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 a 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 name>
  				Enable options only for select slabs
  
  Possible debug options are
  	F		Sanity checks on (enables SLAB_DEBUG_FREE. 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.
  
  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=x		(default 4)
  slub_min_order=x		(default 0)
  slub_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))
  
  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 minim 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:
  
  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. slub_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.
  	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.
  
  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
  
  	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
  
  Christoph Lameter, May 30, 2007