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04f70336 CM |
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 any new | |
20 | unreferenced objects found. To trigger an intermediate scan and display | |
21 | all the possible memory leaks: | |
22 | ||
23 | # mount -t debugfs nodev /sys/kernel/debug/ | |
24 | # cat /sys/kernel/debug/kmemleak | |
25 | ||
26 | Note that the orphan objects are listed in the order they were allocated | |
27 | and one object at the beginning of the list may cause other subsequent | |
28 | objects to be reported as orphan. | |
29 | ||
30 | Memory scanning parameters can be modified at run-time by writing to the | |
31 | /sys/kernel/debug/kmemleak file. The following parameters are supported: | |
32 | ||
33 | off - disable kmemleak (irreversible) | |
34 | stack=on - enable the task stacks scanning | |
35 | stack=off - disable the tasks stacks scanning | |
36 | scan=on - start the automatic memory scanning thread | |
37 | scan=off - stop the automatic memory scanning thread | |
38 | scan=<secs> - set the automatic memory scanning period in seconds (0 | |
39 | to disable it) | |
40 | ||
41 | Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on | |
42 | the kernel command line. | |
43 | ||
44 | Basic Algorithm | |
45 | --------------- | |
46 | ||
47 | The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and | |
48 | friends are traced and the pointers, together with additional | |
49 | information like size and stack trace, are stored in a prio search tree. | |
50 | The corresponding freeing function calls are tracked and the pointers | |
51 | removed from the kmemleak data structures. | |
52 | ||
53 | An allocated block of memory is considered orphan if no pointer to its | |
54 | start address or to any location inside the block can be found by | |
55 | scanning the memory (including saved registers). This means that there | |
56 | might be no way for the kernel to pass the address of the allocated | |
57 | block to a freeing function and therefore the block is considered a | |
58 | memory leak. | |
59 | ||
60 | The scanning algorithm steps: | |
61 | ||
62 | 1. mark all objects as white (remaining white objects will later be | |
63 | considered orphan) | |
64 | 2. scan the memory starting with the data section and stacks, checking | |
65 | the values against the addresses stored in the prio search tree. If | |
66 | a pointer to a white object is found, the object is added to the | |
67 | gray list | |
68 | 3. scan the gray objects for matching addresses (some white objects | |
69 | can become gray and added at the end of the gray list) until the | |
70 | gray set is finished | |
71 | 4. the remaining white objects are considered orphan and reported via | |
72 | /sys/kernel/debug/kmemleak | |
73 | ||
74 | Some allocated memory blocks have pointers stored in the kernel's | |
75 | internal data structures and they cannot be detected as orphans. To | |
76 | avoid this, kmemleak can also store the number of values pointing to an | |
77 | address inside the block address range that need to be found so that the | |
78 | block is not considered a leak. One example is __vmalloc(). | |
79 | ||
80 | Kmemleak API | |
81 | ------------ | |
82 | ||
83 | See the include/linux/kmemleak.h header for the functions prototype. | |
84 | ||
85 | kmemleak_init - initialize kmemleak | |
86 | kmemleak_alloc - notify of a memory block allocation | |
87 | kmemleak_free - notify of a memory block freeing | |
88 | kmemleak_not_leak - mark an object as not a leak | |
89 | kmemleak_ignore - do not scan or report an object as leak | |
90 | kmemleak_scan_area - add scan areas inside a memory block | |
91 | kmemleak_no_scan - do not scan a memory block | |
92 | kmemleak_erase - erase an old value in a pointer variable | |
93 | kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness | |
94 | kmemleak_free_recursive - as kmemleak_free but checks the recursiveness | |
95 | ||
96 | Dealing with false positives/negatives | |
97 | -------------------------------------- | |
98 | ||
99 | The false negatives are real memory leaks (orphan objects) but not | |
100 | reported by kmemleak because values found during the memory scanning | |
101 | point to such objects. To reduce the number of false negatives, kmemleak | |
102 | provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and | |
103 | kmemleak_erase functions (see above). The task stacks also increase the | |
104 | amount of false negatives and their scanning is not enabled by default. | |
105 | ||
106 | The false positives are objects wrongly reported as being memory leaks | |
107 | (orphan). For objects known not to be leaks, kmemleak provides the | |
108 | kmemleak_not_leak function. The kmemleak_ignore could also be used if | |
109 | the memory block is known not to contain other pointers and it will no | |
110 | longer be scanned. | |
111 | ||
112 | Some of the reported leaks are only transient, especially on SMP | |
113 | systems, because of pointers temporarily stored in CPU registers or | |
114 | stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing | |
115 | the minimum age of an object to be reported as a memory leak. | |
116 | ||
117 | Limitations and Drawbacks | |
118 | ------------------------- | |
119 | ||
120 | The main drawback is the reduced performance of memory allocation and | |
121 | freeing. To avoid other penalties, the memory scanning is only performed | |
122 | when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is | |
123 | intended for debugging purposes where the performance might not be the | |
124 | most important requirement. | |
125 | ||
126 | To keep the algorithm simple, kmemleak scans for values pointing to any | |
127 | address inside a block's address range. This may lead to an increased | |
128 | number of false negatives. However, it is likely that a real memory leak | |
129 | will eventually become visible. | |
130 | ||
131 | Another source of false negatives is the data stored in non-pointer | |
132 | values. In a future version, kmemleak could only scan the pointer | |
133 | members in the allocated structures. This feature would solve many of | |
134 | the false negative cases described above. | |
135 | ||
136 | The tool can report false positives. These are cases where an allocated | |
137 | block doesn't need to be freed (some cases in the init_call functions), | |
138 | the pointer is calculated by other methods than the usual container_of | |
139 | macro or the pointer is stored in a location not scanned by kmemleak. | |
140 | ||
141 | Page allocations and ioremap are not tracked. Only the ARM and x86 | |
142 | architectures are currently supported. |