Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 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 (https://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, ppc, mips, s390, metag 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 or free all kmemleak objects if kmemleak has been disabled. 56 dump=<addr> - dump information about the object found at <addr> 57 58 Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on 59 the kernel command line. 60 61 Memory may be allocated or freed before kmemleak is initialised and 62 these actions are stored in an early log buffer. The size of this buffer 63 is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option. 64 65 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is 66 disabled by default. Passing "kmemleak=on" on the kernel command 67 line enables the function. 68 69 Basic Algorithm 70 --------------- 71 72 The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and 73 friends are traced and the pointers, together with additional 74 information like size and stack trace, are stored in a rbtree. 75 The corresponding freeing function calls are tracked and the pointers 76 removed from the kmemleak data structures. 77 78 An allocated block of memory is considered orphan if no pointer to its 79 start address or to any location inside the block can be found by 80 scanning the memory (including saved registers). This means that there 81 might be no way for the kernel to pass the address of the allocated 82 block to a freeing function and therefore the block is considered a 83 memory leak. 84 85 The scanning algorithm steps: 86 87 1. mark all objects as white (remaining white objects will later be 88 considered orphan) 89 2. scan the memory starting with the data section and stacks, checking 90 the values against the addresses stored in the rbtree. If 91 a pointer to a white object is found, the object is added to the 92 gray list 93 3. scan the gray objects for matching addresses (some white objects 94 can become gray and added at the end of the gray list) until the 95 gray set is finished 96 4. the remaining white objects are considered orphan and reported via 97 /sys/kernel/debug/kmemleak 98 99 Some allocated memory blocks have pointers stored in the kernel's 100 internal data structures and they cannot be detected as orphans. To 101 avoid this, kmemleak can also store the number of values pointing to an 102 address inside the block address range that need to be found so that the 103 block is not considered a leak. One example is __vmalloc(). 104 105 Testing specific sections with kmemleak 106 --------------------------------------- 107 108 Upon initial bootup your /sys/kernel/debug/kmemleak output page may be 109 quite extensive. This can also be the case if you have very buggy code 110 when doing development. To work around these situations you can use the 111 'clear' command to clear all reported unreferenced objects from the 112 /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' 113 you can find new unreferenced objects; this should help with testing 114 specific sections of code. 115 116 To test a critical section on demand with a clean kmemleak do: 117 118 # echo clear > /sys/kernel/debug/kmemleak 119 ... test your kernel or modules ... 120 # echo scan > /sys/kernel/debug/kmemleak 121 122 Then as usual to get your report with: 123 124 # cat /sys/kernel/debug/kmemleak 125 126 Freeing kmemleak internal objects 127 --------------------------------- 128 129 To allow access to previously found memory leaks after kmemleak has been 130 disabled by the user or due to an fatal error, internal kmemleak objects 131 won't be freed when kmemleak is disabled, and those objects may occupy 132 a large part of physical memory. 133 134 In this situation, you may reclaim memory with: 135 136 # echo clear > /sys/kernel/debug/kmemleak 137 138 Kmemleak API 139 ------------ 140 141 See the include/linux/kmemleak.h header for the functions prototype. 142 143 kmemleak_init - initialize kmemleak 144 kmemleak_alloc - notify of a memory block allocation 145 kmemleak_alloc_percpu - notify of a percpu memory block allocation 146 kmemleak_free - notify of a memory block freeing 147 kmemleak_free_part - notify of a partial memory block freeing 148 kmemleak_free_percpu - notify of a percpu memory block freeing 149 kmemleak_update_trace - update object allocation stack trace 150 kmemleak_not_leak - mark an object as not a leak 151 kmemleak_ignore - do not scan or report an object as leak 152 kmemleak_scan_area - add scan areas inside a memory block 153 kmemleak_no_scan - do not scan a memory block 154 kmemleak_erase - erase an old value in a pointer variable 155 kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness 156 kmemleak_free_recursive - as kmemleak_free but checks the recursiveness 157 158 Dealing with false positives/negatives 159 -------------------------------------- 160 161 The false negatives are real memory leaks (orphan objects) but not 162 reported by kmemleak because values found during the memory scanning 163 point to such objects. To reduce the number of false negatives, kmemleak 164 provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and 165 kmemleak_erase functions (see above). The task stacks also increase the 166 amount of false negatives and their scanning is not enabled by default. 167 168 The false positives are objects wrongly reported as being memory leaks 169 (orphan). For objects known not to be leaks, kmemleak provides the 170 kmemleak_not_leak function. The kmemleak_ignore could also be used if 171 the memory block is known not to contain other pointers and it will no 172 longer be scanned. 173 174 Some of the reported leaks are only transient, especially on SMP 175 systems, because of pointers temporarily stored in CPU registers or 176 stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing 177 the minimum age of an object to be reported as a memory leak. 178 179 Limitations and Drawbacks 180 ------------------------- 181 182 The main drawback is the reduced performance of memory allocation and 183 freeing. To avoid other penalties, the memory scanning is only performed 184 when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is 185 intended for debugging purposes where the performance might not be the 186 most important requirement. 187 188 To keep the algorithm simple, kmemleak scans for values pointing to any 189 address inside a block's address range. This may lead to an increased 190 number of false negatives. However, it is likely that a real memory leak 191 will eventually become visible. 192 193 Another source of false negatives is the data stored in non-pointer 194 values. In a future version, kmemleak could only scan the pointer 195 members in the allocated structures. This feature would solve many of 196 the false negative cases described above. 197 198 The tool can report false positives. These are cases where an allocated 199 block doesn't need to be freed (some cases in the init_call functions), 200 the pointer is calculated by other methods than the usual container_of 201 macro or the pointer is stored in a location not scanned by kmemleak. 202 203 Page allocations and ioremap are not tracked.