Documentation/kmemleak.txt - linux-4.4 - Gitiles Standalone

 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.
 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.
	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.
	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.