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Documentation / vm / slub.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 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 Documentation/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		-		Switch all debugging off (useful if the kernel is
45				configured with CONFIG_SLUB_DEBUG_ON)
46	
47	F.e. in order to boot just with sanity checks and red zoning one would specify:
48	
49		slub_debug=FZ
50	
51	Trying to find an issue in the dentry cache? Try
52	
53		slub_debug=,dentry
54	
55	to only enable debugging on the dentry cache.
56	
57	Red zoning and tracking may realign the slab.  We can just apply sanity checks
58	to the dentry cache with
59	
60		slub_debug=F,dentry
61	
62	In case you forgot to enable debugging on the kernel command line: It is
63	possible to enable debugging manually when the kernel is up. Look at the
64	contents of:
65	
66	/sys/kernel/slab/<slab name>/
67	
68	Look at the writable files. Writing 1 to them will enable the
69	corresponding debug option. All options can be set on a slab that does
70	not contain objects. If the slab already contains objects then sanity checks
71	and tracing may only be enabled. The other options may cause the realignment
72	of objects.
73	
74	Careful with tracing: It may spew out lots of information and never stop if
75	used on the wrong slab.
76	
77	Slab merging
78	------------
79	
80	If no debug options are specified then SLUB may merge similar slabs together
81	in order to reduce overhead and increase cache hotness of objects.
82	slabinfo -a displays which slabs were merged together.
83	
84	Slab validation
85	---------------
86	
87	SLUB can validate all object if the kernel was booted with slub_debug. In
88	order to do so you must have the slabinfo tool. Then you can do
89	
90	slabinfo -v
91	
92	which will test all objects. Output will be generated to the syslog.
93	
94	This also works in a more limited way if boot was without slab debug.
95	In that case slabinfo -v simply tests all reachable objects. Usually
96	these are in the cpu slabs and the partial slabs. Full slabs are not
97	tracked by SLUB in a non debug situation.
98	
99	Getting more performance
100	------------------------
101	
102	To some degree SLUB's performance is limited by the need to take the
103	list_lock once in a while to deal with partial slabs. That overhead is
104	governed by the order of the allocation for each slab. The allocations
105	can be influenced by kernel parameters:
106	
107	slub_min_objects=x		(default 4)
108	slub_min_order=x		(default 0)
109	slub_max_order=x		(default 1)
110	
111	slub_min_objects allows to specify how many objects must at least fit
112	into one slab in order for the allocation order to be acceptable.
113	In general slub will be able to perform this number of allocations
114	on a slab without consulting centralized resources (list_lock) where
115	contention may occur.
116	
117	slub_min_order specifies a minim order of slabs. A similar effect like
118	slub_min_objects.
119	
120	slub_max_order specified the order at which slub_min_objects should no
121	longer be checked. This is useful to avoid SLUB trying to generate
122	super large order pages to fit slub_min_objects of a slab cache with
123	large object sizes into one high order page.
124	
125	SLUB Debug output
126	-----------------
127	
128	Here is a sample of slub debug output:
129	
130	====================================================================
131	BUG kmalloc-8: Redzone overwritten
132	--------------------------------------------------------------------
133	
134	INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
135	INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
136	INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
137	INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
138	
139	Bytes b4 0xc90f6d10:  00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
140	  Object 0xc90f6d20:  31 30 31 39 2e 30 30 35                         1019.005
141	 Redzone 0xc90f6d28:  00 cc cc cc                                     .
142	 Padding 0xc90f6d50:  5a 5a 5a 5a 5a 5a 5a 5a                         ZZZZZZZZ
143	
144	  [<c010523d>] dump_trace+0x63/0x1eb
145	  [<c01053df>] show_trace_log_lvl+0x1a/0x2f
146	  [<c010601d>] show_trace+0x12/0x14
147	  [<c0106035>] dump_stack+0x16/0x18
148	  [<c017e0fa>] object_err+0x143/0x14b
149	  [<c017e2cc>] check_object+0x66/0x234
150	  [<c017eb43>] __slab_free+0x239/0x384
151	  [<c017f446>] kfree+0xa6/0xc6
152	  [<c02e2335>] get_modalias+0xb9/0xf5
153	  [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
154	  [<c027866a>] dev_uevent+0x1ad/0x1da
155	  [<c0205024>] kobject_uevent_env+0x20a/0x45b
156	  [<c020527f>] kobject_uevent+0xa/0xf
157	  [<c02779f1>] store_uevent+0x4f/0x58
158	  [<c027758e>] dev_attr_store+0x29/0x2f
159	  [<c01bec4f>] sysfs_write_file+0x16e/0x19c
160	  [<c0183ba7>] vfs_write+0xd1/0x15a
161	  [<c01841d7>] sys_write+0x3d/0x72
162	  [<c0104112>] sysenter_past_esp+0x5f/0x99
163	  [<b7f7b410>] 0xb7f7b410
164	  =======================
165	
166	FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
167	
168	If SLUB encounters a corrupted object (full detection requires the kernel
169	to be booted with slub_debug) then the following output will be dumped
170	into the syslog:
171	
172	1. Description of the problem encountered
173	
174	This will be a message in the system log starting with
175	
176	===============================================
177	BUG <slab cache affected>: <What went wrong>
178	-----------------------------------------------
179	
180	INFO: <corruption start>-<corruption_end> <more info>
181	INFO: Slab <address> <slab information>
182	INFO: Object <address> <object information>
183	INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
184		cpu> pid=<pid of the process>
185	INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
186		 pid=<pid of the process>
187	
188	(Object allocation / free information is only available if SLAB_STORE_USER is
189	set for the slab. slub_debug sets that option)
190	
191	2. The object contents if an object was involved.
192	
193	Various types of lines can follow the BUG SLUB line:
194	
195	Bytes b4 <address> : <bytes>
196		Shows a few bytes before the object where the problem was detected.
197		Can be useful if the corruption does not stop with the start of the
198		object.
199	
200	Object <address> : <bytes>
201		The bytes of the object. If the object is inactive then the bytes
202		typically contain poison values. Any non-poison value shows a
203		corruption by a write after free.
204	
205	Redzone <address> : <bytes>
206		The Redzone following the object. The Redzone is used to detect
207		writes after the object. All bytes should always have the same
208		value. If there is any deviation then it is due to a write after
209		the object boundary.
210	
211		(Redzone information is only available if SLAB_RED_ZONE is set.
212		slub_debug sets that option)
213	
214	Padding <address> : <bytes>
215		Unused data to fill up the space in order to get the next object
216		properly aligned. In the debug case we make sure that there are
217		at least 4 bytes of padding. This allows the detection of writes
218		before the object.
219	
220	3. A stackdump
221	
222	The stackdump describes the location where the error was detected. The cause
223	of the corruption is may be more likely found by looking at the function that
224	allocated or freed the object.
225	
226	4. Report on how the problem was dealt with in order to ensure the continued
227	operation of the system.
228	
229	These are messages in the system log beginning with
230	
231	FIX <slab cache affected>: <corrective action taken>
232	
233	In the above sample SLUB found that the Redzone of an active object has
234	been overwritten. Here a string of 8 characters was written into a slab that
235	has the length of 8 characters. However, a 8 character string needs a
236	terminating 0. That zero has overwritten the first byte of the Redzone field.
237	After reporting the details of the issue encountered the FIX SLUB message
238	tell us that SLUB has restored the Redzone to its proper value and then
239	system operations continue.
240	
241	Emergency operations:
242	---------------------
243	
244	Minimal debugging (sanity checks alone) can be enabled by booting with
245	
246		slub_debug=F
247	
248	This will be generally be enough to enable the resiliency features of slub
249	which will keep the system running even if a bad kernel component will
250	keep corrupting objects. This may be important for production systems.
251	Performance will be impacted by the sanity checks and there will be a
252	continual stream of error messages to the syslog but no additional memory
253	will be used (unlike full debugging).
254	
255	No guarantees. The kernel component still needs to be fixed. Performance
256	may be optimized further by locating the slab that experiences corruption
257	and enabling debugging only for that cache
258	
259	I.e.
260	
261		slub_debug=F,dentry
262	
263	If the corruption occurs by writing after the end of the object then it
264	may be advisable to enable a Redzone to avoid corrupting the beginning
265	of other objects.
266	
267		slub_debug=FZ,dentry
268	
269	Christoph Lameter, <clameter[AT]sgi[DOT]com>, May 30, 2007
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