221 lines
9 KiB
ReStructuredText
Executable file
221 lines
9 KiB
ReStructuredText
Executable file
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.
|