1 Kernel Memory Leak Detector
2 ===========================
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.
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:
23 # mount -t debugfs nodev /sys/kernel/debug/
24 # cat /sys/kernel/debug/kmemleak
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.
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:
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
41 Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
42 the kernel command line.
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.
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
60 The scanning algorithm steps:
62 1. mark all objects as white (remaining white objects will later be
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
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
71 4. the remaining white objects are considered orphan and reported via
72 /sys/kernel/debug/kmemleak
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().
83 See the include/linux/kmemleak.h header for the functions prototype.
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
96 Dealing with false positives/negatives
97 --------------------------------------
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.
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
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.
117 Limitations and Drawbacks
118 -------------------------
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.
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.
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.
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.
141 Page allocations and ioremap are not tracked. Only the ARM and x86
142 architectures are currently supported.