summaryrefslogtreecommitdiffstats
path: root/Documentation/kmemleak.txt
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/kmemleak.txt')
-rw-r--r--Documentation/kmemleak.txt187
1 files changed, 0 insertions, 187 deletions
diff --git a/Documentation/kmemleak.txt b/Documentation/kmemleak.txt
deleted file mode 100644
index b6e3973..0000000
--- a/Documentation/kmemleak.txt
+++ /dev/null
@@ -1,187 +0,0 @@
-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.
-
-Please check DEBUG_KMEMLEAK dependencies in lib/Kconfig.debug for supported
-architectures.
-
-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
- 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.
-
-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().
-
-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
-
-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_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.