Root/Documentation/vm/slub.txt

1Short users guide for SLUB
2--------------------------
3
4The basic philosophy of SLUB is very different from SLAB. SLAB
5requires rebuilding the kernel to activate debug options for all
6slab caches. SLUB always includes full debugging but it is off by default.
7SLUB can enable debugging only for selected slabs in order to avoid
8an impact on overall system performance which may make a bug more
9difficult to find.
10
11In order to switch debugging on one can add a option "slub_debug"
12to the kernel command line. That will enable full debugging for
13all slabs.
14
15Typically one would then use the "slabinfo" command to get statistical
16data and perform operation on the slabs. By default slabinfo only lists
17slabs that have data in them. See "slabinfo -h" for more options when
18running the command. slabinfo can be compiled with
19
20gcc -o slabinfo Documentation/vm/slabinfo.c
21
22Some of the modes of operation of slabinfo require that slub debugging
23be enabled on the command line. F.e. no tracking information will be
24available without debugging on and validation can only partially
25be performed if debugging was not switched on.
26
27Some more sophisticated uses of slub_debug:
28-------------------------------------------
29
30Parameters may be given to slub_debug. If none is specified then full
31debugging is enabled. Format:
32
33slub_debug=<Debug-Options> Enable options for all slabs
34slub_debug=<Debug-Options>,<slab name>
35                Enable options only for select slabs
36
37Possible 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
50F.e. in order to boot just with sanity checks and red zoning one would specify:
51
52    slub_debug=FZ
53
54Trying to find an issue in the dentry cache? Try
55
56    slub_debug=,dentry
57
58to only enable debugging on the dentry cache.
59
60Red zoning and tracking may realign the slab. We can just apply sanity checks
61to the dentry cache with
62
63    slub_debug=F,dentry
64
65Debugging options may require the minimum possible slab order to increase as
66a result of storing the metadata (for example, caches with PAGE_SIZE object
67sizes). This has a higher liklihood of resulting in slab allocation errors
68in low memory situations or if there's high fragmentation of memory. To
69switch off debugging for such caches by default, use
70
71    slub_debug=O
72
73In case you forgot to enable debugging on the kernel command line: It is
74possible to enable debugging manually when the kernel is up. Look at the
75contents of:
76
77/sys/kernel/slab/<slab name>/
78
79Look at the writable files. Writing 1 to them will enable the
80corresponding debug option. All options can be set on a slab that does
81not contain objects. If the slab already contains objects then sanity checks
82and tracing may only be enabled. The other options may cause the realignment
83of objects.
84
85Careful with tracing: It may spew out lots of information and never stop if
86used on the wrong slab.
87
88Slab merging
89------------
90
91If no debug options are specified then SLUB may merge similar slabs together
92in order to reduce overhead and increase cache hotness of objects.
93slabinfo -a displays which slabs were merged together.
94
95Slab validation
96---------------
97
98SLUB can validate all object if the kernel was booted with slub_debug. In
99order to do so you must have the slabinfo tool. Then you can do
100
101slabinfo -v
102
103which will test all objects. Output will be generated to the syslog.
104
105This also works in a more limited way if boot was without slab debug.
106In that case slabinfo -v simply tests all reachable objects. Usually
107these are in the cpu slabs and the partial slabs. Full slabs are not
108tracked by SLUB in a non debug situation.
109
110Getting more performance
111------------------------
112
113To some degree SLUB's performance is limited by the need to take the
114list_lock once in a while to deal with partial slabs. That overhead is
115governed by the order of the allocation for each slab. The allocations
116can be influenced by kernel parameters:
117
118slub_min_objects=x (default 4)
119slub_min_order=x (default 0)
120slub_max_order=x (default 1)
121
122slub_min_objects allows to specify how many objects must at least fit
123into one slab in order for the allocation order to be acceptable.
124In general slub will be able to perform this number of allocations
125on a slab without consulting centralized resources (list_lock) where
126contention may occur.
127
128slub_min_order specifies a minim order of slabs. A similar effect like
129slub_min_objects.
130
131slub_max_order specified the order at which slub_min_objects should no
132longer be checked. This is useful to avoid SLUB trying to generate
133super large order pages to fit slub_min_objects of a slab cache with
134large object sizes into one high order page.
135
136SLUB Debug output
137-----------------
138
139Here is a sample of slub debug output:
140
141====================================================================
142BUG kmalloc-8: Redzone overwritten
143--------------------------------------------------------------------
144
145INFO: 0xc90f6d28-0xc90f6d2b. First byte 0x00 instead of 0xcc
146INFO: Slab 0xc528c530 flags=0x400000c3 inuse=61 fp=0xc90f6d58
147INFO: Object 0xc90f6d20 @offset=3360 fp=0xc90f6d58
148INFO: Allocated in get_modalias+0x61/0xf5 age=53 cpu=1 pid=554
149
150Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
151  Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
152 Redzone 0xc90f6d28: 00 cc cc cc .
153 Padding 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
154
155  [<c010523d>] dump_trace+0x63/0x1eb
156  [<c01053df>] show_trace_log_lvl+0x1a/0x2f
157  [<c010601d>] show_trace+0x12/0x14
158  [<c0106035>] dump_stack+0x16/0x18
159  [<c017e0fa>] object_err+0x143/0x14b
160  [<c017e2cc>] check_object+0x66/0x234
161  [<c017eb43>] __slab_free+0x239/0x384
162  [<c017f446>] kfree+0xa6/0xc6
163  [<c02e2335>] get_modalias+0xb9/0xf5
164  [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
165  [<c027866a>] dev_uevent+0x1ad/0x1da
166  [<c0205024>] kobject_uevent_env+0x20a/0x45b
167  [<c020527f>] kobject_uevent+0xa/0xf
168  [<c02779f1>] store_uevent+0x4f/0x58
169  [<c027758e>] dev_attr_store+0x29/0x2f
170  [<c01bec4f>] sysfs_write_file+0x16e/0x19c
171  [<c0183ba7>] vfs_write+0xd1/0x15a
172  [<c01841d7>] sys_write+0x3d/0x72
173  [<c0104112>] sysenter_past_esp+0x5f/0x99
174  [<b7f7b410>] 0xb7f7b410
175  =======================
176
177FIX kmalloc-8: Restoring Redzone 0xc90f6d28-0xc90f6d2b=0xcc
178
179If SLUB encounters a corrupted object (full detection requires the kernel
180to be booted with slub_debug) then the following output will be dumped
181into the syslog:
182
1831. Description of the problem encountered
184
185This will be a message in the system log starting with
186
187===============================================
188BUG <slab cache affected>: <What went wrong>
189-----------------------------------------------
190
191INFO: <corruption start>-<corruption_end> <more info>
192INFO: Slab <address> <slab information>
193INFO: Object <address> <object information>
194INFO: Allocated in <kernel function> age=<jiffies since alloc> cpu=<allocated by
195    cpu> pid=<pid of the process>
196INFO: Freed in <kernel function> age=<jiffies since free> cpu=<freed by cpu>
197     pid=<pid of the process>
198
199(Object allocation / free information is only available if SLAB_STORE_USER is
200set for the slab. slub_debug sets that option)
201
2022. The object contents if an object was involved.
203
204Various types of lines can follow the BUG SLUB line:
205
206Bytes b4 <address> : <bytes>
207    Shows a few bytes before the object where the problem was detected.
208    Can be useful if the corruption does not stop with the start of the
209    object.
210
211Object <address> : <bytes>
212    The bytes of the object. If the object is inactive then the bytes
213    typically contain poison values. Any non-poison value shows a
214    corruption by a write after free.
215
216Redzone <address> : <bytes>
217    The Redzone following the object. The Redzone is used to detect
218    writes after the object. All bytes should always have the same
219    value. If there is any deviation then it is due to a write after
220    the object boundary.
221
222    (Redzone information is only available if SLAB_RED_ZONE is set.
223    slub_debug sets that option)
224
225Padding <address> : <bytes>
226    Unused data to fill up the space in order to get the next object
227    properly aligned. In the debug case we make sure that there are
228    at least 4 bytes of padding. This allows the detection of writes
229    before the object.
230
2313. A stackdump
232
233The stackdump describes the location where the error was detected. The cause
234of the corruption is may be more likely found by looking at the function that
235allocated or freed the object.
236
2374. Report on how the problem was dealt with in order to ensure the continued
238operation of the system.
239
240These are messages in the system log beginning with
241
242FIX <slab cache affected>: <corrective action taken>
243
244In the above sample SLUB found that the Redzone of an active object has
245been overwritten. Here a string of 8 characters was written into a slab that
246has the length of 8 characters. However, a 8 character string needs a
247terminating 0. That zero has overwritten the first byte of the Redzone field.
248After reporting the details of the issue encountered the FIX SLUB message
249tells us that SLUB has restored the Redzone to its proper value and then
250system operations continue.
251
252Emergency operations:
253---------------------
254
255Minimal debugging (sanity checks alone) can be enabled by booting with
256
257    slub_debug=F
258
259This will be generally be enough to enable the resiliency features of slub
260which will keep the system running even if a bad kernel component will
261keep corrupting objects. This may be important for production systems.
262Performance will be impacted by the sanity checks and there will be a
263continual stream of error messages to the syslog but no additional memory
264will be used (unlike full debugging).
265
266No guarantees. The kernel component still needs to be fixed. Performance
267may be optimized further by locating the slab that experiences corruption
268and enabling debugging only for that cache
269
270I.e.
271
272    slub_debug=F,dentry
273
274If the corruption occurs by writing after the end of the object then it
275may be advisable to enable a Redzone to avoid corrupting the beginning
276of other objects.
277
278    slub_debug=FZ,dentry
279
280Christoph Lameter, May 30, 2007
281

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