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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.
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
ELF header is passed to new kernel through command line parameter elfcorehdr=.
On i386, the first 640 KB of physical memory is needed to boot, irrespective
of where the kernel loads. Hence, this region is backed up by kexec just before
rebooting into the new kernel.
In the second kernel, "old memory" can be accessed in two ways.
- The first one is through a /dev/oldmem device interface. A capture utility
can read the device file and write out the memory in raw format. This is raw
dump of memory and analysis/capture tool should be intelligent enough to
determine where to look for the right information. ELF headers (elfcorehdr=)
can become handy here.
- The second interface is through /proc/vmcore. This exports the dump as an ELF
format file which can be written out using any file copy command
(cp, scp, etc). Further, gdb can be used to perform limited debugging on
the dump file. This method ensures methods ensure that there is correct
ordering of the dump pages (corresponding to the first 640 KB that has been
relocated).
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.
2) Download and build the appropriate (latest) kexec/kdump (-mm) kernel
patchset and apply it to the vanilla kernel tree.
Two kernels need to be built in order to get this feature working.
A) First 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
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
Note: Options a) and b) depend upon "Configure standard kernel features
(for small systems)" (under General setup).
Option a) also depends on CONFIG_HIGHMEM (under Processor
type and features).
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:
kexec -p <second-kernel> --crash-dump --args-linux --append="root=<root-dev>
maxcpus=1 init 1"
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 ELF32 format (for i386). This
is sufficient to represent the physical memory up to 4GB. To store
headers in ELF64 format, specifiy "--elf64-core-headers" on the
kexec command line additionally.
iii) For now (or until it is fixed), it's best to build the
second-kernel without multi-processor support, i.e., make it
a uniprocessor kernel.
5) System reboots into the second kernel when a panic occurs. A module can be
written to force the panic, for testing purposes.
6) Write out the dump file using
cp /proc/vmcore <dump-file>
Dump memory can also be accessed as a /dev/oldmem device for a linear/raw
view. To create the device, type:
mknod /dev/oldmem c 1 12
Use "dd" with suitable options for count, bs and skip to access specific
portions of the dump.
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
gdb vmlinux <dump-file>
Stack trace for the task on processor 0, register display, memory display
work fine.
Note: gdb cannot analyse core files generated in ELF64 format for i386.
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.
CONTACT
=======
Hariprasad Nellitheertha - hari at in dot ibm dot com
Vivek Goyal (vgoyal@in.ibm.com)
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