Short users guide for SLUB¶

The basic philosophy of SLUB is very different from SLAB. SLAB requires rebuilding the kernel to activate debug options for all slab caches. SLUB always includes full debugging but it is off by default. SLUB 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 slub_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/vm/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 slub_debug:¶

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

slub_debug=<Debug-Options>: Enable options for all slabs
slub_debug=<Debug-Options>,<slab name1>,<slab name2>,…: Enable options only for select slabs (no spaces after a comma)

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               Toggle 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:

slub_debug=FZ

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

slub_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:

slub_debug=P,kmalloc-*,dentry

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

slub_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 liklihood 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:

slub_debug=O

In case you forgot to enable debugging on the kernel command line: It is possible to enable debugging manually when the kernel is up. Look at the contents of:

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

Look at the writable files. Writing 1 to them will enable the corresponding debug option. All options can be set on a slab that does not contain objects. If the slab already contains objects then sanity checks and tracing may only be enabled. The other options may cause the realignment of objects.

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 slub_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:

slub_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.
slub_min_order: specifies a minimum order of slabs. A similar effect like slub_min_objects.
slub_max_order: specified the order at which slub_min_objects should no longer be checked. This is useful to avoid SLUB trying to generate super large order pages to fit slub_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 slub_max_order to 0, what cause minimum possible order of slabs allocation.

SLUB Debug output¶

Here is a sample of slub debug output:

====================================================================
BUG kmalloc-8: 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 slub_debug) then the following output will be dumped into the syslog:

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. slub_debug sets that option)

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. slub_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.
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.
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:

slub_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.:

slub_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:

slub_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:

collect slabinfo extended records, for example:

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

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:

Collect as many STATS1, STATS2, .. STATSN files as you need:
```
while [ 1 ]; do slabinfo -X >> STATS<X>; sleep 1; done
```

Pre-process those STATS files:

slabinfo-gnuplot.sh STATS1 STATS2 .. STATSN

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’:
1. -s %d,%d – overwrites the default image width and heigh
2. -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).

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