orphan if no pointer to its
79start address or to any location inside the block can be found by
80scanning the memory (including saved registers). This means that there
81might be no way for the kernel to pass the address of the allocated
82block to a freeing function and therefore the block is considered a
83memory leak.
84
85The 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
4762c984 90 the values against the addresses stored in the rbtree. If
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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
99Some allocated memory blocks have pointers stored in the kernel's
100internal data structures and they cannot be detected as orphans. To
101avoid this, kmemleak can also store the number of values pointing to an
102address inside the block address range that need to be found so that the
103block is not considered a leak. One example is __vmalloc().
104
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105Testing specific sections with kmemleak
106---------------------------------------
107
108Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
109quite extensive. This can also be the case if you have very buggy code
110when 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'
113you can find new unreferenced objects; this should help with testing
114specific sections of code.
115
116To 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
122Then as usual to get your report with:
123
124 # cat /sys/kernel/debug/kmemleak
125
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126Freeing kmemleak internal objects
127---------------------------------
128
abb3b1f8 129To allow access to previously found memory leaks after kmemleak has been
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130disabled by the user or due to an fatal error, internal kmemleak objects
131won't be freed when kmemleak is disabled, and those objects may occupy
132a large part of physical memory.
133
134In this situation, you may reclaim memory with:
135
136 # echo clear > /sys/kernel/debug/kmemleak
137
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138Kmemleak API
139------------
140
141See the include/linux/kmemleak.h header for the functions prototype.
142
143kmemleak_init - initialize kmemleak
144kmemleak_alloc - notify of a memory block allocation
f528f0b8 145kmemleak_alloc_percpu - notify of a percpu memory block allocation
04f70336 146kmemleak_free - notify of a memory block freeing
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147kmemleak_free_part - notify of a partial memory block freeing
148kmemleak_free_percpu - notify of a percpu memory block freeing
ffe2c748 149kmemleak_update_trace - update object allocation stack trace
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150kmemleak_not_leak - mark an object as not a leak
151kmemleak_ignore - do not scan or report an object as leak
152kmemleak_scan_area - add scan areas inside a memory block
153kmemleak_no_scan - do not scan a memory block
154kmemleak_erase - erase an old value in a pointer variable
155kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
156kmemleak_free_recursive - as kmemleak_free but checks the recursiveness
157
158Dealing with false positives/negatives
159--------------------------------------
160
161The false negatives are real memory leaks (orphan objects) but not
162reported by kmemleak because values found during the memory scanning
163point to such objects. To reduce the number of false negatives, kmemleak
164provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
165kmemleak_erase functions (see above). The task stacks also increase the
166amount of false negatives and their scanning is not enabled by default.
167
168The false positives are objects wrongly reported as being memory leaks
169(orphan). For objects known not to be leaks, kmemleak provides the
170kmemleak_not_leak function. The kmemleak_ignore could also be used if
171the memory block is known not to contain other pointers and it will no
172longer be scanned.
173
174Some of the reported leaks are only transient, especially on SMP
175systems, because of pointers temporarily stored in CPU registers or
176stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
177the minimum age of an object to be reported as a memory leak.
178
179Limitations and Drawbacks
180-------------------------
181
182The main drawback is the reduced performance of memory allocation and
183freeing. To avoid other penalties, the memory scanning is only performed
184when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
185intended for debugging purposes where the performance might not be the
186most important requirement.
187
188To keep the algorithm simple, kmemleak scans for values pointing to any
189address inside a block's address range. This may lead to an increased
190number of false negatives. However, it is likely that a real memory leak
191will eventually become visible.
192
193Another source of false negatives is the data stored in non-pointer
194values. In a future version, kmemleak could only scan the pointer
195members in the allocated structures. This feature would solve many of
196the false negative cases described above.
197
198The tool can report false positives. These are cases where an allocated
199block doesn't need to be freed (some cases in the init_call functions),
200the pointer is calculated by other methods than the usual container_of
201macro or the pointer is stored in a location not scanned by kmemleak.
202
21b86bd5 203Page allocations and ioremap are not tracked.