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authorChristoph Lameter <clameter@sgi.com>2007-05-31 03:40:47 -0400
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-05-31 10:58:13 -0400
commitc1aee215d760175601b820bd1e2f0364e844ff8c (patch)
treeafbbbe64b57552fa55a0ee95b01922f64ecec0a1 /Documentation/vm
parent9af20376ee65cd2d13f7eb587fab70879d8c355b (diff)
SLUB: More documentation
Update documentation to describe how to read a SLUB error report. Add slub parameters to Documentation/kernel-parameters. Signed-off-by: Christoph Lameter <clameter@sgi.com> Cc: "Randy.Dunlap" <rdunlap@xenotime.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/vm')
-rw-r--r--Documentation/vm/slub.txt135
1 files changed, 124 insertions, 11 deletions
diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt
index 727c8d81aeaf..1523320abd87 100644
--- a/Documentation/vm/slub.txt
+++ b/Documentation/vm/slub.txt
@@ -1,13 +1,9 @@
1Short users guide for SLUB 1Short users guide for SLUB
2-------------------------- 2--------------------------
3 3
4First of all slub should transparently replace SLAB. If you enable
5SLUB then everything should work the same (Note the word "should".
6There is likely not much value in that word at this point).
7
8The basic philosophy of SLUB is very different from SLAB. SLAB 4The basic philosophy of SLUB is very different from SLAB. SLAB
9requires rebuilding the kernel to activate debug options for all 5requires rebuilding the kernel to activate debug options for all
10SLABS. SLUB always includes full debugging but its off by default. 6slab caches. SLUB always includes full debugging but it is off by default.
11SLUB can enable debugging only for selected slabs in order to avoid 7SLUB can enable debugging only for selected slabs in order to avoid
12an impact on overall system performance which may make a bug more 8an impact on overall system performance which may make a bug more
13difficult to find. 9difficult to find.
@@ -76,13 +72,28 @@ of objects.
76Careful with tracing: It may spew out lots of information and never stop if 72Careful with tracing: It may spew out lots of information and never stop if
77used on the wrong slab. 73used on the wrong slab.
78 74
79SLAB Merging 75Slab merging
80------------ 76------------
81 77
82If no debugging is specified then SLUB may merge similar slabs together 78If no debug options are specified then SLUB may merge similar slabs together
83in order to reduce overhead and increase cache hotness of objects. 79in order to reduce overhead and increase cache hotness of objects.
84slabinfo -a displays which slabs were merged together. 80slabinfo -a displays which slabs were merged together.
85 81
82Slab validation
83---------------
84
85SLUB can validate all object if the kernel was booted with slub_debug. In
86order to do so you must have the slabinfo tool. Then you can do
87
88slabinfo -v
89
90which will test all objects. Output will be generated to the syslog.
91
92This also works in a more limited way if boot was without slab debug.
93In that case slabinfo -v simply tests all reachable objects. Usually
94these are in the cpu slabs and the partial slabs. Full slabs are not
95tracked by SLUB in a non debug situation.
96
86Getting more performance 97Getting more performance
87------------------------ 98------------------------
88 99
@@ -91,9 +102,9 @@ list_lock once in a while to deal with partial slabs. That overhead is
91governed by the order of the allocation for each slab. The allocations 102governed by the order of the allocation for each slab. The allocations
92can be influenced by kernel parameters: 103can be influenced by kernel parameters:
93 104
94slub_min_objects=x (default 8) 105slub_min_objects=x (default 4)
95slub_min_order=x (default 0) 106slub_min_order=x (default 0)
96slub_max_order=x (default 4) 107slub_max_order=x (default 1)
97 108
98slub_min_objects allows to specify how many objects must at least fit 109slub_min_objects allows to specify how many objects must at least fit
99into one slab in order for the allocation order to be acceptable. 110into one slab in order for the allocation order to be acceptable.
@@ -109,5 +120,107 @@ longer be checked. This is useful to avoid SLUB trying to generate
109super large order pages to fit slub_min_objects of a slab cache with 120super large order pages to fit slub_min_objects of a slab cache with
110large object sizes into one high order page. 121large object sizes into one high order page.
111 122
112 123SLUB Debug output
113Christoph Lameter, <clameter@sgi.com>, April 10, 2007 124-----------------
125
126Here is a sample of slub debug output:
127
128*** SLUB kmalloc-8: Redzone Active@0xc90f6d20 slab 0xc528c530 offset=3360 flags=0x400000c3 inuse=61 freelist=0xc90f6d58
129 Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
130 Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
131 Redzone 0xc90f6d28: 00 cc cc cc .
132FreePointer 0xc90f6d2c -> 0xc90f6d58
133Last alloc: get_modalias+0x61/0xf5 jiffies_ago=53 cpu=1 pid=554
134Filler 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
135 [<c010523d>] dump_trace+0x63/0x1eb
136 [<c01053df>] show_trace_log_lvl+0x1a/0x2f
137 [<c010601d>] show_trace+0x12/0x14
138 [<c0106035>] dump_stack+0x16/0x18
139 [<c017e0fa>] object_err+0x143/0x14b
140 [<c017e2cc>] check_object+0x66/0x234
141 [<c017eb43>] __slab_free+0x239/0x384
142 [<c017f446>] kfree+0xa6/0xc6
143 [<c02e2335>] get_modalias+0xb9/0xf5
144 [<c02e23b7>] dmi_dev_uevent+0x27/0x3c
145 [<c027866a>] dev_uevent+0x1ad/0x1da
146 [<c0205024>] kobject_uevent_env+0x20a/0x45b
147 [<c020527f>] kobject_uevent+0xa/0xf
148 [<c02779f1>] store_uevent+0x4f/0x58
149 [<c027758e>] dev_attr_store+0x29/0x2f
150 [<c01bec4f>] sysfs_write_file+0x16e/0x19c
151 [<c0183ba7>] vfs_write+0xd1/0x15a
152 [<c01841d7>] sys_write+0x3d/0x72
153 [<c0104112>] sysenter_past_esp+0x5f/0x99
154 [<b7f7b410>] 0xb7f7b410
155 =======================
156@@@ SLUB kmalloc-8: Restoring redzone (0xcc) from 0xc90f6d28-0xc90f6d2b
157
158
159
160If SLUB encounters a corrupted object then it will perform the following
161actions:
162
1631. Isolation and report of the issue
164
165This will be a message in the system log starting with
166
167*** SLUB <slab cache affected>: <What went wrong>@<object address>
168offset=<offset of object into slab> flags=<slabflags>
169inuse=<objects in use in this slab> freelist=<first free object in slab>
170
1712. Report on how the problem was dealt with in order to ensure the continued
172operation of the system.
173
174These are messages in the system log beginning with
175
176@@@ SLUB <slab cache affected>: <corrective action taken>
177
178
179In the above sample SLUB found that the Redzone of an active object has
180been overwritten. Here a string of 8 characters was written into a slab that
181has the length of 8 characters. However, a 8 character string needs a
182terminating 0. That zero has overwritten the first byte of the Redzone field.
183After reporting the details of the issue encountered the @@@ SLUB message
184tell us that SLUB has restored the redzone to its proper value and then
185system operations continue.
186
187Various types of lines can follow the @@@ SLUB line:
188
189Bytes b4 <address> : <bytes>
190 Show a few bytes before the object where the problem was detected.
191 Can be useful if the corruption does not stop with the start of the
192 object.
193
194Object <address> : <bytes>
195 The bytes of the object. If the object is inactive then the bytes
196 typically contain poisoning values. Any non-poison value shows a
197 corruption by a write after free.
198
199Redzone <address> : <bytes>
200 The redzone following the object. The redzone is used to detect
201 writes after the object. All bytes should always have the same
202 value. If there is any deviation then it is due to a write after
203 the object boundary.
204
205Freepointer
206 The pointer to the next free object in the slab. May become
207 corrupted if overwriting continues after the red zone.
208
209Last alloc:
210Last free:
211 Shows the address from which the object was allocated/freed last.
212 We note the pid, the time and the CPU that did so. This is usually
213 the most useful information to figure out where things went wrong.
214 Here get_modalias() did an kmalloc(8) instead of a kmalloc(9).
215
216Filler <address> : <bytes>
217 Unused data to fill up the space in order to get the next object
218 properly aligned. In the debug case we make sure that there are
219 at least 4 bytes of filler. This allow for the detection of writes
220 before the object.
221
222Following the filler will be a stackdump. That stackdump describes the
223location where the error was detected. The cause of the corruption is more
224likely to be found by looking at the information about the last alloc / free.
225
226Christoph Lameter, <clameter@sgi.com>, May 23, 2007