Documentation/vm/slub.txt - kernel/msm-4.19 - Gitiles

 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 Documentation/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)

 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_cache

 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_cache

 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/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 1)

 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.

 SLUB Debug output
 -----------------

 Here is a sample of slub debug output:

 *** SLUB kmalloc-8: Redzone Active@0xc90f6d20 slab 0xc528c530 offset=3360 flags=0x400000c3 inuse=61 freelist=0xc90f6d58
   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                                     .
 FreePointer 0xc90f6d2c -> 0xc90f6d58
 Last alloc: get_modalias+0x61/0xf5 jiffies_ago=53 cpu=1 pid=554
 Filler 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
   =======================
 @@@ SLUB kmalloc-8: Restoring redzone (0xcc) from 0xc90f6d28-0xc90f6d2b


 If SLUB encounters a corrupted object then it will perform the following
 actions:

 1. Isolation and report of the issue

 This will be a message in the system log starting with

 *** SLUB <slab cache affected>: <What went wrong>@<object address>
 offset=<offset of object into slab> flags=<slabflags>
 inuse=<objects in use in this slab> freelist=<first free object in slab>

 2. 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

 @@@ SLUB <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 @@@ SLUB message
 tell us that SLUB has restored the redzone to its proper value and then
 system operations continue.

 Various types of lines can follow the @@@ SLUB line:

 Bytes b4 <address> : <bytes>
 	Show 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 poisoning 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.

 Freepointer
 	The pointer to the next free object in the slab. May become
 	corrupted if overwriting continues after the red zone.

 Last alloc:
 Last free:
 	Shows the address from which the object was allocated/freed last.
 	We note the pid, the time and the CPU that did so. This is usually
 	the most useful information to figure out where things went wrong.
 	Here get_modalias() did an kmalloc(8) instead of a kmalloc(9).

 Filler <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 filler. This allow for the detection of writes
 	before the object.

 Following the filler will be a stackdump. That stackdump describes the
 location where the error was detected. The cause of the corruption is more
 likely to be found by looking at the information about the last alloc / free.

 Christoph Lameter, <clameter@sgi.com>, May 23, 2007
	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 Documentation/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)

	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_cache

	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_cache

	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/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 1)

	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.

	SLUB Debug output
	-----------------

	Here is a sample of slub debug output:

	*** SLUB kmalloc-8: Redzone Active@0xc90f6d20 slab 0xc528c530 offset=3360 flags=0x400000c3 inuse=61 freelist=0xc90f6d58
	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 .
	FreePointer 0xc90f6d2c -> 0xc90f6d58
	Last alloc: get_modalias+0x61/0xf5 jiffies_ago=53 cpu=1 pid=554
	Filler 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
	=======================
	@@@ SLUB kmalloc-8: Restoring redzone (0xcc) from 0xc90f6d28-0xc90f6d2b



	If SLUB encounters a corrupted object then it will perform the following
	actions:

	1. Isolation and report of the issue

	This will be a message in the system log starting with

	*** SLUB <slab cache affected>: <What went wrong>@<object address>
	offset=<offset of object into slab> flags=<slabflags>
	inuse=<objects in use in this slab> freelist=<first free object in slab>

	2. 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

	@@@ SLUB <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 @@@ SLUB message
	tell us that SLUB has restored the redzone to its proper value and then
	system operations continue.

	Various types of lines can follow the @@@ SLUB line:

	Bytes b4 <address> : <bytes>
	Show 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 poisoning 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.

	Freepointer
	The pointer to the next free object in the slab. May become
	corrupted if overwriting continues after the red zone.

	Last alloc:
	Last free:
	Shows the address from which the object was allocated/freed last.
	We note the pid, the time and the CPU that did so. This is usually
	the most useful information to figure out where things went wrong.
	Here get_modalias() did an kmalloc(8) instead of a kmalloc(9).

	Filler <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 filler. This allow for the detection of writes
	before the object.

	Following the filler will be a stackdump. That stackdump describes the
	location where the error was detected. The cause of the corruption is more
	likely to be found by looking at the information about the last alloc / free.

	Christoph Lameter, <clameter@sgi.com>, May 23, 2007