bab4a34afc
Because of false positives, the memory scanning thread may print too much information. This patch changes the scanning thread to only print the number of newly suspected leaks. Further information can be read from the /sys/kernel/debug/kmemleak file. Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
146 lines
6.4 KiB
Text
146 lines
6.4 KiB
Text
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 the
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number of new unreferenced objects found. To trigger an intermediate
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scan and display the details of 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 (default)
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stack=off - disable the tasks stacks scanning
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scan=on - start the automatic memory scanning thread (default)
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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
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(default 600, 0 to stop the automatic scanning)
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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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Memory may be allocated or freed before kmemleak is initialised and
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these actions are stored in an early log buffer. The size of this buffer
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is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
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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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