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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>
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Christoph Lameter
Linus Torvalds
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c1aee215d7
@ -1132,9 +1132,9 @@ and is between 256 and 4096 characters. It is defined in the file
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when set.
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Format: <int>
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noaliencache [MM, NUMA] Disables the allcoation of alien caches in
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the slab allocator. Saves per-node memory, but will
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impact performance on real NUMA hardware.
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noaliencache [MM, NUMA, SLAB] Disables the allocation of alien
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caches in the slab allocator. Saves per-node memory,
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but will impact performance.
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noalign [KNL,ARM]
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@ -1613,6 +1613,37 @@ and is between 256 and 4096 characters. It is defined in the file
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slram= [HW,MTD]
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slub_debug [MM, SLUB]
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Enabling slub_debug allows one to determine the culprit
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if slab objects become corrupted. Enabling slub_debug
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creates guard zones around objects and poisons objects
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when not in use. Also tracks the last alloc / free.
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For more information see Documentation/vm/slub.txt.
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slub_max_order= [MM, SLUB]
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Determines the maximum allowed order for slabs. Setting
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this too high may cause fragmentation.
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For more information see Documentation/vm/slub.txt.
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slub_min_objects= [MM, SLUB]
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The minimum objects per slab. SLUB will increase the
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slab order up to slub_max_order to generate a
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sufficiently big slab to satisfy the number of objects.
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The higher the number of objects the smaller the overhead
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of tracking slabs.
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For more information see Documentation/vm/slub.txt.
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slub_min_order= [MM, SLUB]
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Determines the mininum page order for slabs. Must be
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lower than slub_max_order
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For more information see Documentation/vm/slub.txt.
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slub_nomerge [MM, SLUB]
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Disable merging of slabs of similar size. May be
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necessary if there is some reason to distinguish
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allocs to different slabs.
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For more information see Documentation/vm/slub.txt.
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smart2= [HW]
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Format: <io1>[,<io2>[,...,<io8>]]
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@ -1,13 +1,9 @@
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Short users guide for SLUB
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--------------------------
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First of all slub should transparently replace SLAB. If you enable
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SLUB then everything should work the same (Note the word "should".
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There is likely not much value in that word at this point).
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The basic philosophy of SLUB is very different from SLAB. SLAB
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requires rebuilding the kernel to activate debug options for all
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SLABS. SLUB always includes full debugging but its off by default.
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slab caches. SLUB always includes full debugging but it is off by default.
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SLUB can enable debugging only for selected slabs in order to avoid
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an impact on overall system performance which may make a bug more
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difficult to find.
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@ -76,13 +72,28 @@ of objects.
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Careful with tracing: It may spew out lots of information and never stop if
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used on the wrong slab.
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SLAB Merging
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Slab merging
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------------
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If no debugging is specified then SLUB may merge similar slabs together
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If no debug options are specified then SLUB may merge similar slabs together
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in order to reduce overhead and increase cache hotness of objects.
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slabinfo -a displays which slabs were merged together.
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Slab validation
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---------------
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SLUB can validate all object if the kernel was booted with slub_debug. In
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order to do so you must have the slabinfo tool. Then you can do
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slabinfo -v
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which will test all objects. Output will be generated to the syslog.
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This also works in a more limited way if boot was without slab debug.
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In that case slabinfo -v simply tests all reachable objects. Usually
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these are in the cpu slabs and the partial slabs. Full slabs are not
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tracked by SLUB in a non debug situation.
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Getting more performance
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------------------------
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@ -91,9 +102,9 @@ list_lock once in a while to deal with partial slabs. That overhead is
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governed by the order of the allocation for each slab. The allocations
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can be influenced by kernel parameters:
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slub_min_objects=x (default 8)
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slub_min_objects=x (default 4)
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slub_min_order=x (default 0)
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slub_max_order=x (default 4)
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slub_max_order=x (default 1)
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slub_min_objects allows to specify how many objects must at least fit
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into one slab in order for the allocation order to be acceptable.
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@ -109,5 +120,107 @@ longer be checked. This is useful to avoid SLUB trying to generate
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super large order pages to fit slub_min_objects of a slab cache with
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large object sizes into one high order page.
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SLUB Debug output
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-----------------
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Christoph Lameter, <clameter@sgi.com>, April 10, 2007
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Here is a sample of slub debug output:
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*** SLUB kmalloc-8: Redzone Active@0xc90f6d20 slab 0xc528c530 offset=3360 flags=0x400000c3 inuse=61 freelist=0xc90f6d58
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Bytes b4 0xc90f6d10: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
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Object 0xc90f6d20: 31 30 31 39 2e 30 30 35 1019.005
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Redzone 0xc90f6d28: 00 cc cc cc .
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FreePointer 0xc90f6d2c -> 0xc90f6d58
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Last alloc: get_modalias+0x61/0xf5 jiffies_ago=53 cpu=1 pid=554
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Filler 0xc90f6d50: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
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[<c010523d>] dump_trace+0x63/0x1eb
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[<c01053df>] show_trace_log_lvl+0x1a/0x2f
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[<c010601d>] show_trace+0x12/0x14
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[<c0106035>] dump_stack+0x16/0x18
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[<c017e0fa>] object_err+0x143/0x14b
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[<c017e2cc>] check_object+0x66/0x234
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[<c017eb43>] __slab_free+0x239/0x384
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[<c017f446>] kfree+0xa6/0xc6
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[<c02e2335>] get_modalias+0xb9/0xf5
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[<c02e23b7>] dmi_dev_uevent+0x27/0x3c
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[<c027866a>] dev_uevent+0x1ad/0x1da
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[<c0205024>] kobject_uevent_env+0x20a/0x45b
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[<c020527f>] kobject_uevent+0xa/0xf
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[<c02779f1>] store_uevent+0x4f/0x58
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[<c027758e>] dev_attr_store+0x29/0x2f
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[<c01bec4f>] sysfs_write_file+0x16e/0x19c
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[<c0183ba7>] vfs_write+0xd1/0x15a
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[<c01841d7>] sys_write+0x3d/0x72
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[<c0104112>] sysenter_past_esp+0x5f/0x99
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[<b7f7b410>] 0xb7f7b410
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=======================
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@@@ SLUB kmalloc-8: Restoring redzone (0xcc) from 0xc90f6d28-0xc90f6d2b
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If SLUB encounters a corrupted object then it will perform the following
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actions:
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1. Isolation and report of the issue
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This will be a message in the system log starting with
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*** SLUB <slab cache affected>: <What went wrong>@<object address>
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offset=<offset of object into slab> flags=<slabflags>
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inuse=<objects in use in this slab> freelist=<first free object in slab>
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2. Report on how the problem was dealt with in order to ensure the continued
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operation of the system.
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These are messages in the system log beginning with
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@@@ SLUB <slab cache affected>: <corrective action taken>
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In the above sample SLUB found that the Redzone of an active object has
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been overwritten. Here a string of 8 characters was written into a slab that
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has the length of 8 characters. However, a 8 character string needs a
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terminating 0. That zero has overwritten the first byte of the Redzone field.
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After reporting the details of the issue encountered the @@@ SLUB message
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tell us that SLUB has restored the redzone to its proper value and then
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system operations continue.
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Various types of lines can follow the @@@ SLUB line:
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Bytes b4 <address> : <bytes>
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Show a few bytes before the object where the problem was detected.
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Can be useful if the corruption does not stop with the start of the
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object.
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Object <address> : <bytes>
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The bytes of the object. If the object is inactive then the bytes
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typically contain poisoning values. Any non-poison value shows a
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corruption by a write after free.
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Redzone <address> : <bytes>
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The redzone following the object. The redzone is used to detect
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writes after the object. All bytes should always have the same
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value. If there is any deviation then it is due to a write after
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the object boundary.
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Freepointer
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The pointer to the next free object in the slab. May become
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corrupted if overwriting continues after the red zone.
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Last alloc:
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Last free:
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Shows the address from which the object was allocated/freed last.
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We note the pid, the time and the CPU that did so. This is usually
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the most useful information to figure out where things went wrong.
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Here get_modalias() did an kmalloc(8) instead of a kmalloc(9).
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Filler <address> : <bytes>
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Unused data to fill up the space in order to get the next object
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properly aligned. In the debug case we make sure that there are
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at least 4 bytes of filler. This allow for the detection of writes
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before the object.
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Following the filler will be a stackdump. That stackdump describes the
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location where the error was detected. The cause of the corruption is more
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likely to be found by looking at the information about the last alloc / free.
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Christoph Lameter, <clameter@sgi.com>, May 23, 2007
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