1Kernel Memory Leak Detector 2=========================== 3 4Introduction 5------------ 6 7Kmemleak provides a way of detecting possible kernel memory leaks in a 8way similar to a tracing garbage collector 9(http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors), 10with the difference that the orphan objects are not freed but only 11reported via /sys/kernel/debug/kmemleak. A similar method is used by the 12Valgrind tool (memcheck --leak-check) to detect the memory leaks in 13user-space applications. 14 15Usage 16----- 17 18CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel 19thread scans the memory every 10 minutes (by default) and prints the 20number of new unreferenced objects found. To display the details of all 21the possible memory leaks: 22 23 # mount -t debugfs nodev /sys/kernel/debug/ 24 # cat /sys/kernel/debug/kmemleak 25 26To trigger an intermediate memory scan: 27 28 # echo scan > /sys/kernel/debug/kmemleak 29 30To clear the list of all current possible memory leaks: 31 32 # echo clear > /sys/kernel/debug/kmemleak 33 34New leaks will then come up upon reading /sys/kernel/debug/kmemleak 35again. 36 37Note that the orphan objects are listed in the order they were allocated 38and one object at the beginning of the list may cause other subsequent 39objects to be reported as orphan. 40 41Memory scanning parameters can be modified at run-time by writing to the 42/sys/kernel/debug/kmemleak file. The following parameters are supported: 43 44 off - disable kmemleak (irreversible) 45 stack=on - enable the task stacks scanning (default) 46 stack=off - disable the tasks stacks scanning 47 scan=on - start the automatic memory scanning thread (default) 48 scan=off - stop the automatic memory scanning thread 49 scan=<secs> - set the automatic memory scanning period in seconds 50 (default 600, 0 to stop the automatic scanning) 51 scan - trigger a memory scan 52 clear - clear list of current memory leak suspects, done by 53 marking all current reported unreferenced objects grey 54 dump=<addr> - dump information about the object found at <addr> 55 56Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on 57the kernel command line. 58 59Memory may be allocated or freed before kmemleak is initialised and 60these actions are stored in an early log buffer. The size of this buffer 61is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option. 62 63Basic Algorithm 64--------------- 65 66The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and 67friends are traced and the pointers, together with additional 68information like size and stack trace, are stored in a prio search tree. 69The corresponding freeing function calls are tracked and the pointers 70removed from the kmemleak data structures. 71 72An allocated block of memory is considered orphan if no pointer to its 73start address or to any location inside the block can be found by 74scanning the memory (including saved registers). This means that there 75might be no way for the kernel to pass the address of the allocated 76block to a freeing function and therefore the block is considered a 77memory leak. 78 79The scanning algorithm steps: 80 81 1. mark all objects as white (remaining white objects will later be 82 considered orphan) 83 2. scan the memory starting with the data section and stacks, checking 84 the values against the addresses stored in the prio search tree. If 85 a pointer to a white object is found, the object is added to the 86 gray list 87 3. scan the gray objects for matching addresses (some white objects 88 can become gray and added at the end of the gray list) until the 89 gray set is finished 90 4. the remaining white objects are considered orphan and reported via 91 /sys/kernel/debug/kmemleak 92 93Some allocated memory blocks have pointers stored in the kernel's 94internal data structures and they cannot be detected as orphans. To 95avoid this, kmemleak can also store the number of values pointing to an 96address inside the block address range that need to be found so that the 97block is not considered a leak. One example is __vmalloc(). 98 99Testing specific sections with kmemleak 100--------------------------------------- 101 102Upon initial bootup your /sys/kernel/debug/kmemleak output page may be 103quite extensive. This can also be the case if you have very buggy code 104when doing development. To work around these situations you can use the 105'clear' command to clear all reported unreferenced objects from the 106/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' 107you can find new unreferenced objects; this should help with testing 108specific sections of code. 109 110To test a critical section on demand with a clean kmemleak do: 111 112 # echo clear > /sys/kernel/debug/kmemleak 113 ... test your kernel or modules ... 114 # echo scan > /sys/kernel/debug/kmemleak 115 116Then as usual to get your report with: 117 118 # cat /sys/kernel/debug/kmemleak 119 120Kmemleak API 121------------ 122 123See the include/linux/kmemleak.h header for the functions prototype. 124 125kmemleak_init - initialize kmemleak 126kmemleak_alloc - notify of a memory block allocation 127kmemleak_free - notify of a memory block freeing 128kmemleak_not_leak - mark an object as not a leak 129kmemleak_ignore - do not scan or report an object as leak 130kmemleak_scan_area - add scan areas inside a memory block 131kmemleak_no_scan - do not scan a memory block 132kmemleak_erase - erase an old value in a pointer variable 133kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness 134kmemleak_free_recursive - as kmemleak_free but checks the recursiveness 135 136Dealing with false positives/negatives 137-------------------------------------- 138 139The false negatives are real memory leaks (orphan objects) but not 140reported by kmemleak because values found during the memory scanning 141point to such objects. To reduce the number of false negatives, kmemleak 142provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and 143kmemleak_erase functions (see above). The task stacks also increase the 144amount of false negatives and their scanning is not enabled by default. 145 146The false positives are objects wrongly reported as being memory leaks 147(orphan). For objects known not to be leaks, kmemleak provides the 148kmemleak_not_leak function. The kmemleak_ignore could also be used if 149the memory block is known not to contain other pointers and it will no 150longer be scanned. 151 152Some of the reported leaks are only transient, especially on SMP 153systems, because of pointers temporarily stored in CPU registers or 154stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing 155the minimum age of an object to be reported as a memory leak. 156 157Limitations and Drawbacks 158------------------------- 159 160The main drawback is the reduced performance of memory allocation and 161freeing. To avoid other penalties, the memory scanning is only performed 162when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is 163intended for debugging purposes where the performance might not be the 164most important requirement. 165 166To keep the algorithm simple, kmemleak scans for values pointing to any 167address inside a block's address range. This may lead to an increased 168number of false negatives. However, it is likely that a real memory leak 169will eventually become visible. 170 171Another source of false negatives is the data stored in non-pointer 172values. In a future version, kmemleak could only scan the pointer 173members in the allocated structures. This feature would solve many of 174the false negative cases described above. 175 176The tool can report false positives. These are cases where an allocated 177block doesn't need to be freed (some cases in the init_call functions), 178the pointer is calculated by other methods than the usual container_of 179macro or the pointer is stored in a location not scanned by kmemleak. 180 181Page allocations and ioremap are not tracked. Only the ARM and x86 182architectures are currently supported. 183