Kernel Memory Leak Detector
  ===========================
  
  Introduction
  ------------
  
  Kmemleak provides a way of detecting possible kernel memory leaks in a
  way similar to a tracing garbage collector
  (http://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.
  
  Usage
  -----
  
  CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
  thread scans the memory every 10 minutes (by default) and prints any new
  unreferenced objects found. To trigger an intermediate scan and display
  all the possible memory leaks:
  
    # mount -t debugfs nodev /sys/kernel/debug/
    # cat /sys/kernel/debug/kmemleak
  
  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
    stack=off	- disable the tasks stacks scanning
    scan=on	- start the automatic memory scanning thread
    scan=off	- stop the automatic memory scanning thread
    scan=<secs>	- set the automatic memory scanning period in seconds (0
  		  to disable it)
  
  Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
  the kernel command line.
  
  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 prio search tree.
  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 prio search tree. 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().
  
  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_free		 - notify of a memory block freeing
  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. Only the ARM and x86
  architectures are currently supported.