Messages in this thread Patch in this message | | | Subject | [PATCH 02/15] kmemleak: Add documentation on the memory leak detector | From | Catalin Marinas <> | Date | Wed, 10 Dec 2008 18:27:05 +0000 |
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This patch adds the Documentation/kmemleak.txt file with some information about how kmemleak works.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> --- Documentation/kmemleak.txt | 127 ++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 127 insertions(+), 0 deletions(-) create mode 100644 Documentation/kmemleak.txt
diff --git a/Documentation/kmemleak.txt b/Documentation/kmemleak.txt new file mode 100644 index 0000000..6617ce8 --- /dev/null +++ b/Documentation/kmemleak.txt @@ -0,0 +1,127 @@ +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/memleak. A similar method is used by the +Valgrind tool (memcheck --leak-check) to detect the memory leaks in +user-space applications. + +Usage +----- + +CONFIG_DEBUG_MEMLEAK in "Kernel hacking" has to be enabled. A kernel +thread scans the memory every 10 min (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/memleak + +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. + +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/memleak + +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/memleak.h header for the functions prototype. + +memleak_init - initialize kmemleak +memleak_alloc - notify of a memory block allocation +memleak_free - notify of a memory block freeing +memleak_not_leak - mark an object as not a leak +memleak_ignore - do not scan or report an object as leak +memleak_scan_area - add scan areas inside a memory block +memleak_erase - erase an old value in a pointer variable +memleak_alloc_recursive - as memleak_alloc but checks the recursiveness +memleak_free_recursive - as memleak_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 memleak_ignore, memleak_scan_area and memleak_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 +memleak_not_leak function. The memleak_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/memleak 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 i386 +architectures are currently supported.
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