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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:05 EST.

1	Short users guide for SLUB
2	--------------------------
3	
4	The basic philosophy of SLUB is very different from SLAB. SLAB
5	requires rebuilding the kernel to activate debug options for all
6	slab caches. SLUB always includes full debugging but it is off by default.
7	SLUB can enable debugging only for selected slabs in order to avoid
8	an impact on overall system performance which may make a bug more
9	difficult to find.
10	
11	In order to switch debugging on one can add a option "slub_debug"
12	to the kernel command line. That will enable full debugging for
13	all slabs.
14	
15	Typically one would then use the "slabinfo" command to get statistical
16	data and perform operation on the slabs. By default slabinfo only lists
17	slabs that have data in them. See "slabinfo -h" for more options when
18	running the command. slabinfo can be compiled with
19	
20	gcc -o slabinfo tools/vm/slabinfo.c
21	
22	Some of the modes of operation of slabinfo require that slub debugging
23	be enabled on the command line. F.e. no tracking information will be
24	available without debugging on and validation can only partially
25	be performed if debugging was not switched on.
26	
27	Some more sophisticated uses of slub_debug:
28	-------------------------------------------
29	
30	Parameters may be given to slub_debug. If none is specified then full
31	debugging is enabled. Format:
32	
33	slub_debug=<Debug-Options>       Enable options for all slabs
34	slub_debug=<Debug-Options>,<slab name>
35					Enable options only for select slabs
36	
37	Possible debug options are
38		F		Sanity checks on (enables SLAB_DEBUG_FREE. Sorry
39				SLAB legacy issues)
40		Z		Red zoning
41		P		Poisoning (object and padding)
42		U		User tracking (free and alloc)
43		T		Trace (please only use on single slabs)
44		A		Toggle failslab filter mark for the cache
45		O		Switch debugging off for caches that would have
46				caused higher minimum slab orders
47		-		Switch all debugging off (useful if the kernel is
48				configured with CONFIG_SLUB_DEBUG_ON)
49	
50	F.e. in order to boot just with sanity checks and red zoning one would specify:
51	
52		slub_debug=FZ
53	
54	Trying to find an issue in the dentry cache? Try
55	
56		slub_debug=,dentry
57	
58	to only enable debugging on the dentry cache.
59	
60	Red zoning and tracking may realign the slab.  We can just apply sanity checks
61	to the dentry cache with
62	
63		slub_debug=F,dentry
64	
65	Debugging options may require the minimum possible slab order to increase as
66	a result of storing the metadata (for example, caches with PAGE_SIZE object
67	sizes).  This has a higher liklihood of resulting in slab allocation errors
68	in low memory situations or if there's high fragmentation of memory.  To
69	switch off debugging for such caches by default, use
70	
71		slub_debug=O
72	
73	In case you forgot to enable debugging on the kernel command line: It is
74	possible to enable debugging manually when the kernel is up. Look at the
75	contents of:
76	
77	/sys/kernel/slab/<slab name>/
78	
79	Look at the writable files. Writing 1 to them will enable the
80	corresponding debug option. All options can be set on a slab that does
81	not contain objects. If the slab already contains objects then sanity checks
82	and tracing may only be enabled. The other options may cause the realignment
83	of objects.
84	
85	Careful with tracing: It may spew out lots of information and never stop if
86	used on the wrong slab.
87	
88	Slab merging
89	------------
90	
91	If no debug options are specified then SLUB may merge similar slabs together
92	in order to reduce overhead and increase cache hotness of objects.
93	slabinfo -a displays which slabs were merged together.
94	
95	Slab validation
96	---------------
97	
98	SLUB can validate all object if the kernel was booted with slub_debug. In
99	order to do so you must have the slabinfo tool. Then you can do
100	
101	slabinfo -v
102	
103	which will test all objects. Output will be generated to the syslog.
104	
105	This also works in a more limited way if boot was without slab debug.
106	In that case slabinfo -v simply tests all reachable objects. Usually
107	these are in the cpu slabs and the partial slabs. Full slabs are not
108	tracked by SLUB in a non debug situation.
109	
110	Getting more performance
111	------------------------
112	
113	To some degree SLUB's performance is limited by the need to take the
114	list_lock once in a while to deal with partial slabs. That overhead is
115	governed by the order of the allocation for each slab. The allocations
116	can be influenced by kernel parameters:
117	
118	slub_min_objects=x		(default 4)
119	slub_min_order=x		(default 0)
120	slub_max_order=x		(default 3 (PAGE_ALLOC_COSTLY_ORDER))
121	
122	slub_min_objects allows to specify how many objects must at least fit
123	into one slab in order for the allocation order to be acceptable.
124	In general slub will be able to perform this number of allocations
125	on a slab without consulting centralized resources (list_lock) where
126	contention may occur.
127	
128	slub_min_order specifies a minim order of slabs. A similar effect like
129	slub_min_objects.
130	
131	slub_max_order specified the order at which slub_min_objects should no
132	longer be checked. This is useful to avoid SLUB trying to generate
133	super large order pages to fit slub_min_objects of a slab cache with
134	large object sizes into one high order page. Setting command line
135	parameter debug_guardpage_minorder=N (N > 0), forces setting
136	slub_max_order to 0, what cause minimum possible order of slabs
137	allocation.
138	
139	SLUB Debug output
140	-----------------
141	
142	Here is a sample of slub debug output:
143	
144	====================================================================
145	BUG kmalloc-8: Redzone overwritten
146	--------------------------------------------------------------------
147	
148	INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
149	INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
150	INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
151	INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
152	
153	Bytes b4 0xc90f6d10:  00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
154	  Object 0xc90f6d20:  31 30 31 39 2e 30 30 35                         1019.005
155	 Redzone 0xc90f6d28:  00 cc cc cc                                     .
156	 Padding 0xc90f6d50:  5a 5a 5a 5a 5a 5a 5a 5a                         ZZZZZZZZ
157	
158	  [<c010523d>] dump_trace+0x63/0x1eb
159	  [<c01053df>] show_trace_log_lvl+0x1a/0x2f
160	  [<c010601d>] show_trace+0x12/0x14
161	  [<c0106035>] dump_stack+0x16/0x18
162	  [<c017e0fa>] object_err+0x143/0x14b
163	  [<c017e2cc>] check_object+0x66/0x234
164	  [<c017eb43>] __slab_free+0x239/0x384
165	  [<c017f446>] kfree+0xa6/0xc6
166	  [<c02e2335>] get_modalias+0xb9/0xf5
167	  [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
168	  [<c027866a>] dev_uevent+0x1ad/0x1da
169	  [<c0205024>] kobject_uevent_env+0x20a/0x45b
170	  [<c020527f>] kobject_uevent+0xa/0xf
171	  [<c02779f1>] store_uevent+0x4f/0x58
172	  [<c027758e>] dev_attr_store+0x29/0x2f
173	  [<c01bec4f>] sysfs_write_file+0x16e/0x19c
174	  [<c0183ba7>] vfs_write+0xd1/0x15a
175	  [<c01841d7>] sys_write+0x3d/0x72
176	  [<c0104112>] sysenter_past_esp+0x5f/0x99
177	  [<b7f7b410>] 0xb7f7b410
178	  =======================
179	
180	FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
181	
182	If SLUB encounters a corrupted object (full detection requires the kernel
183	to be booted with slub_debug) then the following output will be dumped
184	into the syslog:
185	
186	1. Description of the problem encountered
187	
188	This will be a message in the system log starting with
189	
190	===============================================
191	BUG <slab cache affected>: <What went wrong>
192	-----------------------------------------------
193	
194	INFO: <corruption start>-<corruption_end> <more info>
195	INFO: Slab <address> <slab information>
196	INFO: Object <address> <object information>
197	INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
198		cpu> pid=<pid of the process>
199	INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
200		 pid=<pid of the process>
201	
202	(Object allocation / free information is only available if SLAB_STORE_USER is
203	set for the slab. slub_debug sets that option)
204	
205	2. The object contents if an object was involved.
206	
207	Various types of lines can follow the BUG SLUB line:
208	
209	Bytes b4 <address> : <bytes>
210		Shows a few bytes before the object where the problem was detected.
211		Can be useful if the corruption does not stop with the start of the
212		object.
213	
214	Object <address> : <bytes>
215		The bytes of the object. If the object is inactive then the bytes
216		typically contain poison values. Any non-poison value shows a
217		corruption by a write after free.
218	
219	Redzone <address> : <bytes>
220		The Redzone following the object. The Redzone is used to detect
221		writes after the object. All bytes should always have the same
222		value. If there is any deviation then it is due to a write after
223		the object boundary.
224	
225		(Redzone information is only available if SLAB_RED_ZONE is set.
226		slub_debug sets that option)
227	
228	Padding <address> : <bytes>
229		Unused data to fill up the space in order to get the next object
230		properly aligned. In the debug case we make sure that there are
231		at least 4 bytes of padding. This allows the detection of writes
232		before the object.
233	
234	3. A stackdump
235	
236	The stackdump describes the location where the error was detected. The cause
237	of the corruption is may be more likely found by looking at the function that
238	allocated or freed the object.
239	
240	4. Report on how the problem was dealt with in order to ensure the continued
241	operation of the system.
242	
243	These are messages in the system log beginning with
244	
245	FIX <slab cache affected>: <corrective action taken>
246	
247	In the above sample SLUB found that the Redzone of an active object has
248	been overwritten. Here a string of 8 characters was written into a slab that
249	has the length of 8 characters. However, a 8 character string needs a
250	terminating 0. That zero has overwritten the first byte of the Redzone field.
251	After reporting the details of the issue encountered the FIX SLUB message
252	tells us that SLUB has restored the Redzone to its proper value and then
253	system operations continue.
254	
255	Emergency operations:
256	---------------------
257	
258	Minimal debugging (sanity checks alone) can be enabled by booting with
259	
260		slub_debug=F
261	
262	This will be generally be enough to enable the resiliency features of slub
263	which will keep the system running even if a bad kernel component will
264	keep corrupting objects. This may be important for production systems.
265	Performance will be impacted by the sanity checks and there will be a
266	continual stream of error messages to the syslog but no additional memory
267	will be used (unlike full debugging).
268	
269	No guarantees. The kernel component still needs to be fixed. Performance
270	may be optimized further by locating the slab that experiences corruption
271	and enabling debugging only for that cache
272	
273	I.e.
274	
275		slub_debug=F,dentry
276	
277	If the corruption occurs by writing after the end of the object then it
278	may be advisable to enable a Redzone to avoid corrupting the beginning
279	of other objects.
280	
281		slub_debug=FZ,dentry
282	
283	Christoph Lameter, May 30, 2007
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