221 lines
9.0 KiB
ReStructuredText
221 lines
9.0 KiB
ReStructuredText
Kernel Memory Leak Detector
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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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(https://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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Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile.
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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 display the details of all
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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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To trigger an intermediate memory scan::
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# echo scan > /sys/kernel/debug/kmemleak
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To clear the list of all current possible memory leaks::
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# echo clear > /sys/kernel/debug/kmemleak
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New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak``
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again.
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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
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disable kmemleak (irreversible)
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- stack=on
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enable the task stacks scanning (default)
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- stack=off
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disable the tasks stacks scanning
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- scan=on
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start the automatic memory scanning thread (default)
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- scan=off
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stop the automatic memory scanning thread
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- scan=<secs>
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set the automatic memory scanning period in seconds
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(default 600, 0 to stop the automatic scanning)
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- scan
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trigger a memory scan
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- clear
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clear list of current memory leak suspects, done by
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marking all current reported unreferenced objects grey,
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or free all kmemleak objects if kmemleak has been disabled.
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- dump=<addr>
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dump information about the object found at <addr>
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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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If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
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disabled by default. Passing ``kmemleak=on`` on the kernel command
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line enables the function.
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Basic Algorithm
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---------------
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The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`,
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:c:func:`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 rbtree.
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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 rbtree. 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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Testing specific sections with kmemleak
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---------------------------------------
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Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
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quite extensive. This can also be the case if you have very buggy code
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when doing development. To work around these situations you can use the
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'clear' command to clear all reported unreferenced objects from the
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/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
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you can find new unreferenced objects; this should help with testing
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specific sections of code.
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To test a critical section on demand with a clean kmemleak do::
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# echo clear > /sys/kernel/debug/kmemleak
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... test your kernel or modules ...
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# echo scan > /sys/kernel/debug/kmemleak
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Then as usual to get your report with::
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# cat /sys/kernel/debug/kmemleak
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Freeing kmemleak internal objects
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---------------------------------
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To allow access to previously found memory leaks after kmemleak has been
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disabled by the user or due to an fatal error, internal kmemleak objects
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won't be freed when kmemleak is disabled, and those objects may occupy
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a large part of physical memory.
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In this situation, you may reclaim memory with::
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# echo clear > /sys/kernel/debug/kmemleak
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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_alloc_percpu`` - notify of a percpu memory block allocation
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- ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation
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- ``kmemleak_free`` - notify of a memory block freeing
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- ``kmemleak_free_part`` - notify of a partial memory block freeing
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- ``kmemleak_free_percpu`` - notify of a percpu memory block freeing
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- ``kmemleak_update_trace`` - update object allocation stack trace
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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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The following functions take a physical address as the object pointer
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and only perform the corresponding action if the address has a lowmem
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mapping:
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- ``kmemleak_alloc_phys``
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- ``kmemleak_free_part_phys``
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- ``kmemleak_not_leak_phys``
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- ``kmemleak_ignore_phys``
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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.
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