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