Kernel Memory Leak Detector
  ===========================
  
  Introduction
  ------------
  
  Kmemleak provides a way of detecting possible kernel memory leaks in a
  way similar to a tracing garbage collector

  (https://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),

  with the difference that the orphan objects are not freed but only
  reported via /sys/kernel/debug/kmemleak. A similar method is used by the
  Valgrind tool (memcheck --leak-check) to detect the memory leaks in
  user-space applications.

  Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390, metag and tile.

  
  Usage
  -----
  
  CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel

  thread scans the memory every 10 minutes (by default) and prints the

  number of new unreferenced objects found. To display the details of all
  the possible memory leaks:

  
    # mount -t debugfs nodev /sys/kernel/debug/
    # cat /sys/kernel/debug/kmemleak

  To trigger an intermediate memory scan:
  
    # echo scan > /sys/kernel/debug/kmemleak

  To clear the list of all current possible memory leaks:
  
    # echo clear > /sys/kernel/debug/kmemleak
  
  New leaks will then come up upon reading /sys/kernel/debug/kmemleak
  again.

  Note that the orphan objects are listed in the order they were allocated
  and one object at the beginning of the list may cause other subsequent
  objects to be reported as orphan.
  
  Memory scanning parameters can be modified at run-time by writing to the
  /sys/kernel/debug/kmemleak file. The following parameters are supported:
  
    off		- disable kmemleak (irreversible)

    stack=on	- enable the task stacks scanning (default)

    stack=off	- disable the tasks stacks scanning

    scan=on	- start the automatic memory scanning thread (default)

    scan=off	- stop the automatic memory scanning thread

    scan=<secs>	- set the automatic memory scanning period in seconds
  		  (default 600, 0 to stop the automatic scanning)

    scan		- trigger a memory scan

    clear		- clear list of current memory leak suspects, done by

  		  marking all current reported unreferenced objects grey,
  		  or free all kmemleak objects if kmemleak has been disabled.

    dump=<addr>	- dump information about the object found at <addr>

  
  Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
  the kernel command line.

  Memory may be allocated or freed before kmemleak is initialised and
  these actions are stored in an early log buffer. The size of this buffer
  is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.

  If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
  disabled by default. Passing "kmemleak=on" on the kernel command
  line enables the function. 

  Basic Algorithm
  ---------------
  
  The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
  friends are traced and the pointers, together with additional

  information like size and stack trace, are stored in a rbtree.

  The corresponding freeing function calls are tracked and the pointers
  removed from the kmemleak data structures.
  
  An allocated block of memory is considered orphan if no pointer to its
  start address or to any location inside the block can be found by
  scanning the memory (including saved registers). This means that there
  might be no way for the kernel to pass the address of the allocated
  block to a freeing function and therefore the block is considered a
  memory leak.
  
  The scanning algorithm steps:
  
    1. mark all objects as white (remaining white objects will later be
       considered orphan)
    2. scan the memory starting with the data section and stacks, checking

       the values against the addresses stored in the rbtree. If

       a pointer to a white object is found, the object is added to the
       gray list
    3. scan the gray objects for matching addresses (some white objects
       can become gray and added at the end of the gray list) until the
       gray set is finished
    4. the remaining white objects are considered orphan and reported via
       /sys/kernel/debug/kmemleak
  
  Some allocated memory blocks have pointers stored in the kernel's
  internal data structures and they cannot be detected as orphans. To
  avoid this, kmemleak can also store the number of values pointing to an
  address inside the block address range that need to be found so that the
  block is not considered a leak. One example is __vmalloc().

  Testing specific sections with kmemleak
  ---------------------------------------
  
  Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
  quite extensive. This can also be the case if you have very buggy code
  when doing development. To work around these situations you can use the
  'clear' command to clear all reported unreferenced objects from the
  /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
  you can find new unreferenced objects; this should help with testing
  specific sections of code.
  
  To test a critical section on demand with a clean kmemleak do:
  
    # echo clear > /sys/kernel/debug/kmemleak
    ... test your kernel or modules ...
    # echo scan > /sys/kernel/debug/kmemleak
  
  Then as usual to get your report with:
  
    # cat /sys/kernel/debug/kmemleak

  Freeing kmemleak internal objects
  ---------------------------------

  To allow access to previously found memory leaks after kmemleak has been

  disabled by the user or due to an fatal error, internal kmemleak objects
  won't be freed when kmemleak is disabled, and those objects may occupy
  a large part of physical memory.
  
  In this situation, you may reclaim memory with:
  
    # echo clear > /sys/kernel/debug/kmemleak

  Kmemleak API
  ------------
  
  See the include/linux/kmemleak.h header for the functions prototype.
  
  kmemleak_init		 - initialize kmemleak
  kmemleak_alloc		 - notify of a memory block allocation

  kmemleak_alloc_percpu	 - notify of a percpu memory block allocation

  kmemleak_free		 - notify of a memory block freeing

  kmemleak_free_part	 - notify of a partial memory block freeing
  kmemleak_free_percpu	 - notify of a percpu memory block freeing

  kmemleak_update_trace	 - update object allocation stack trace

  kmemleak_not_leak	 - mark an object as not a leak
  kmemleak_ignore		 - do not scan or report an object as leak
  kmemleak_scan_area	 - add scan areas inside a memory block
  kmemleak_no_scan	 - do not scan a memory block
  kmemleak_erase		 - erase an old value in a pointer variable
  kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
  kmemleak_free_recursive	 - as kmemleak_free but checks the recursiveness
  
  Dealing with false positives/negatives
  --------------------------------------
  
  The false negatives are real memory leaks (orphan objects) but not
  reported by kmemleak because values found during the memory scanning
  point to such objects. To reduce the number of false negatives, kmemleak
  provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
  kmemleak_erase functions (see above). The task stacks also increase the
  amount of false negatives and their scanning is not enabled by default.
  
  The false positives are objects wrongly reported as being memory leaks
  (orphan). For objects known not to be leaks, kmemleak provides the
  kmemleak_not_leak function. The kmemleak_ignore could also be used if
  the memory block is known not to contain other pointers and it will no
  longer be scanned.
  
  Some of the reported leaks are only transient, especially on SMP
  systems, because of pointers temporarily stored in CPU registers or
  stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
  the minimum age of an object to be reported as a memory leak.
  
  Limitations and Drawbacks
  -------------------------
  
  The main drawback is the reduced performance of memory allocation and
  freeing. To avoid other penalties, the memory scanning is only performed
  when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
  intended for debugging purposes where the performance might not be the
  most important requirement.
  
  To keep the algorithm simple, kmemleak scans for values pointing to any
  address inside a block's address range. This may lead to an increased
  number of false negatives. However, it is likely that a real memory leak
  will eventually become visible.
  
  Another source of false negatives is the data stored in non-pointer
  values. In a future version, kmemleak could only scan the pointer
  members in the allocated structures. This feature would solve many of
  the false negative cases described above.
  
  The tool can report false positives. These are cases where an allocated
  block doesn't need to be freed (some cases in the init_call functions),
  the pointer is calculated by other methods than the usual container_of
  macro or the pointer is stored in a location not scanned by kmemleak.

  Page allocations and ioremap are not tracked.