Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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 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 Basic Algorithm 66 --------------- 67 68 The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and 69 friends are traced and the pointers, together with additional 70 information like size and stack trace, are stored in a rbtree. 71 The corresponding freeing function calls are tracked and the pointers 72 removed from the kmemleak data structures. 73 74 An allocated block of memory is considered orphan if no pointer to its 75 start address or to any location inside the block can be found by 76 scanning the memory (including saved registers). This means that there 77 might be no way for the kernel to pass the address of the allocated 78 block to a freeing function and therefore the block is considered a 79 memory leak. 80 81 The scanning algorithm steps: 82 83 1. mark all objects as white (remaining white objects will later be 84 considered orphan) 85 2. scan the memory starting with the data section and stacks, checking 86 the values against the addresses stored in the rbtree. If 87 a pointer to a white object is found, the object is added to the 88 gray list 89 3. scan the gray objects for matching addresses (some white objects 90 can become gray and added at the end of the gray list) until the 91 gray set is finished 92 4. the remaining white objects are considered orphan and reported via 93 /sys/kernel/debug/kmemleak 94 95 Some allocated memory blocks have pointers stored in the kernel's 96 internal data structures and they cannot be detected as orphans. To 97 avoid this, kmemleak can also store the number of values pointing to an 98 address inside the block address range that need to be found so that the 99 block is not considered a leak. One example is __vmalloc(). 100 101 Testing specific sections with kmemleak 102 --------------------------------------- 103 104 Upon initial bootup your /sys/kernel/debug/kmemleak output page may be 105 quite extensive. This can also be the case if you have very buggy code 106 when doing development. To work around these situations you can use the 107 'clear' command to clear all reported unreferenced objects from the 108 /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' 109 you can find new unreferenced objects; this should help with testing 110 specific sections of code. 111 112 To test a critical section on demand with a clean kmemleak do: 113 114 # echo clear > /sys/kernel/debug/kmemleak 115 ... test your kernel or modules ... 116 # echo scan > /sys/kernel/debug/kmemleak 117 118 Then as usual to get your report with: 119 120 # cat /sys/kernel/debug/kmemleak 121 122 Freeing kmemleak internal objects 123 --------------------------------- 124 125 To allow access to previosuly found memory leaks after kmemleak has been 126 disabled by the user or due to an fatal error, internal kmemleak objects 127 won't be freed when kmemleak is disabled, and those objects may occupy 128 a large part of physical memory. 129 130 In this situation, you may reclaim memory with: 131 132 # echo clear > /sys/kernel/debug/kmemleak 133 134 Kmemleak API 135 ------------ 136 137 See the include/linux/kmemleak.h header for the functions prototype. 138 139 kmemleak_init - initialize kmemleak 140 kmemleak_alloc - notify of a memory block allocation 141 kmemleak_alloc_percpu - notify of a percpu memory block allocation 142 kmemleak_free - notify of a memory block freeing 143 kmemleak_free_part - notify of a partial memory block freeing 144 kmemleak_free_percpu - notify of a percpu memory block freeing 145 kmemleak_not_leak - mark an object as not a leak 146 kmemleak_ignore - do not scan or report an object as leak 147 kmemleak_scan_area - add scan areas inside a memory block 148 kmemleak_no_scan - do not scan a memory block 149 kmemleak_erase - erase an old value in a pointer variable 150 kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness 151 kmemleak_free_recursive - as kmemleak_free but checks the recursiveness 152 153 Dealing with false positives/negatives 154 -------------------------------------- 155 156 The false negatives are real memory leaks (orphan objects) but not 157 reported by kmemleak because values found during the memory scanning 158 point to such objects. To reduce the number of false negatives, kmemleak 159 provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and 160 kmemleak_erase functions (see above). The task stacks also increase the 161 amount of false negatives and their scanning is not enabled by default. 162 163 The false positives are objects wrongly reported as being memory leaks 164 (orphan). For objects known not to be leaks, kmemleak provides the 165 kmemleak_not_leak function. The kmemleak_ignore could also be used if 166 the memory block is known not to contain other pointers and it will no 167 longer be scanned. 168 169 Some of the reported leaks are only transient, especially on SMP 170 systems, because of pointers temporarily stored in CPU registers or 171 stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing 172 the minimum age of an object to be reported as a memory leak. 173 174 Limitations and Drawbacks 175 ------------------------- 176 177 The main drawback is the reduced performance of memory allocation and 178 freeing. To avoid other penalties, the memory scanning is only performed 179 when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is 180 intended for debugging purposes where the performance might not be the 181 most important requirement. 182 183 To keep the algorithm simple, kmemleak scans for values pointing to any 184 address inside a block's address range. This may lead to an increased 185 number of false negatives. However, it is likely that a real memory leak 186 will eventually become visible. 187 188 Another source of false negatives is the data stored in non-pointer 189 values. In a future version, kmemleak could only scan the pointer 190 members in the allocated structures. This feature would solve many of 191 the false negative cases described above. 192 193 The tool can report false positives. These are cases where an allocated 194 block doesn't need to be freed (some cases in the init_call functions), 195 the pointer is calculated by other methods than the usual container_of 196 macro or the pointer is stored in a location not scanned by kmemleak. 197 198 Page allocations and ioremap are not tracked.