143 lines
6.2 KiB
Plaintext
143 lines
6.2 KiB
Plaintext
Kernel Memory Leak Detector
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===========================
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Introduction
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------------
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Kmemleak provides a way of detecting possible kernel memory leaks in a
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way similar to a tracing garbage collector
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(http://en.wikipedia.org/wiki/Garbage_collection_%28computer_science%29#Tracing_garbage_collectors),
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with the difference that the orphan objects are not freed but only
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reported via /sys/kernel/debug/kmemleak. A similar method is used by the
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Valgrind tool (memcheck --leak-check) to detect the memory leaks in
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user-space applications.
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Usage
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-----
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CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
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thread scans the memory every 10 minutes (by default) and prints any new
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unreferenced objects found. To trigger an intermediate scan and display
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all the possible memory leaks:
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# mount -t debugfs nodev /sys/kernel/debug/
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# cat /sys/kernel/debug/kmemleak
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Note that the orphan objects are listed in the order they were allocated
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and one object at the beginning of the list may cause other subsequent
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objects to be reported as orphan.
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Memory scanning parameters can be modified at run-time by writing to the
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/sys/kernel/debug/kmemleak file. The following parameters are supported:
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off - disable kmemleak (irreversible)
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stack=on - enable the task stacks scanning
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stack=off - disable the tasks stacks scanning
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scan=on - start the automatic memory scanning thread
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scan=off - stop the automatic memory scanning thread
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scan=<secs> - set the automatic memory scanning period in seconds (0
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to disable it)
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Kmemleak can also be disabled at boot-time by passing "kmemleak=off" on
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the kernel command line.
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Basic Algorithm
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---------------
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The memory allocations via kmalloc, vmalloc, kmem_cache_alloc and
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friends are traced and the pointers, together with additional
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information like size and stack trace, are stored in a prio search tree.
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The corresponding freeing function calls are tracked and the pointers
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removed from the kmemleak data structures.
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An allocated block of memory is considered orphan if no pointer to its
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start address or to any location inside the block can be found by
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scanning the memory (including saved registers). This means that there
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might be no way for the kernel to pass the address of the allocated
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block to a freeing function and therefore the block is considered a
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memory leak.
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The scanning algorithm steps:
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1. mark all objects as white (remaining white objects will later be
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considered orphan)
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2. scan the memory starting with the data section and stacks, checking
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the values against the addresses stored in the prio search tree. If
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a pointer to a white object is found, the object is added to the
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gray list
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3. scan the gray objects for matching addresses (some white objects
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can become gray and added at the end of the gray list) until the
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gray set is finished
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4. the remaining white objects are considered orphan and reported via
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/sys/kernel/debug/kmemleak
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Some allocated memory blocks have pointers stored in the kernel's
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internal data structures and they cannot be detected as orphans. To
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avoid this, kmemleak can also store the number of values pointing to an
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address inside the block address range that need to be found so that the
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block is not considered a leak. One example is __vmalloc().
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Kmemleak API
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------------
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See the include/linux/kmemleak.h header for the functions prototype.
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kmemleak_init - initialize kmemleak
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kmemleak_alloc - notify of a memory block allocation
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kmemleak_free - notify of a memory block freeing
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kmemleak_not_leak - mark an object as not a leak
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kmemleak_ignore - do not scan or report an object as leak
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kmemleak_scan_area - add scan areas inside a memory block
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kmemleak_no_scan - do not scan a memory block
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kmemleak_erase - erase an old value in a pointer variable
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kmemleak_alloc_recursive - as kmemleak_alloc but checks the recursiveness
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kmemleak_free_recursive - as kmemleak_free but checks the recursiveness
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Dealing with false positives/negatives
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--------------------------------------
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The false negatives are real memory leaks (orphan objects) but not
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reported by kmemleak because values found during the memory scanning
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point to such objects. To reduce the number of false negatives, kmemleak
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provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
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kmemleak_erase functions (see above). The task stacks also increase the
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amount of false negatives and their scanning is not enabled by default.
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The false positives are objects wrongly reported as being memory leaks
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(orphan). For objects known not to be leaks, kmemleak provides the
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kmemleak_not_leak function. The kmemleak_ignore could also be used if
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the memory block is known not to contain other pointers and it will no
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longer be scanned.
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Some of the reported leaks are only transient, especially on SMP
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systems, because of pointers temporarily stored in CPU registers or
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stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
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the minimum age of an object to be reported as a memory leak.
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Limitations and Drawbacks
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-------------------------
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The main drawback is the reduced performance of memory allocation and
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freeing. To avoid other penalties, the memory scanning is only performed
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when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
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intended for debugging purposes where the performance might not be the
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most important requirement.
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To keep the algorithm simple, kmemleak scans for values pointing to any
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address inside a block's address range. This may lead to an increased
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number of false negatives. However, it is likely that a real memory leak
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will eventually become visible.
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Another source of false negatives is the data stored in non-pointer
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values. In a future version, kmemleak could only scan the pointer
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members in the allocated structures. This feature would solve many of
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the false negative cases described above.
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The tool can report false positives. These are cases where an allocated
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block doesn't need to be freed (some cases in the init_call functions),
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the pointer is calculated by other methods than the usual container_of
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macro or the pointer is stored in a location not scanned by kmemleak.
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Page allocations and ioremap are not tracked. Only the ARM and x86
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architectures are currently supported.
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