linux/Documentation/kmemleak.txt
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   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
  15Please check DEBUG_KMEMLEAK dependencies in lib/Kconfig.debug for supported
  16architectures.
  17
  18Usage
  19-----
  20
  21CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
  22thread scans the memory every 10 minutes (by default) and prints the
  23number of new unreferenced objects found. To display the details of all
  24the possible memory leaks:
  25
  26  # mount -t debugfs nodev /sys/kernel/debug/
  27  # cat /sys/kernel/debug/kmemleak
  28
  29To trigger an intermediate memory scan:
  30
  31  # echo scan > /sys/kernel/debug/kmemleak
  32
  33To clear the list of all current possible memory leaks:
  34
  35  # echo clear > /sys/kernel/debug/kmemleak
  36
  37New leaks will then come up upon reading /sys/kernel/debug/kmemleak
  38again.
  39
  40Note that the orphan objects are listed in the order they were allocated
  41and one object at the beginning of the list may cause other subsequent
  42objects to be reported as orphan.
  43
  44Memory 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
  59Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
  60the kernel command line.
  61
  62Memory may be allocated or freed before kmemleak is initialised and
  63these actions are stored in an early log buffer. The size of this buffer
  64is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
  65
  66Basic Algorithm
  67---------------
  68
  69The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
  70friends are traced and the pointers, together with additional
  71information like size and stack trace, are stored in a prio search tree.
  72The corresponding freeing function calls are tracked and the pointers
  73removed from the kmemleak data structures.
  74
  75An allocated block of memory is considered orphan if no pointer to its
  76start address or to any location inside the block can be found by
  77scanning the memory (including saved registers). This means that there
  78might be no way for the kernel to pass the address of the allocated
  79block to a freeing function and therefore the block is considered a
  80memory leak.
  81
  82The 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
  96Some allocated memory blocks have pointers stored in the kernel's
  97internal data structures and they cannot be detected as orphans. To
  98avoid this, kmemleak can also store the number of values pointing to an
  99address inside the block address range that need to be found so that the
 100block is not considered a leak. One example is __vmalloc().
 101
 102Testing specific sections with kmemleak
 103---------------------------------------
 104
 105Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
 106quite extensive. This can also be the case if you have very buggy code
 107when 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'
 110you can find new unreferenced objects; this should help with testing
 111specific sections of code.
 112
 113To 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
 119Then as usual to get your report with:
 120
 121  # cat /sys/kernel/debug/kmemleak
 122
 123Kmemleak API
 124------------
 125
 126See the include/linux/kmemleak.h header for the functions prototype.
 127
 128kmemleak_init            - initialize kmemleak
 129kmemleak_alloc           - notify of a memory block allocation
 130kmemleak_free            - notify of a memory block freeing
 131kmemleak_not_leak        - mark an object as not a leak
 132kmemleak_ignore          - do not scan or report an object as leak
 133kmemleak_scan_area       - add scan areas inside a memory block
 134kmemleak_no_scan         - do not scan a memory block
 135kmemleak_erase           - erase an old value in a pointer variable
 136kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
 137kmemleak_free_recursive  - as kmemleak_free but checks the recursiveness
 138
 139Dealing with false positives/negatives
 140--------------------------------------
 141
 142The false negatives are real memory leaks (orphan objects) but not
 143reported by kmemleak because values found during the memory scanning
 144point to such objects. To reduce the number of false negatives, kmemleak
 145provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
 146kmemleak_erase functions (see above). The task stacks also increase the
 147amount of false negatives and their scanning is not enabled by default.
 148
 149The false positives are objects wrongly reported as being memory leaks
 150(orphan). For objects known not to be leaks, kmemleak provides the
 151kmemleak_not_leak function. The kmemleak_ignore could also be used if
 152the memory block is known not to contain other pointers and it will no
 153longer be scanned.
 154
 155Some of the reported leaks are only transient, especially on SMP
 156systems, because of pointers temporarily stored in CPU registers or
 157stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
 158the minimum age of an object to be reported as a memory leak.
 159
 160Limitations and Drawbacks
 161-------------------------
 162
 163The main drawback is the reduced performance of memory allocation and
 164freeing. To avoid other penalties, the memory scanning is only performed
 165when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
 166intended for debugging purposes where the performance might not be the
 167most important requirement.
 168
 169To keep the algorithm simple, kmemleak scans for values pointing to any
 170address inside a block's address range. This may lead to an increased
 171number of false negatives. However, it is likely that a real memory leak
 172will eventually become visible.
 173
 174Another source of false negatives is the data stored in non-pointer
 175values. In a future version, kmemleak could only scan the pointer
 176members in the allocated structures. This feature would solve many of
 177the false negative cases described above.
 178
 179The tool can report false positives. These are cases where an allocated
 180block doesn't need to be freed (some cases in the init_call functions),
 181the pointer is calculated by other methods than the usual container_of
 182macro or the pointer is stored in a location not scanned by kmemleak.
 183
 184Page allocations and ioremap are not tracked.
 185
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