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diff --git a/Documentation/dev-tools/kmemleak.rst b/Documentation/dev-tools/kmemleak.rst new file mode 100644 index 000000000..e6f51260f --- /dev/null +++ b/Documentation/dev-tools/kmemleak.rst @@ -0,0 +1,220 @@ +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 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_vmalloc`` - notify of a vmalloc() memory 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 + +The following functions take a physical address as the object pointer +and only perform the corresponding action if the address has a lowmem +mapping: + +- ``kmemleak_alloc_phys`` +- ``kmemleak_free_part_phys`` +- ``kmemleak_not_leak_phys`` +- ``kmemleak_ignore_phys`` + +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. |