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    Subject[PATCH 2.6.19 02/10] Kmemleak documentation
    Signed-off-by: Catalin Marinas <>

    Documentation/kmemleak.txt | 161 ++++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 161 insertions(+), 0 deletions(-)

    diff --git a/Documentation/kmemleak.txt b/Documentation/kmemleak.txt
    new file mode 100644
    index 0000000..4689f85
    --- /dev/null
    +++ b/Documentation/kmemleak.txt
    @@ -0,0 +1,161 @@
    +Kernel Memory Leak Detector
    +Kmemleak provides a way of detecting possible kernel memory leaks in a
    +way similar to a tracing garbage collector
    +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.
    +CONFIG_DEBUG_MEMLEAK has to be enabled. For additional config options,
    +look in:
    + -> Kernel hacking
    + -> Kernel debugging
    + -> Debug slab memory allocations
    + -> Kernel memory leak detector
    +To display the possible memory leaks:
    + # mount -t debugfs nodev /sys/kernel/debug/
    + # cat /sys/kernel/debug/memleak
    +In order to reduce the run-time overhead, memory scanning is only
    +performed when reading the /sys/kernel/debug/memleak file. 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 tracked and the pointers, together with additional
    +information like size and stack trace, are stored in a hash table. The
    +corresponding freeing function calls are tracked and the pointers
    +removed from the hash table.
    +An allocated block of memory is considered orphan if no pointer to its
    +start address or to an alias (pointer aliases are explained later) 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 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 hash
    + table. If a pointer to a white object is found, the object is
    + added to the grey list
    + 3. scan the grey objects for matching addresses (some white objects
    + can become grey and added at the end of the grey list) until the
    + grey set is finished
    + 4. the remaining white objects are considered orphan and reported
    + via /sys/kernel/debug/memleak
    +Because the Linux kernel calculates many pointers at run-time via the
    +container_of macro (see the lists implementation), a lot of false
    +positives would be reported. This tool re-writes the container_of
    +macro so that the offset and type information is stored in the
    +.init.memleak_offsets section. The memleak_init() function creates a
    +radix tree with corresponding offsets for every encountered block
    +type. The memory allocations hook stores the pointer address together
    +with its aliases based on the type of the allocated block.
    +While one level of offsets should be enough for most cases, a second
    +level, i.e. container_of(container_of(...)), can be enabled via the
    +configuration options (one false positive is the "struct socket_alloc"
    +allocation in the sock_alloc_inode() function).
    +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 equal to the
    +pointer (or aliases) that need to be found so that the block is not
    +considered a leak. One example is __vmalloc().
    +Limitations and Drawbacks
    +The biggest 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.
    +Kmemleak currently approximates the type id using the sizeof()
    +compiler built-in function. This is not accurate and can lead to false
    +negatives. The aim is to gradually change the kernel and kmemleak to
    +do more precise type identification.
    +Another source of false negatives is the data stored in non-pointer
    +values. Together with the more precise type identification, kmemleak
    +could only scan the pointer members in the allocated structures.
    +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
    +container_of macro or the pointer is stored in a location not scanned
    +by kmemleak. If the "member" argument in the offsetof(type, member)
    +call is not constant, kmemleak considers the offset as zero since it
    +cannot be determined at compilation time.
    +Page allocations and ioremap are not tracked. Only the ARM and i386
    +architectures are currently supported.
    +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_padding - mark the boundaries of the data inside the block
    +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_insert_aliases - add aliases for a given type
    +memleak_erase - erase an old value in a pointer variable
    +memleak_typeid_raw - set the typeid for an allocated block
    +memleak_container - statically declare a pointer alias
    +memleak_typeid - set the typeid for an allocated block (takes
    + a type rather than typeid as argument)
    +Dealing with false positives/negatives
    +To reduce the false negatives, kmemleak provides the memleak_ignore,
    +memleak_scan_area and memleak_erase functions. The task stacks also
    +increase the amount of false negatives and their scanning is not
    +enabled by default.
    +To eliminate the false positives caused by code allocating a different
    +size from the object one (either for alignment or for extra memory
    +after the end of the structure), kmemleak provides the memleak_padding
    +and memleak_typeid functions.
    +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.
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