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