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[PATCH] Kdump documentation update
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Update the kdump documentation to reflect the changes due to recent kernel
config option changes for kexec and kdump.

Signed-off-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Maneesh Soni authored and Linus Torvalds committed Jan 10, 2006
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149 changes: 79 additions & 70 deletions Documentation/kdump/kdump.txt
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Expand Up @@ -4,10 +4,10 @@ Documentation for kdump - the kexec-based crash dumping solution
DESIGN
======

Kdump uses kexec to reboot to a second kernel whenever a dump needs to be taken.
This second kernel is booted with very little memory. The first kernel reserves
the section of memory that the second kernel uses. This ensures that on-going
DMA from the first kernel does not corrupt the second kernel.
Kdump uses kexec to reboot to a second kernel whenever a dump needs to be
taken. This second kernel is booted with very little memory. The first kernel
reserves the section of memory that the second kernel uses. This ensures that
on-going DMA from the first kernel does not corrupt the second kernel.

All the necessary information about Core image is encoded in ELF format and
stored in reserved area of memory before crash. Physical address of start of
Expand Down Expand Up @@ -35,77 +35,82 @@ In the second kernel, "old memory" can be accessed in two ways.
SETUP
=====

1) Download http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
and apply http://lse.sourceforge.net/kdump/patches/kexec-tools-1.101-kdump.patch
and after that build the source.
1) Download the upstream kexec-tools userspace package from
http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz.

2) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernel.
Apply the latest consolidated kdump patch on top of kexec-tools-1.101
from http://lse.sourceforge.net/kdump/. This arrangment has been made
till all the userspace patches supporting kdump are integrated with
upstream kexec-tools userspace.

2) Download and build the appropriate (2.6.13-rc1 onwards) vanilla kernels.
Two kernels need to be built in order to get this feature working.
Following are the steps to properly configure the two kernels specific
to kexec and kdump features:

A) First kernel:
A) First kernel or regular kernel:
----------------------------------
a) Enable "kexec system call" feature (in Processor type and features).
CONFIG_KEXEC=y
b) This kernel's physical load address should be the default value of
0x100000 (0x100000, 1 MB) (in Processor type and features).
CONFIG_PHYSICAL_START=0x100000
c) Enable "sysfs file system support" (in Pseudo filesystems).
CONFIG_SYSFS=y
CONFIG_KEXEC=y
b) Enable "sysfs file system support" (in Pseudo filesystems).
CONFIG_SYSFS=y
c) make
d) Boot into first kernel with the command line parameter "crashkernel=Y@X".
Use appropriate values for X and Y. Y denotes how much memory to reserve
for the second kernel, and X denotes at what physical address the reserved
memory section starts. For example: "crashkernel=64M@16M".

B) Second kernel:
a) Enable "kernel crash dumps" feature (in Processor type and features).
CONFIG_CRASH_DUMP=y
b) Specify a suitable value for "Physical address where the kernel is
loaded" (in Processor type and features). Typically this value
should be same as X (See option d) above, e.g., 16 MB or 0x1000000.
CONFIG_PHYSICAL_START=0x1000000
c) Enable "/proc/vmcore support" (Optional, in Pseudo filesystems).
CONFIG_PROC_VMCORE=y
d) Disable SMP support and build a UP kernel (Until it is fixed).
CONFIG_SMP=n
e) Enable "Local APIC support on uniprocessors".
CONFIG_X86_UP_APIC=y
f) Enable "IO-APIC support on uniprocessors"
CONFIG_X86_UP_IOAPIC=y

Note: i) Options a) and b) depend upon "Configure standard kernel features
(for small systems)" (under General setup).
ii) Option a) also depends on CONFIG_HIGHMEM (under Processor
type and features).
iii) Both option a) and b) are under "Processor type and features".

3) Boot into the first kernel. You are now ready to try out kexec-based crash
dumps.

4) Load the second kernel to be booted using:
for the second kernel, and X denotes at what physical address the
reserved memory section starts. For example: "crashkernel=64M@16M".


B) Second kernel or dump capture kernel:
---------------------------------------
a) For i386 architecture enable Highmem support
CONFIG_HIGHMEM=y
b) Enable "kernel crash dumps" feature (under "Processor type and features")
CONFIG_CRASH_DUMP=y
c) Make sure a suitable value for "Physical address where the kernel is
loaded" (under "Processor type and features"). By default this value
is 0x1000000 (16MB) and it should be same as X (See option d above),
e.g., 16 MB or 0x1000000.
CONFIG_PHYSICAL_START=0x1000000
d) Enable "/proc/vmcore support" (Optional, under "Pseudo filesystems").
CONFIG_PROC_VMCORE=y

3) After booting to regular kernel or first kernel, load the second kernel
using the following command:

kexec -p <second-kernel> --args-linux --elf32-core-headers
--append="root=<root-dev> init 1 irqpoll"

Note: i) <second-kernel> has to be a vmlinux image. bzImage will not work,
as of now.
ii) By default ELF headers are stored in ELF64 format. Option
--elf32-core-headers forces generation of ELF32 headers. gdb can
not open ELF64 headers on 32 bit systems. So creating ELF32
headers can come handy for users who have got non-PAE systems and
hence have memory less than 4GB.
iii) Specify "irqpoll" as command line parameter. This reduces driver
initialization failures in second kernel due to shared interrupts.
iv) <root-dev> needs to be specified in a format corresponding to
the root device name in the output of mount command.
v) If you have built the drivers required to mount root file
system as modules in <second-kernel>, then, specify
--initrd=<initrd-for-second-kernel>.

5) System reboots into the second kernel when a panic occurs. A module can be
written to force the panic or "ALT-SysRq-c" can be used initiate a crash
dump for testing purposes.

6) Write out the dump file using
--append="root=<root-dev> init 1 irqpoll maxcpus=1"

Notes:
======
i) <second-kernel> has to be a vmlinux image ie uncompressed elf image.
bzImage will not work, as of now.
ii) --args-linux has to be speicfied as if kexec it loading an elf image,
it needs to know that the arguments supplied are of linux type.
iii) By default ELF headers are stored in ELF64 format to support systems
with more than 4GB memory. Option --elf32-core-headers forces generation
of ELF32 headers. The reason for this option being, as of now gdb can
not open vmcore file with ELF64 headers on a 32 bit systems. So ELF32
headers can be used if one has non-PAE systems and hence memory less
than 4GB.
iv) Specify "irqpoll" as command line parameter. This reduces driver
initialization failures in second kernel due to shared interrupts.
v) <root-dev> needs to be specified in a format corresponding to the root
device name in the output of mount command.
vi) If you have built the drivers required to mount root file system as
modules in <second-kernel>, then, specify
--initrd=<initrd-for-second-kernel>.
vii) Specify maxcpus=1 as, if during first kernel run, if panic happens on
non-boot cpus, second kernel doesn't seem to be boot up all the cpus.
The other option is to always built the second kernel without SMP
support ie CONFIG_SMP=n

4) After successfully loading the second kernel as above, if a panic occurs
system reboots into the second kernel. A module can be written to force
the panic or "ALT-SysRq-c" can be used initiate a crash dump for testing
purposes.

5) Once the second kernel has booted, write out the dump file using

cp /proc/vmcore <dump-file>

Expand All @@ -119,9 +124,9 @@ SETUP

Entire memory: dd if=/dev/oldmem of=oldmem.001


ANALYSIS
========

Limited analysis can be done using gdb on the dump file copied out of
/proc/vmcore. Use vmlinux built with -g and run

Expand All @@ -132,15 +137,19 @@ work fine.

Note: gdb cannot analyse core files generated in ELF64 format for i386.

Latest "crash" (crash-4.0-2.18) as available on Dave Anderson's site
http://people.redhat.com/~anderson/ works well with kdump format.


TODO
====

1) Provide a kernel pages filtering mechanism so that core file size is not
insane on systems having huge memory banks.
2) Modify "crash" tool to make it recognize this dump.
2) Relocatable kernel can help in maintaining multiple kernels for crashdump
and same kernel as the first kernel can be used to capture the dump.


CONTACT
=======

Vivek Goyal (vgoyal@in.ibm.com)
Maneesh Soni (maneesh@in.ibm.com)

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