docs: sphinxify kmemleak.txt and move it to dev-tools

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 210 insertions, 0 deletions

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+Kernel Memory Leak Detector
+===========================
+
+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 :c:func:`kmalloc`, :c:func:`vmalloc`,
+:c:func:`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.