tools/memleak_example.txt

Demonstrations of memleak.


memleak traces and matches memory allocation and deallocation requests, and
collects call stacks for each allocation. memleak can then print a summary
of which call stacks performed allocations that weren't subsequently freed.
For example:

# ./memleak -p $(pidof allocs)
Attaching to pid 5193, Ctrl+C to quit.
[11:16:33] Top 2 stacks with outstanding allocations:
        80 bytes in 5 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]

[11:16:34] Top 2 stacks with outstanding allocations:
        160 bytes in 10 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]


Each entry printed is a set of allocations that originate from the same call
stack, and that weren't freed yet. The number of bytes and number of allocs
are followed by the call stack, top to bottom, of the allocation site.

As time goes on, it becomes apparent that the main function in the allocs
process is leaking memory, 16 bytes at a time. Fortunately, you don't have to
inspect each allocation individually -- you get a nice summary of which stack
is responsible for a large leak.

Occasionally, you do want the individual allocation details. Perhaps the same
stack is allocating various sizes and you want to confirm which sizes are 
prevalent. Use the -a switch:

# ./memleak -p $(pidof allocs) -a
Attaching to pid 5193, Ctrl+C to quit.
[11:16:33] Top 2 stacks with outstanding allocations:
        addr = 948cd0 size = 16
        addr = 948d10 size = 16
        addr = 948d30 size = 16
        addr = 948cf0 size = 16
        64 bytes in 4 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]

[11:16:34] Top 2 stacks with outstanding allocations:
        addr = 948d50 size = 16
        addr = 948cd0 size = 16
        addr = 948d10 size = 16
        addr = 948d30 size = 16
        addr = 948cf0 size = 16
        addr = 948dd0 size = 16
        addr = 948d90 size = 16
        addr = 948db0 size = 16
        addr = 948d70 size = 16
        addr = 948df0 size = 16
        160 bytes in 10 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]


When using the -p switch, memleak traces the libc allocations of a particular
process. Without this switch, kernel allocations are traced instead.
For example:

# ./memleak
Attaching to kernel allocators, Ctrl+C to quit.
...
        248 bytes in 4 allocations from stack
                 bpf_prog_load [kernel]
                 sys_bpf [kernel]

        328 bytes in 1 allocations from stack
                 perf_mmap [kernel]
                 mmap_region [kernel]
                 do_mmap [kernel]
                 vm_mmap_pgoff [kernel]
                 sys_mmap_pgoff [kernel]
                 sys_mmap [kernel]

        464 bytes in 1 allocations from stack
                 traceprobe_command [kernel]
                 traceprobe_probes_write [kernel]
                 probes_write [kernel]
                 __vfs_write [kernel]
                 vfs_write [kernel]
                 sys_write [kernel]
                 entry_SYSCALL_64_fastpath [kernel]

        8192 bytes in 1 allocations from stack
                 alloc_and_copy_ftrace_hash.constprop.59 [kernel]
                 ftrace_set_hash [kernel]
                 ftrace_set_filter_ip [kernel]
                 arm_kprobe [kernel]
                 enable_kprobe [kernel]
                 kprobe_register [kernel]
                 perf_trace_init [kernel]
                 perf_tp_event_init [kernel]


Here you can see that arming the kprobe to which our eBPF program is attached
consumed 8KB of memory. Loading the BPF program also consumed a couple hundred
bytes (in bpf_prog_load).

memleak stores each allocated block along with its size, timestamp, and the
stack that allocated it. When the block is deleted, this information is freed
to reduce the memory overhead.

To avoid false positives, allocations younger than a certain age (500ms by
default) are not printed. To change this threshold, use the -o switch.

By default, memleak prints its output every 5 seconds. To change this 
interval, pass the interval as a positional parameter to memleak. You can 
also control the number of times the output will be printed before exiting.
For example:

# ./memleak 1 10

... will print the outstanding allocation statistics every second, for ten
times, and then exit. 

memleak may introduce considerable overhead if your application or kernel is
allocating and freeing memory at a very high rate. In that case, you can 
control the overhead by sampling every N-th allocation. For example, to sample
roughly 10% of the allocations and print the outstanding allocations every 5
seconds, 3 times before quitting:

# ./memleak -p $(pidof allocs) -s 10 5 3
Attaching to pid 2614, Ctrl+C to quit.
[11:16:33] Top 2 stacks with outstanding allocations:
        16 bytes in 1 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]

[11:16:38] Top 2 stacks with outstanding allocations:
        16 bytes in 1 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]

[11:16:43] Top 2 stacks with outstanding allocations:
        32 bytes in 2 allocations from stack
                 main+0x6d [allocs]
                 __libc_start_main+0xf0 [libc-2.21.so]

Note that even though the application leaks 16 bytes of memory every second, 
the report (printed every 5 seconds) doesn't "see" all the allocations because
of the sampling rate applied. 

Profiling in memory part is hard to be accurate because of BPF infrastructure.
memleak keeps misjudging memory leak on the complicated environment which has
the action of free in hard/soft irq.
Add workaround to alleviate misjudgments when free is missing:

# ./memleak --wa-missing-free
Attaching to kernel allocators, Ctrl+C to quit.
...
        248 bytes in 4 allocations from stack
                 bpf_prog_load [kernel]
                 sys_bpf [kernel]

        328 bytes in 1 allocations from stack
                 perf_mmap [kernel]
                 mmap_region [kernel]
                 do_mmap [kernel]
                 vm_mmap_pgoff [kernel]
                 sys_mmap_pgoff [kernel]
                 sys_mmap [kernel]


USAGE message:

# ./memleak -h
usage: memleak.py [-h] [-p PID] [-t] [-a] [-o OLDER] [-c COMMAND]
                  [--combined-only] [--wa-missing-free] [-s SAMPLE_RATE]
                  [-T TOP] [-z MIN_SIZE] [-Z MAX_SIZE] [-O OBJ]
                  [interval] [count]

Trace outstanding memory allocations that weren't freed.
Supports both user-mode allocations made with libc functions and kernel-mode
allocations made with kmalloc/kmem_cache_alloc/get_free_pages and corresponding
memory release functions.

positional arguments:
  interval              interval in seconds to print outstanding allocations
  count                 number of times to print the report before exiting

optional arguments:
  -h, --help            show this help message and exit
  -p PID, --pid PID     the PID to trace; if not specified, trace kernel
                        allocs
  -t, --trace           print trace messages for each alloc/free call
  -a, --show-allocs     show allocation addresses and sizes as well as call
                        stacks
  -o OLDER, --older OLDER
                        prune allocations younger than this age in
                        milliseconds
  -c COMMAND, --command COMMAND
                        execute and trace the specified command
  --combined-only       show combined allocation statistics only
  --wa-missing-free     Workaround to alleviate misjudgments when free is
                        missing
  -s SAMPLE_RATE, --sample-rate SAMPLE_RATE
                        sample every N-th allocation to decrease the overhead
  -T TOP, --top TOP     display only this many top allocating stacks (by size)
  -z MIN_SIZE, --min-size MIN_SIZE
                        capture only allocations larger than this size
  -Z MAX_SIZE, --max-size MAX_SIZE
                        capture only allocations smaller than this size
  -O OBJ, --obj OBJ     attach to allocator functions in the specified object

EXAMPLES:

./memleak -p $(pidof allocs)
        Trace allocations and display a summary of "leaked" (outstanding)
        allocations every 5 seconds
./memleak -p $(pidof allocs) -t
        Trace allocations and display each individual allocator function call
./memleak -ap $(pidof allocs) 10
        Trace allocations and display allocated addresses, sizes, and stacks
        every 10 seconds for outstanding allocations
./memleak -c "./allocs"
        Run the specified command and trace its allocations
./memleak
        Trace allocations in kernel mode and display a summary of outstanding
        allocations every 5 seconds
./memleak -o 60000
        Trace allocations in kernel mode and display a summary of outstanding
        allocations that are at least one minute (60 seconds) old
./memleak -s 5
        Trace roughly every 5th allocation, to reduce overhead