Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.
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 Please check DEBUG_KMEMLEAK dependencies in lib/Kconfig.debug for supported 16 architectures. 17 18 Usage 19 ----- 20 21 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel 22 thread scans the memory every 10 minutes (by default) and prints the 23 number of new unreferenced objects found. To display the details of all 24 the possible memory leaks: 25 26 # mount -t debugfs nodev /sys/kernel/debug/ 27 # cat /sys/kernel/debug/kmemleak 28 29 To trigger an intermediate memory scan: 30 31 # echo scan > /sys/kernel/debug/kmemleak 32 33 To clear the list of all current possible memory leaks: 34 35 # echo clear > /sys/kernel/debug/kmemleak 36 37 New leaks will then come up upon reading /sys/kernel/debug/kmemleak 38 again. 39 40 Note that the orphan objects are listed in the order they were allocated 41 and one object at the beginning of the list may cause other subsequent 42 objects to be reported as orphan. 43 44 Memory scanning parameters can be modified at run-time by writing to the 45 /sys/kernel/debug/kmemleak file. The following parameters are supported: 46 47 off - disable kmemleak (irreversible) 48 stack=on - enable the task stacks scanning (default) 49 stack=off - disable the tasks stacks scanning 50 scan=on - start the automatic memory scanning thread (default) 51 scan=off - stop the automatic memory scanning thread 52 scan=<secs> - set the automatic memory scanning period in seconds 53 (default 600, 0 to stop the automatic scanning) 54 scan - trigger a memory scan 55 clear - clear list of current memory leak suspects, done by 56 marking all current reported unreferenced objects grey 57 dump=<addr> - dump information about the object found at <addr> 58 59 Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on 60 the kernel command line. 61 62 Memory may be allocated or freed before kmemleak is initialised and 63 these actions are stored in an early log buffer. The size of this buffer 64 is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option. 65 66 Basic Algorithm 67 --------------- 68 69 The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and 70 friends are traced and the pointers, together with additional 71 information like size and stack trace, are stored in a prio search tree. 72 The corresponding freeing function calls are tracked and the pointers 73 removed from the kmemleak data structures. 74 75 An allocated block of memory is considered orphan if no pointer to its 76 start address or to any location inside the block can be found by 77 scanning the memory (including saved registers). This means that there 78 might be no way for the kernel to pass the address of the allocated 79 block to a freeing function and therefore the block is considered a 80 memory leak. 81 82 The scanning algorithm steps: 83 84 1. mark all objects as white (remaining white objects will later be 85 considered orphan) 86 2. scan the memory starting with the data section and stacks, checking 87 the values against the addresses stored in the prio search tree. If 88 a pointer to a white object is found, the object is added to the 89 gray list 90 3. scan the gray objects for matching addresses (some white objects 91 can become gray and added at the end of the gray list) until the 92 gray set is finished 93 4. the remaining white objects are considered orphan and reported via 94 /sys/kernel/debug/kmemleak 95 96 Some allocated memory blocks have pointers stored in the kernel's 97 internal data structures and they cannot be detected as orphans. To 98 avoid this, kmemleak can also store the number of values pointing to an 99 address inside the block address range that need to be found so that the 100 block is not considered a leak. One example is __vmalloc(). 101 102 Testing specific sections with kmemleak 103 --------------------------------------- 104 105 Upon initial bootup your /sys/kernel/debug/kmemleak output page may be 106 quite extensive. This can also be the case if you have very buggy code 107 when doing development. To work around these situations you can use the 108 'clear' command to clear all reported unreferenced objects from the 109 /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' 110 you can find new unreferenced objects; this should help with testing 111 specific sections of code. 112 113 To test a critical section on demand with a clean kmemleak do: 114 115 # echo clear > /sys/kernel/debug/kmemleak 116 ... test your kernel or modules ... 117 # echo scan > /sys/kernel/debug/kmemleak 118 119 Then as usual to get your report with: 120 121 # cat /sys/kernel/debug/kmemleak 122 123 Kmemleak API 124 ------------ 125 126 See the include/linux/kmemleak.h header for the functions prototype. 127 128 kmemleak_init - initialize kmemleak 129 kmemleak_alloc - notify of a memory block allocation 130 kmemleak_alloc_percpu - notify of a percpu memory block allocation 131 kmemleak_free - notify of a memory block freeing 132 kmemleak_free_part - notify of a partial memory block freeing 133 kmemleak_free_percpu - notify of a percpu memory block freeing 134 kmemleak_not_leak - mark an object as not a leak 135 kmemleak_ignore - do not scan or report an object as leak 136 kmemleak_scan_area - add scan areas inside a memory block 137 kmemleak_no_scan - do not scan a memory block 138 kmemleak_erase - erase an old value in a pointer variable 139 kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness 140 kmemleak_free_recursive - as kmemleak_free but checks the recursiveness 141 142 Dealing with false positives/negatives 143 -------------------------------------- 144 145 The false negatives are real memory leaks (orphan objects) but not 146 reported by kmemleak because values found during the memory scanning 147 point to such objects. To reduce the number of false negatives, kmemleak 148 provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and 149 kmemleak_erase functions (see above). The task stacks also increase the 150 amount of false negatives and their scanning is not enabled by default. 151 152 The false positives are objects wrongly reported as being memory leaks 153 (orphan). For objects known not to be leaks, kmemleak provides the 154 kmemleak_not_leak function. The kmemleak_ignore could also be used if 155 the memory block is known not to contain other pointers and it will no 156 longer be scanned. 157 158 Some of the reported leaks are only transient, especially on SMP 159 systems, because of pointers temporarily stored in CPU registers or 160 stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing 161 the minimum age of an object to be reported as a memory leak. 162 163 Limitations and Drawbacks 164 ------------------------- 165 166 The main drawback is the reduced performance of memory allocation and 167 freeing. To avoid other penalties, the memory scanning is only performed 168 when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is 169 intended for debugging purposes where the performance might not be the 170 most important requirement. 171 172 To keep the algorithm simple, kmemleak scans for values pointing to any 173 address inside a block's address range. This may lead to an increased 174 number of false negatives. However, it is likely that a real memory leak 175 will eventually become visible. 176 177 Another source of false negatives is the data stored in non-pointer 178 values. In a future version, kmemleak could only scan the pointer 179 members in the allocated structures. This feature would solve many of 180 the false negative cases described above. 181 182 The tool can report false positives. These are cases where an allocated 183 block doesn't need to be freed (some cases in the init_call functions), 184 the pointer is calculated by other methods than the usual container_of 185 macro or the pointer is stored in a location not scanned by kmemleak. 186 187 Page allocations and ioremap are not tracked.