linux-next/Documentation/kmemleak.txt

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.