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1 | Kernel Memory Leak Detector |
2 | =========================== | |
3 | ||
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4 | Kmemleak provides a way of detecting possible kernel memory leaks in a |
5 | way similar to a tracing garbage collector | |
ae13c65b | 6 | (https://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors), |
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7 | with the difference that the orphan objects are not freed but only |
8 | reported via /sys/kernel/debug/kmemleak. A similar method is used by the | |
ca90a7a3 | 9 | Valgrind tool (``memcheck --leak-check``) to detect the memory leaks in |
04f70336 | 10 | user-space applications. |
6a767685 | 11 | Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile. |
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12 | |
13 | Usage | |
14 | ----- | |
15 | ||
16 | CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel | |
bab4a34a | 17 | thread scans the memory every 10 minutes (by default) and prints the |
4698c1f2 | 18 | number of new unreferenced objects found. To display the details of all |
ca90a7a3 | 19 | the possible memory leaks:: |
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20 | |
21 | # mount -t debugfs nodev /sys/kernel/debug/ | |
22 | # cat /sys/kernel/debug/kmemleak | |
23 | ||
ca90a7a3 | 24 | To trigger an intermediate memory scan:: |
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25 | |
26 | # echo scan > /sys/kernel/debug/kmemleak | |
27 | ||
ca90a7a3 | 28 | To clear the list of all current possible memory leaks:: |
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29 | |
30 | # echo clear > /sys/kernel/debug/kmemleak | |
31 | ||
ca90a7a3 | 32 | New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak`` |
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33 | again. |
34 | ||
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35 | Note that the orphan objects are listed in the order they were allocated |
36 | and one object at the beginning of the list may cause other subsequent | |
37 | objects to be reported as orphan. | |
38 | ||
39 | Memory scanning parameters can be modified at run-time by writing to the | |
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40 | ``/sys/kernel/debug/kmemleak`` file. The following parameters are supported: |
41 | ||
42 | - off | |
43 | disable kmemleak (irreversible) | |
44 | - stack=on | |
45 | enable the task stacks scanning (default) | |
46 | - stack=off | |
47 | disable the tasks stacks scanning | |
48 | - scan=on | |
49 | start the automatic memory scanning thread (default) | |
50 | - scan=off | |
51 | stop the automatic memory scanning thread | |
52 | - scan=<secs> | |
53 | set the automatic memory scanning period in seconds | |
54 | (default 600, 0 to stop the automatic scanning) | |
55 | - scan | |
56 | trigger a memory scan | |
57 | - clear | |
58 | clear list of current memory leak suspects, done by | |
59 | marking all current reported unreferenced objects grey, | |
60 | or free all kmemleak objects if kmemleak has been disabled. | |
61 | - dump=<addr> | |
62 | dump information about the object found at <addr> | |
63 | ||
64 | Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on | |
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65 | the kernel command line. |
66 | ||
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67 | Memory may be allocated or freed before kmemleak is initialised and |
68 | these actions are stored in an early log buffer. The size of this buffer | |
69 | is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option. | |
70 | ||
6808a40d | 71 | If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is |
ca90a7a3 | 72 | disabled by default. Passing ``kmemleak=on`` on the kernel command |
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73 | line enables the function. |
74 | ||
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75 | Basic Algorithm |
76 | --------------- | |
77 | ||
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78 | The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`, |
79 | :c:func:`kmem_cache_alloc` and | |
04f70336 | 80 | friends are traced and the pointers, together with additional |
4762c984 | 81 | information like size and stack trace, are stored in a rbtree. |
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82 | The corresponding freeing function calls are tracked and the pointers |
83 | removed from the kmemleak data structures. | |
84 | ||
85 | An allocated block of memory is considered orphan if no pointer to its | |
86 | start address or to any location inside the block can be found by | |
87 | scanning the memory (including saved registers). This means that there | |
88 | might be no way for the kernel to pass the address of the allocated | |
89 | block to a freeing function and therefore the block is considered a | |
90 | memory leak. | |
91 | ||
92 | The scanning algorithm steps: | |
93 | ||
94 | 1. mark all objects as white (remaining white objects will later be | |
95 | considered orphan) | |
96 | 2. scan the memory starting with the data section and stacks, checking | |
4762c984 | 97 | the values against the addresses stored in the rbtree. If |
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98 | a pointer to a white object is found, the object is added to the |
99 | gray list | |
100 | 3. scan the gray objects for matching addresses (some white objects | |
101 | can become gray and added at the end of the gray list) until the | |
102 | gray set is finished | |
103 | 4. the remaining white objects are considered orphan and reported via | |
104 | /sys/kernel/debug/kmemleak | |
105 | ||
106 | Some allocated memory blocks have pointers stored in the kernel's | |
107 | internal data structures and they cannot be detected as orphans. To | |
108 | avoid this, kmemleak can also store the number of values pointing to an | |
109 | address inside the block address range that need to be found so that the | |
110 | block is not considered a leak. One example is __vmalloc(). | |
111 | ||
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112 | Testing specific sections with kmemleak |
113 | --------------------------------------- | |
114 | ||
115 | Upon initial bootup your /sys/kernel/debug/kmemleak output page may be | |
116 | quite extensive. This can also be the case if you have very buggy code | |
117 | when doing development. To work around these situations you can use the | |
118 | 'clear' command to clear all reported unreferenced objects from the | |
119 | /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' | |
120 | you can find new unreferenced objects; this should help with testing | |
121 | specific sections of code. | |
122 | ||
ca90a7a3 | 123 | To test a critical section on demand with a clean kmemleak do:: |
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124 | |
125 | # echo clear > /sys/kernel/debug/kmemleak | |
126 | ... test your kernel or modules ... | |
127 | # echo scan > /sys/kernel/debug/kmemleak | |
128 | ||
ca90a7a3 | 129 | Then as usual to get your report with:: |
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130 | |
131 | # cat /sys/kernel/debug/kmemleak | |
132 | ||
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133 | Freeing kmemleak internal objects |
134 | --------------------------------- | |
135 | ||
abb3b1f8 | 136 | To allow access to previously found memory leaks after kmemleak has been |
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137 | disabled by the user or due to an fatal error, internal kmemleak objects |
138 | won't be freed when kmemleak is disabled, and those objects may occupy | |
139 | a large part of physical memory. | |
140 | ||
ca90a7a3 | 141 | In this situation, you may reclaim memory with:: |
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142 | |
143 | # echo clear > /sys/kernel/debug/kmemleak | |
144 | ||
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145 | Kmemleak API |
146 | ------------ | |
147 | ||
148 | See the include/linux/kmemleak.h header for the functions prototype. | |
149 | ||
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150 | - ``kmemleak_init`` - initialize kmemleak |
151 | - ``kmemleak_alloc`` - notify of a memory block allocation | |
152 | - ``kmemleak_alloc_percpu`` - notify of a percpu memory block allocation | |
94f4a161 | 153 | - ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation |
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154 | - ``kmemleak_free`` - notify of a memory block freeing |
155 | - ``kmemleak_free_part`` - notify of a partial memory block freeing | |
156 | - ``kmemleak_free_percpu`` - notify of a percpu memory block freeing | |
157 | - ``kmemleak_update_trace`` - update object allocation stack trace | |
158 | - ``kmemleak_not_leak`` - mark an object as not a leak | |
159 | - ``kmemleak_ignore`` - do not scan or report an object as leak | |
160 | - ``kmemleak_scan_area`` - add scan areas inside a memory block | |
161 | - ``kmemleak_no_scan`` - do not scan a memory block | |
162 | - ``kmemleak_erase`` - erase an old value in a pointer variable | |
163 | - ``kmemleak_alloc_recursive`` - as kmemleak_alloc but checks the recursiveness | |
164 | - ``kmemleak_free_recursive`` - as kmemleak_free but checks the recursiveness | |
04f70336 | 165 | |
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166 | The following functions take a physical address as the object pointer |
167 | and only perform the corresponding action if the address has a lowmem | |
168 | mapping: | |
169 | ||
170 | - ``kmemleak_alloc_phys`` | |
171 | - ``kmemleak_free_part_phys`` | |
172 | - ``kmemleak_not_leak_phys`` | |
173 | - ``kmemleak_ignore_phys`` | |
174 | ||
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175 | Dealing with false positives/negatives |
176 | -------------------------------------- | |
177 | ||
178 | The false negatives are real memory leaks (orphan objects) but not | |
179 | reported by kmemleak because values found during the memory scanning | |
180 | point to such objects. To reduce the number of false negatives, kmemleak | |
181 | provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and | |
182 | kmemleak_erase functions (see above). The task stacks also increase the | |
183 | amount of false negatives and their scanning is not enabled by default. | |
184 | ||
185 | The false positives are objects wrongly reported as being memory leaks | |
186 | (orphan). For objects known not to be leaks, kmemleak provides the | |
187 | kmemleak_not_leak function. The kmemleak_ignore could also be used if | |
188 | the memory block is known not to contain other pointers and it will no | |
189 | longer be scanned. | |
190 | ||
191 | Some of the reported leaks are only transient, especially on SMP | |
192 | systems, because of pointers temporarily stored in CPU registers or | |
193 | stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing | |
194 | the minimum age of an object to be reported as a memory leak. | |
195 | ||
196 | Limitations and Drawbacks | |
197 | ------------------------- | |
198 | ||
199 | The main drawback is the reduced performance of memory allocation and | |
200 | freeing. To avoid other penalties, the memory scanning is only performed | |
201 | when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is | |
202 | intended for debugging purposes where the performance might not be the | |
203 | most important requirement. | |
204 | ||
205 | To keep the algorithm simple, kmemleak scans for values pointing to any | |
206 | address inside a block's address range. This may lead to an increased | |
207 | number of false negatives. However, it is likely that a real memory leak | |
208 | will eventually become visible. | |
209 | ||
210 | Another source of false negatives is the data stored in non-pointer | |
211 | values. In a future version, kmemleak could only scan the pointer | |
212 | members in the allocated structures. This feature would solve many of | |
213 | the false negative cases described above. | |
214 | ||
215 | The tool can report false positives. These are cases where an allocated | |
216 | block doesn't need to be freed (some cases in the init_call functions), | |
217 | the pointer is calculated by other methods than the usual container_of | |
218 | macro or the pointer is stored in a location not scanned by kmemleak. | |
219 | ||
21b86bd5 | 220 | Page allocations and ioremap are not tracked. |