kernel / Documentation / admin-guide / mm / slab.rst

========================================
Short users guide for the slab allocator
========================================

The slab allocator includes full debugging support (when built with
CONFIG_SLUB_DEBUG=y) but it is off by default (unless built with
CONFIG_SLUB_DEBUG_ON=y).  You can enable debugging only for selected
slabs in order to avoid an impact on overall system performance which
may make a bug more difficult to find.

In order to switch debugging on one can add an option ``slab_debug``
to the kernel command line. That will enable full debugging for
all slabs.

Typically one would then use the ``slabinfo`` command to get statistical
data and perform operation on the slabs. By default ``slabinfo`` only lists
slabs that have data in them. See "slabinfo -h" for more options when
running the command. ``slabinfo`` can be compiled with
::

	gcc -o slabinfo tools/mm/slabinfo.c

Some of the modes of operation of ``slabinfo`` require that slub debugging
be enabled on the command line. F.e. no tracking information will be
available without debugging on and validation can only partially
be performed if debugging was not switched on.

Some more sophisticated uses of slab_debug:
-------------------------------------------

Parameters may be given to ``slab_debug``. If none is specified then full
debugging is enabled. Format:

slab_debug=<Debug-Options>
	Enable options for all slabs

slab_debug=<Debug-Options>,<slab name1>,<slab name2>,...
	Enable options only for select slabs (no spaces
	after a comma)

Multiple blocks of options for all slabs or selected slabs can be given, with
blocks of options delimited by ';'. The last of "all slabs" blocks is applied
to all slabs except those that match one of the "select slabs" block. Options
of the first "select slabs" blocks that matches the slab's name are applied.

Possible debug options are::

	F		Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
			Sorry SLAB legacy issues)
	Z		Red zoning
	P		Poisoning (object and padding)
	U		User tracking (free and alloc)
	T		Trace (please only use on single slabs)
	A		Enable failslab filter mark for the cache
	O		Switch debugging off for caches that would have
			caused higher minimum slab orders
	-		Switch all debugging off (useful if the kernel is
			configured with CONFIG_SLUB_DEBUG_ON)

F.e. in order to boot just with sanity checks and red zoning one would specify::

	slab_debug=FZ

Trying to find an issue in the dentry cache? Try::

	slab_debug=,dentry

to only enable debugging on the dentry cache.  You may use an asterisk at the
end of the slab name, in order to cover all slabs with the same prefix.  For
example, here's how you can poison the dentry cache as well as all kmalloc
slabs::

	slab_debug=P,kmalloc-*,dentry

Red zoning and tracking may realign the slab.  We can just apply sanity checks
to the dentry cache with::

	slab_debug=F,dentry

Debugging options may require the minimum possible slab order to increase as
a result of storing the metadata (for example, caches with PAGE_SIZE object
sizes).  This has a higher likelihood of resulting in slab allocation errors
in low memory situations or if there's high fragmentation of memory.  To
switch off debugging for such caches by default, use::

	slab_debug=O

You can apply different options to different list of slab names, using blocks
of options. This will enable red zoning for dentry and user tracking for
kmalloc. All other slabs will not get any debugging enabled::

	slab_debug=Z,dentry;U,kmalloc-*

You can also enable options (e.g. sanity checks and poisoning) for all caches
except some that are deemed too performance critical and don't need to be
debugged by specifying global debug options followed by a list of slab names
with "-" as options::

	slab_debug=FZ;-,zs_handle,zspage

The state of each debug option for a slab can be found in the respective files
under::

	/sys/kernel/slab/<slab name>/

If the file contains 1, the option is enabled, 0 means disabled. The debug
options from the ``slab_debug`` parameter translate to the following files::

	F	sanity_checks
	Z	red_zone
	P	poison
	U	store_user
	T	trace
	A	failslab

failslab file is writable, so writing 1 or 0 will enable or disable
the option at runtime. Write returns -EINVAL if cache is an alias.
Careful with tracing: It may spew out lots of information and never stop if
used on the wrong slab.

Slab merging
============

If no debug options are specified then SLUB may merge similar slabs together
in order to reduce overhead and increase cache hotness of objects.
``slabinfo -a`` displays which slabs were merged together.

Slab validation
===============

SLUB can validate all object if the kernel was booted with slab_debug. In
order to do so you must have the ``slabinfo`` tool. Then you can do
::

	slabinfo -v

which will test all objects. Output will be generated to the syslog.

This also works in a more limited way if boot was without slab debug.
In that case ``slabinfo -v`` simply tests all reachable objects. Usually
these are in the cpu slabs and the partial slabs. Full slabs are not
tracked by SLUB in a non debug situation.

Getting more performance
========================

To some degree SLUB's performance is limited by the need to take the
list_lock once in a while to deal with partial slabs. That overhead is
governed by the order of the allocation for each slab. The allocations
can be influenced by kernel parameters:

.. slab_min_objects=x		(default: automatically scaled by number of cpus)
.. slab_min_order=x		(default 0)
.. slab_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))

``slab_min_objects``
	allows to specify how many objects must at least fit into one
	slab in order for the allocation order to be acceptable.  In
	general slub will be able to perform this number of
	allocations on a slab without consulting centralized resources
	(list_lock) where contention may occur.

``slab_min_order``
	specifies a minimum order of slabs. A similar effect like
	``slab_min_objects``.

``slab_max_order``
	specified the order at which ``slab_min_objects`` should no
	longer be checked. This is useful to avoid SLUB trying to
	generate super large order pages to fit ``slab_min_objects``
	of a slab cache with large object sizes into one high order
	page. Setting command line parameter
	``debug_guardpage_minorder=N`` (N > 0), forces setting
	``slab_max_order`` to 0, what cause minimum possible order of
	slabs allocation.

``slab_strict_numa``
        Enables the application of memory policies on each
        allocation. This results in more accurate placement of
        objects which may result in the reduction of accesses
        to remote nodes. The default is to only apply memory
        policies at the folio level when a new folio is acquired
        or a folio is retrieved from the lists. Enabling this
        option reduces the fastpath performance of the slab allocator.

SLUB Debug output
=================

Here is a sample of slub debug output::

 ====================================================================
 BUG kmalloc-8: Right Redzone overwritten
 --------------------------------------------------------------------

 INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
 INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
 INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
 INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554

 Bytes b4 (0xc90f6d10): 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
 Object   (0xc90f6d20): 31 30 31 39 2e 30 30 35                         1019.005
 Redzone  (0xc90f6d28): 00 cc cc cc                                     .
 Padding  (0xc90f6d50): 5a 5a 5a 5a 5a 5a 5a 5a                         ZZZZZZZZ

   [<c010523d>] dump_trace+0x63/0x1eb
   [<c01053df>] show_trace_log_lvl+0x1a/0x2f
   [<c010601d>] show_trace+0x12/0x14
   [<c0106035>] dump_stack+0x16/0x18
   [<c017e0fa>] object_err+0x143/0x14b
   [<c017e2cc>] check_object+0x66/0x234
   [<c017eb43>] __slab_free+0x239/0x384
   [<c017f446>] kfree+0xa6/0xc6
   [<c02e2335>] get_modalias+0xb9/0xf5
   [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
   [<c027866a>] dev_uevent+0x1ad/0x1da
   [<c0205024>] kobject_uevent_env+0x20a/0x45b
   [<c020527f>] kobject_uevent+0xa/0xf
   [<c02779f1>] store_uevent+0x4f/0x58
   [<c027758e>] dev_attr_store+0x29/0x2f
   [<c01bec4f>] sysfs_write_file+0x16e/0x19c
   [<c0183ba7>] vfs_write+0xd1/0x15a
   [<c01841d7>] sys_write+0x3d/0x72
   [<c0104112>] sysenter_past_esp+0x5f/0x99
   [<b7f7b410>] 0xb7f7b410
   =======================

 FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc

If SLUB encounters a corrupted object (full detection requires the kernel
to be booted with slab_debug) then the following output will be dumped
into the syslog:

1. Description of the problem encountered

   This will be a message in the system log starting with::

     ===============================================
     BUG <slab cache affected>: <What went wrong>
     -----------------------------------------------

     INFO: <corruption start>-<corruption_end> <more info>
     INFO: Slab <address> <slab information>
     INFO: Object <address> <object information>
     INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
	cpu> pid=<pid of the process>
     INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
	pid=<pid of the process>

   (Object allocation / free information is only available if SLAB_STORE_USER is
   set for the slab. slab_debug sets that option)

2. The object contents if an object was involved.

   Various types of lines can follow the BUG SLUB line:

   Bytes b4 <address> : <bytes>
	Shows a few bytes before the object where the problem was detected.
	Can be useful if the corruption does not stop with the start of the
	object.

   Object <address> : <bytes>
	The bytes of the object. If the object is inactive then the bytes
	typically contain poison values. Any non-poison value shows a
	corruption by a write after free.

   Redzone <address> : <bytes>
	The Redzone following the object. The Redzone is used to detect
	writes after the object. All bytes should always have the same
	value. If there is any deviation then it is due to a write after
	the object boundary.

	(Redzone information is only available if SLAB_RED_ZONE is set.
	slab_debug sets that option)

   Padding <address> : <bytes>
	Unused data to fill up the space in order to get the next object
	properly aligned. In the debug case we make sure that there are
	at least 4 bytes of padding. This allows the detection of writes
	before the object.

3. A stackdump

   The stackdump describes the location where the error was detected. The cause
   of the corruption is may be more likely found by looking at the function that
   allocated or freed the object.

4. Report on how the problem was dealt with in order to ensure the continued
   operation of the system.

   These are messages in the system log beginning with::

	FIX <slab cache affected>: <corrective action taken>

   In the above sample SLUB found that the Redzone of an active object has
   been overwritten. Here a string of 8 characters was written into a slab that
   has the length of 8 characters. However, a 8 character string needs a
   terminating 0. That zero has overwritten the first byte of the Redzone field.
   After reporting the details of the issue encountered the FIX SLUB message
   tells us that SLUB has restored the Redzone to its proper value and then
   system operations continue.

Emergency operations
====================

Minimal debugging (sanity checks alone) can be enabled by booting with::

	slab_debug=F

This will be generally be enough to enable the resiliency features of slub
which will keep the system running even if a bad kernel component will
keep corrupting objects. This may be important for production systems.
Performance will be impacted by the sanity checks and there will be a
continual stream of error messages to the syslog but no additional memory
will be used (unlike full debugging).

No guarantees. The kernel component still needs to be fixed. Performance
may be optimized further by locating the slab that experiences corruption
and enabling debugging only for that cache

I.e.::

	slab_debug=F,dentry

If the corruption occurs by writing after the end of the object then it
may be advisable to enable a Redzone to avoid corrupting the beginning
of other objects::

	slab_debug=FZ,dentry

Extended slabinfo mode and plotting
===================================

The ``slabinfo`` tool has a special 'extended' ('-X') mode that includes:
 - Slabcache Totals
 - Slabs sorted by size (up to -N <num> slabs, default 1)
 - Slabs sorted by loss (up to -N <num> slabs, default 1)

Additionally, in this mode ``slabinfo`` does not dynamically scale
sizes (G/M/K) and reports everything in bytes (this functionality is
also available to other slabinfo modes via '-B' option) which makes
reporting more precise and accurate. Moreover, in some sense the `-X'
mode also simplifies the analysis of slabs' behaviour, because its
output can be plotted using the ``slabinfo-gnuplot.sh`` script. So it
pushes the analysis from looking through the numbers (tons of numbers)
to something easier -- visual analysis.

To generate plots:

a) collect slabinfo extended records, for example::

	while [ 1 ]; do slabinfo -X >> FOO_STATS; sleep 1; done

b) pass stats file(-s) to ``slabinfo-gnuplot.sh`` script::

	slabinfo-gnuplot.sh FOO_STATS [FOO_STATS2 .. FOO_STATSN]

   The ``slabinfo-gnuplot.sh`` script will pre-processes the collected records
   and generates 3 png files (and 3 pre-processing cache files) per STATS
   file:
   - Slabcache Totals: FOO_STATS-totals.png
   - Slabs sorted by size: FOO_STATS-slabs-by-size.png
   - Slabs sorted by loss: FOO_STATS-slabs-by-loss.png

Another use case, when ``slabinfo-gnuplot.sh`` can be useful, is when you
need to compare slabs' behaviour "prior to" and "after" some code
modification.  To help you out there, ``slabinfo-gnuplot.sh`` script
can 'merge' the `Slabcache Totals` sections from different
measurements. To visually compare N plots:

a) Collect as many STATS1, STATS2, .. STATSN files as you need::

	while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done

b) Pre-process those STATS files::

	slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN

c) Execute ``slabinfo-gnuplot.sh`` in '-t' mode, passing all of the
   generated pre-processed \*-totals::

	slabinfo-gnuplot.sh -t STATS1-totals STATS2-totals .. STATSN-totals

   This will produce a single plot (png file).

   Plots, expectedly, can be large so some fluctuations or small spikes
   can go unnoticed. To deal with that, ``slabinfo-gnuplot.sh`` has two
   options to 'zoom-in'/'zoom-out':

   a) ``-s %d,%d`` -- overwrites the default image width and height
   b) ``-r %d,%d`` -- specifies a range of samples to use (for example,
      in ``slabinfo -X >> FOO_STATS; sleep 1;`` case, using a ``-r
      40,60`` range will plot only samples collected between 40th and
      60th seconds).


DebugFS files for SLUB
======================

For more information about current state of SLUB caches with the user tracking
debug option enabled, debugfs files are available, typically under
/sys/kernel/debug/slab/<cache>/ (created only for caches with enabled user
tracking). There are 2 types of these files with the following debug
information:

1. alloc_traces::

    Prints information about unique allocation traces of the currently
    allocated objects. The output is sorted by frequency of each trace.

    Information in the output:
    Number of objects, allocating function, possible memory wastage of
    kmalloc objects(total/per-object), minimal/average/maximal jiffies
    since alloc, pid range of the allocating processes, cpu mask of
    allocating cpus, numa node mask of origins of memory, and stack trace.

    Example:::

    338 pci_alloc_dev+0x2c/0xa0 waste=521872/1544 age=290837/291891/293509 pid=1 cpus=106 nodes=0-1
        __kmem_cache_alloc_node+0x11f/0x4e0
        kmalloc_trace+0x26/0xa0
        pci_alloc_dev+0x2c/0xa0
        pci_scan_single_device+0xd2/0x150
        pci_scan_slot+0xf7/0x2d0
        pci_scan_child_bus_extend+0x4e/0x360
        acpi_pci_root_create+0x32e/0x3b0
        pci_acpi_scan_root+0x2b9/0x2d0
        acpi_pci_root_add.cold.11+0x110/0xb0a
        acpi_bus_attach+0x262/0x3f0
        device_for_each_child+0xb7/0x110
        acpi_dev_for_each_child+0x77/0xa0
        acpi_bus_attach+0x108/0x3f0
        device_for_each_child+0xb7/0x110
        acpi_dev_for_each_child+0x77/0xa0
        acpi_bus_attach+0x108/0x3f0

2. free_traces::

    Prints information about unique freeing traces of the currently allocated
    objects. The freeing traces thus come from the previous life-cycle of the
    objects and are reported as not available for objects allocated for the first
    time. The output is sorted by frequency of each trace.

    Information in the output:
    Number of objects, freeing function, minimal/average/maximal jiffies since free,
    pid range of the freeing processes, cpu mask of freeing cpus, and stack trace.

    Example:::

    1980 <not-available> age=4294912290 pid=0 cpus=0
    51 acpi_ut_update_ref_count+0x6a6/0x782 age=236886/237027/237772 pid=1 cpus=1
	kfree+0x2db/0x420
	acpi_ut_update_ref_count+0x6a6/0x782
	acpi_ut_update_object_reference+0x1ad/0x234
	acpi_ut_remove_reference+0x7d/0x84
	acpi_rs_get_prt_method_data+0x97/0xd6
	acpi_get_irq_routing_table+0x82/0xc4
	acpi_pci_irq_find_prt_entry+0x8e/0x2e0
	acpi_pci_irq_lookup+0x3a/0x1e0
	acpi_pci_irq_enable+0x77/0x240
	pcibios_enable_device+0x39/0x40
	do_pci_enable_device.part.0+0x5d/0xe0
	pci_enable_device_flags+0xfc/0x120
	pci_enable_device+0x13/0x20
	virtio_pci_probe+0x9e/0x170
	local_pci_probe+0x48/0x80
	pci_device_probe+0x105/0x1c0

Christoph Lameter, May 30, 2007
Sergey Senozhatsky, October 23, 2015