Kmemleak

1. Article purpose

This article provides the basic information needed to start using the Linux kernel tool: kmemleak.

2. Introduction

The following table provides a brief description of the tool, as well as its availability depending on the software packages:

: this tool is either present (ready to use or to be activated), or can be integrated and activated on the software package.

: this tool is not present and cannot be integrated, or it is present but cannot be activated on the software package.

Tool			STM32MPU Embedded Software distribution			STM32MPU Embedded Software distribution for Android™
Name	Category	Purpose	Starter Package	Developer Package	Distribution Package	Starter Package	Developer Package	Distribution Package
kmemleak	Monitoring tools	kmemleak provides a means to detect possible kernel memory leaks in a similar way to a tracing garbage collector, with the difference that the orphan objects are not freed, but only reported via /sys/kernel/debug/kmemleak.

3. Installing the trace and debug tool on your target board

In order to use kmemleak, the Linux kernel configuration must activate CONFIG_DEBUG_KMEMLEAK:

Symbol: DEBUG_KMEMLEAK
Location:
  Kernel Hacking --->
    Memory Debugging -->
      [*] Kernel memory leak detector

For ecosystem release ≤ v1.1.0 :

Since memory may be allocated or freed before kmemleak is initialised, an early log buffer is used to store these actions.
In this ecosystem release, the default buffer size is limited, and this is recommended to increase it.

Warning

If kmemleak reports "early log buffer exceeded" and debugfs entry is not present, you can increase the log buffer size by changing the configuration value, for example to 5000 Symbol: DEBUG_KMEMLEAK_EARLY_LOG_SIZE [=400] Location: Range: [200 40000] Kernel Hacking ---> Memory Debugging --> [*] Kernel memory leak detector (400) Maximum kmemleak early log entries

3.1. Using STM32MPU Embedded Software Distribution

3.1.1. Developer Package

To enable CONFIG_DEBUG_KMEMLEAK in the Linux kernel configuration, please refer to the Menuconfig or how to configure kernel article to find instructions for modification of the configuration and recompiling Linux kernel image in Developer Package context.

3.1.2. Distribution Package

To enable CONFIG_DEBUG_KMEMLEAK in the Linux kernel configuration, please refer to the Menuconfig or how to configure kernel article to find instructions for modifying the configuration and recompiling the Linux kernel image in the Distribution Package context.

3.2. Using STM32MPU Embedded Software Distribution for Android™

3.2.1. Distribution Package

To enable CONFIG_DEBUG_KMEMLEAK in the Linux kernel configuration, please refer to the How to customize kernel for Android article to find instructions for modification of the configuration and recompiling Linux kernel image in Distribution Package context.

4. Getting started

Below information is related to the Android™ distribution Need to enable root access rights Using ADB shell is ADB link available: adb root adb shell ... Using uart console shell: su ...

4.1. Reading kmemleak report

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:

 cat /sys/kernel/debug/kmemleak

Note: debugfs is mounted by default, otherwise you can mount it using the following command. Please refer to the Debugfs article.

Kmemleak result example: contains an extract of the memory content, and backtrace of function calls, to help you when debugging.

unreferenced object 0xed638800 (size 64):
  comm "swapper/0", pid 1, jiffies 4294937542 (age 197.490s)
  hex dump (first 32 bytes):
    01 00 00 00 77 f9 ff 7a cf 37 dd e3 01 00 00 00  ....w..z.7......
    00 92 63 ed 40 00 00 00 00 00 00 00 01 00 00 00  ..c.@...........
  backtrace:
    [<c048d130>] pinconf_generic_parse_dt_config+0x10c/0x13c
    [<c0490f88>] stm32_pctrl_dt_node_to_map+0x90/0x3f4
    [<c048ca58>] pinctrl_dt_to_map+0x130/0x35c
    [<c04895a0>] create_pinctrl+0x60/0x3b0
    [<c0489a08>] devm_pinctrl_get+0x38/0x68
    [<c0582a24>] pinctrl_bind_pins+0x48/0x280
    [<c055f7f8>] driver_probe_device+0xc0/0x470
    [<c055fca8>] __driver_attach+0x100/0x11c
    [<c055db08>] bus_for_each_dev+0x4c/0x9c
    [<c055ecc4>] bus_add_driver+0x1c0/0x264
    [<c0560910>] driver_register+0x78/0xf4
    [<c0101c48>] do_one_initcall+0x44/0x168
    [<c0f00e74>] kernel_init_freeable+0x1c0/0x24c
    [<c0a65ac4>] kernel_init+0x8/0x110
    [<c0108b50>] ret_from_fork+0x14/0x24
    [<ffffffff>] 0xffffffff

4.2. Trigerring an intermediate memory scan

 echo scan > /sys/kernel/debug/kmemleak

4.3. Clearing the list of all current possible memory leaks

 echo clear > /sys/kernel/debug/kmemleak

4.4. All kmemleak commands

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 in grey, or freeing all kmemleak objects if kmemleak is disabled.
dump=<addr> - dump information about the object found at <addr>

5. To go further

If enabled in the Linux kernel configuration, Kmemleak can also be disabled at boot time by passing kmemleak=off on the kernel command line.

Conversely, if CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF is enabled in the Linux kernel configuration, kmemleak is disabled by default.

Symbol: DEBUG_KMEMLEAK_DEFAULT_OFF
Location:
  Kernel Hacking --->
    Memory Debugging -->
      [*] Kernel memory leak detector
        [*] Default kmemleak to off

Passing kmemleak=on on the kernel command line enables the function.

6. References

• Useful external links

Document link	Document Type	Description
Kernel Memory Leak Detector	Standard	Documentation from kernel.org

Debug File System (See https://en.wikipedia.org/wiki/Debugfs for more details)

Android debug bridge (Android specific)

Debug File System (See https://en.wikipedia.org/wiki/Debugfs for more details)

Android debug bridge (Android specific)