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.
  14Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390, metag and tile.
  15
  16Usage
  17-----
  18
  19CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
  20thread scans the memory every 10 minutes (by default) and prints the
  21number of new unreferenced objects found. To display the details of all
  22the possible memory leaks:
  23
  24  # mount -t debugfs nodev /sys/kernel/debug/
  25  # cat /sys/kernel/debug/kmemleak
  26
  27To trigger an intermediate memory scan:
  28
  29  # echo scan > /sys/kernel/debug/kmemleak
  30
  31To clear the list of all current possible memory leaks:
  32
  33  # echo clear > /sys/kernel/debug/kmemleak
  34
  35New leaks will then come up upon reading /sys/kernel/debug/kmemleak
  36again.
  37
  38Note that the orphan objects are listed in the order they were allocated
  39and one object at the beginning of the list may cause other subsequent
  40objects to be reported as orphan.
  41
  42Memory 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
  58Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
  59the kernel command line.
  60
  61Memory may be allocated or freed before kmemleak is initialised and
  62these actions are stored in an early log buffer. The size of this buffer
  63is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
  64
  65If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
  66disabled by default. Passing "kmemleak=on" on the kernel command
  67line enables the function. 
  68
  69Basic Algorithm
  70---------------
  71
  72The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
  73friends are traced and the pointers, together with additional
  74information like size and stack trace, are stored in a rbtree.
  75The corresponding freeing function calls are tracked and the pointers
  76removed from the kmemleak data structures.
  77
  78An allocated block of memory is considered orphan if no pointer to its
  79start address or to any location inside the block can be found by
  80scanning the memory (including saved registers). This means that there
  81might be no way for the kernel to pass the address of the allocated
  82block to a freeing function and therefore the block is considered a
  83memory leak.
  84
  85The 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
  99Some allocated memory blocks have pointers stored in the kernel's
 100internal data structures and they cannot be detected as orphans. To
 101avoid this, kmemleak can also store the number of values pointing to an
 102address inside the block address range that need to be found so that the
 103block is not considered a leak. One example is __vmalloc().
 104
 105Testing specific sections with kmemleak
 106---------------------------------------
 107
 108Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
 109quite extensive. This can also be the case if you have very buggy code
 110when 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'
 113you can find new unreferenced objects; this should help with testing
 114specific sections of code.
 115
 116To 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
 122Then as usual to get your report with:
 123
 124  # cat /sys/kernel/debug/kmemleak
 125
 126Freeing kmemleak internal objects
 127---------------------------------
 128
 129To allow access to previously found memory leaks after kmemleak has been
 130disabled by the user or due to an fatal error, internal kmemleak objects
 131won't be freed when kmemleak is disabled, and those objects may occupy
 132a large part of physical memory.
 133
 134In this situation, you may reclaim memory with:
 135
 136  # echo clear > /sys/kernel/debug/kmemleak
 137
 138Kmemleak API
 139------------
 140
 141See the include/linux/kmemleak.h header for the functions prototype.
 142
 143kmemleak_init            - initialize kmemleak
 144kmemleak_alloc           - notify of a memory block allocation
 145kmemleak_alloc_percpu    - notify of a percpu memory block allocation
 146kmemleak_free            - notify of a memory block freeing
 147kmemleak_free_part       - notify of a partial memory block freeing
 148kmemleak_free_percpu     - notify of a percpu memory block freeing
 149kmemleak_update_trace    - update object allocation stack trace
 150kmemleak_not_leak        - mark an object as not a leak
 151kmemleak_ignore          - do not scan or report an object as leak
 152kmemleak_scan_area       - add scan areas inside a memory block
 153kmemleak_no_scan         - do not scan a memory block
 154kmemleak_erase           - erase an old value in a pointer variable
 155kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
 156kmemleak_free_recursive  - as kmemleak_free but checks the recursiveness
 157
 158Dealing with false positives/negatives
 159--------------------------------------
 160
 161The false negatives are real memory leaks (orphan objects) but not
 162reported by kmemleak because values found during the memory scanning
 163point to such objects. To reduce the number of false negatives, kmemleak
 164provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
 165kmemleak_erase functions (see above). The task stacks also increase the
 166amount of false negatives and their scanning is not enabled by default.
 167
 168The false positives are objects wrongly reported as being memory leaks
 169(orphan). For objects known not to be leaks, kmemleak provides the
 170kmemleak_not_leak function. The kmemleak_ignore could also be used if
 171the memory block is known not to contain other pointers and it will no
 172longer be scanned.
 173
 174Some of the reported leaks are only transient, especially on SMP
 175systems, because of pointers temporarily stored in CPU registers or
 176stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
 177the minimum age of an object to be reported as a memory leak.
 178
 179Limitations and Drawbacks
 180-------------------------
 181
 182The main drawback is the reduced performance of memory allocation and
 183freeing. To avoid other penalties, the memory scanning is only performed
 184when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
 185intended for debugging purposes where the performance might not be the
 186most important requirement.
 187
 188To keep the algorithm simple, kmemleak scans for values pointing to any
 189address inside a block's address range. This may lead to an increased
 190number of false negatives. However, it is likely that a real memory leak
 191will eventually become visible.
 192
 193Another source of false negatives is the data stored in non-pointer
 194values. In a future version, kmemleak could only scan the pointer
 195members in the allocated structures. This feature would solve many of
 196the false negative cases described above.
 197
 198The tool can report false positives. These are cases where an allocated
 199block doesn't need to be freed (some cases in the init_call functions),
 200the pointer is calculated by other methods than the usual container_of
 201macro or the pointer is stored in a location not scanned by kmemleak.
 202
 203Page allocations and ioremap are not tracked.
 204
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