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Documentation for Kdump - The kexec-based Crash Dumping Solution
This document includes overview, setup, installation, and analysis
Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
dump of the system kernel's memory needs to be taken (for example, when
the system panics). The system kernel's memory image is preserved across
the reboot and is accessible to the dump-capture kernel.
You can use common commands, such as cp, scp or makedumpfile to copy
the memory image to a dump file on the local disk, or across the network
to a remote system.
Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
s390x, arm and arm64 architectures.
When the system kernel boots, it reserves a small section of memory for
the dump-capture kernel. This ensures that ongoing Direct Memory Access
(DMA) from the system kernel does not corrupt the dump-capture kernel.
The kexec -p command loads the dump-capture kernel into this reserved
On x86 machines, the first 640 KB of physical memory is needed for boot,
regardless of where the kernel loads. For simpler handling, the whole
low 1M is reserved to avoid any later kernel or device driver writing
data into this area. Like this, the low 1M can be reused as system RAM
by kdump kernel without extra handling.
On PPC64 machines first 32KB of physical memory is needed for booting
regardless of where the kernel is loaded and to support 64K page size
kexec backs up the first 64KB memory.
For s390x, when kdump is triggered, the crashkernel region is exchanged
with the region [0, crashkernel region size] and then the kdump kernel
runs in [0, crashkernel region size]. Therefore no relocatable kernel is
needed for s390x.
All of the necessary information about the system kernel's core image is
encoded in the ELF format, and stored in a reserved area of memory
before a crash. The physical address of the start of the ELF header is
passed to the dump-capture kernel through the elfcorehdr= boot
parameter. Optionally the size of the ELF header can also be passed
when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
With the dump-capture kernel, you can access the memory image through
/proc/vmcore. This exports the dump as an ELF-format file that you can
write out using file copy commands such as cp or scp. You can also use
makedumpfile utility to analyze and write out filtered contents with
options, e.g with '-d 31' it will only write out kernel data. Further,
you can use analysis tools such as the GNU Debugger (GDB) and the Crash
tool to debug the dump file. This method ensures that the dump pages are
correctly ordered.
Setup and Installation
Install kexec-tools
1) Login as the root user.
2) Download the kexec-tools user-space package from the following URL:
This is a symlink to the latest version.
The latest kexec-tools git tree is available at:
- git://
There is also a gitweb interface available at
More information about kexec-tools can be found at
3) Unpack the tarball with the tar command, as follows::
tar xvpzf kexec-tools.tar.gz
4) Change to the kexec-tools directory, as follows::
cd kexec-tools-VERSION
5) Configure the package, as follows::
6) Compile the package, as follows::
7) Install the package, as follows::
make install
Build the system and dump-capture kernels
There are two possible methods of using Kdump.
1) Build a separate custom dump-capture kernel for capturing the
kernel core dump.
2) Or use the system kernel binary itself as dump-capture kernel and there is
no need to build a separate dump-capture kernel. This is possible
only with the architectures which support a relocatable kernel. As
of today, i386, x86_64, ppc64, ia64, arm and arm64 architectures support
relocatable kernel.
Building a relocatable kernel is advantageous from the point of view that
one does not have to build a second kernel for capturing the dump. But
at the same time one might want to build a custom dump capture kernel
suitable to his needs.
Following are the configuration setting required for system and
dump-capture kernels for enabling kdump support.
System kernel config options
1) Enable "kexec system call" or "kexec file based system call" in
"Processor type and features."::
And both of them will select KEXEC_CORE::
Subsequently, CRASH_CORE is selected by KEXEC_CORE::
2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
filesystems." This is usually enabled by default::
Note that "sysfs file system support" might not appear in the "Pseudo
filesystems" menu if "Configure standard kernel features (for small
systems)" is not enabled in "General Setup." In this case, check the
.config file itself to ensure that sysfs is turned on, as follows::
grep 'CONFIG_SYSFS' .config
3) Enable "Compile the kernel with debug info" in "Kernel hacking."::
This causes the kernel to be built with debug symbols. The dump
analysis tools require a vmlinux with debug symbols in order to read
and analyze a dump file.
Dump-capture kernel config options (Arch Independent)
1) Enable "kernel crash dumps" support under "Processor type and
2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems"::
(CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
1) On i386, enable high memory support under "Processor type and
2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
command line when loading the dump-capture kernel because one
CPU is enough for kdump kernel to dump vmcore on most of systems.
However, you can also specify nr_cpus=X to enable multiple processors
in kdump kernel. In this case, "disable_cpu_apicid=" is needed to
tell kdump kernel which cpu is 1st kernel's BSP. Please refer to
admin-guide/kernel-parameters.txt for more details.
With CONFIG_SMP=n, the above things are not related.
3) A relocatable kernel is suggested to be built by default. If not yet,
enable "Build a relocatable kernel" support under "Processor type and
4) Use a suitable value for "Physical address where the kernel is
loaded" (under "Processor type and features"). This only appears when
"kernel crash dumps" is enabled. A suitable value depends upon
whether kernel is relocatable or not.
If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
This will compile the kernel for physical address 1MB, but given the fact
kernel is relocatable, it can be run from any physical address hence
kexec boot loader will load it in memory region reserved for dump-capture
Otherwise it should be the start of memory region reserved for
second kernel using boot parameter "crashkernel=Y@X". Here X is
start of memory region reserved for dump-capture kernel.
Generally X is 16MB (0x1000000). So you can set
5) Make and install the kernel and its modules. DO NOT add this kernel
to the boot loader configuration files.
Dump-capture kernel config options (Arch Dependent, ppc64)
1) Enable "Build a kdump crash kernel" support under "Kernel" options::
2) Enable "Build a relocatable kernel" support::
Make and install the kernel and its modules.
Dump-capture kernel config options (Arch Dependent, ia64)
- No specific options are required to create a dump-capture kernel
for ia64, other than those specified in the arch independent section
above. This means that it is possible to use the system kernel
as a dump-capture kernel if desired.
The crashkernel region can be automatically placed by the system
kernel at runtime. This is done by specifying the base address as 0,
or omitting it all together::
Dump-capture kernel config options (Arch Dependent, arm)
- To use a relocatable kernel,
Enable "AUTO_ZRELADDR" support under "Boot" options::
Dump-capture kernel config options (Arch Dependent, arm64)
- Please note that kvm of the dump-capture kernel will not be enabled
on non-VHE systems even if it is configured. This is because the CPU
will not be reset to EL2 on panic.
crashkernel syntax
1) crashkernel=size@offset
Here 'size' specifies how much memory to reserve for the dump-capture kernel
and 'offset' specifies the beginning of this reserved memory. For example,
"crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
The crashkernel region can be automatically placed by the system
kernel at run time. This is done by specifying the base address as 0,
or omitting it all together::
If the start address is specified, note that the start address of the
kernel will be aligned to a value (which is Arch dependent), so if the
start address is not then any space below the alignment point will be
2) range1:size1[,range2:size2,...][@offset]
While the "crashkernel=size[@offset]" syntax is sufficient for most
configurations, sometimes it's handy to have the reserved memory dependent
on the value of System RAM -- that's mostly for distributors that pre-setup
the kernel command line to avoid a unbootable system after some memory has
been removed from the machine.
The syntax is::
For example::
This would mean:
1) if the RAM is smaller than 512M, then don't reserve anything
(this is the "rescue" case)
2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
3) if the RAM size is larger than 2G, then reserve 128M
3) crashkernel=size,high and crashkernel=size,low
If memory above 4G is preferred, crashkernel=size,high can be used to
fulfill that. With it, physical memory is allowed to be allocated from top,
so could be above 4G if system has more than 4G RAM installed. Otherwise,
memory region will be allocated below 4G if available.
When crashkernel=X,high is passed, kernel could allocate physical memory
region above 4G, low memory under 4G is needed in this case. There are
three ways to get low memory:
1) Kernel will allocate at least 256M memory below 4G automatically
if crashkernel=Y,low is not specified.
2) Let user specify low memory size instead.
3) Specified value 0 will disable low memory allocation::
Boot into System Kernel
1) Update the boot loader (such as grub, yaboot, or lilo) configuration
files as necessary.
2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the
start address 'X' is not necessary, kernel will search a suitable
area. Unless an explicit start address is expected.
On ppc64, use "crashkernel=128M@32M".
On ia64, 256M@256M is a generous value that typically works.
The region may be automatically placed on ia64, see the
dump-capture kernel config option notes above.
If use sparse memory, the size should be rounded to GRANULE boundaries.
On s390x, typically use "crashkernel=xxM". The value of xx is dependent
on the memory consumption of the kdump system. In general this is not
dependent on the memory size of the production system.
On arm, the use of "crashkernel=Y@X" is no longer necessary; the
kernel will automatically locate the crash kernel image within the
first 512MB of RAM if X is not given.
On arm64, use "crashkernel=Y[@X]". Note that the start address of
the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
Load the Dump-capture Kernel
After booting to the system kernel, dump-capture kernel needs to be
Based on the architecture and type of image (relocatable or not), one
can choose to load the uncompressed vmlinux or compressed bzImage/vmlinuz
of dump-capture kernel. Following is the summary.
For i386 and x86_64:
- Use bzImage/vmlinuz if kernel is relocatable.
- Use vmlinux if kernel is not relocatable.
For ppc64:
- Use vmlinux
For ia64:
- Use vmlinux or vmlinuz.gz
For s390x:
- Use image or bzImage
For arm:
- Use zImage
For arm64:
- Use vmlinux or Image
If you are using an uncompressed vmlinux image then use following command
to load dump-capture kernel::
kexec -p <dump-capture-kernel-vmlinux-image> \
--initrd=<initrd-for-dump-capture-kernel> --args-linux \
--append="root=<root-dev> <arch-specific-options>"
If you are using a compressed bzImage/vmlinuz, then use following command
to load dump-capture kernel::
kexec -p <dump-capture-kernel-bzImage> \
--initrd=<initrd-for-dump-capture-kernel> \
--append="root=<root-dev> <arch-specific-options>"
If you are using a compressed zImage, then use following command
to load dump-capture kernel::
kexec --type zImage -p <dump-capture-kernel-bzImage> \
--initrd=<initrd-for-dump-capture-kernel> \
--dtb=<dtb-for-dump-capture-kernel> \
--append="root=<root-dev> <arch-specific-options>"
If you are using an uncompressed Image, then use following command
to load dump-capture kernel::
kexec -p <dump-capture-kernel-Image> \
--initrd=<initrd-for-dump-capture-kernel> \
--append="root=<root-dev> <arch-specific-options>"
Please note, that --args-linux does not need to be specified for ia64.
It is planned to make this a no-op on that architecture, but for now
it should be omitted
Following are the arch specific command line options to be used while
loading dump-capture kernel.
For i386, x86_64 and ia64:
"1 irqpoll nr_cpus=1 reset_devices"
For ppc64:
"1 maxcpus=1 noirqdistrib reset_devices"
For s390x:
"1 nr_cpus=1 cgroup_disable=memory"
For arm:
"1 maxcpus=1 reset_devices"
For arm64:
"1 nr_cpus=1 reset_devices"
Notes on loading the dump-capture kernel:
* By default, the ELF headers are stored in ELF64 format to support
systems with more than 4GB memory. On i386, kexec automatically checks if
the physical RAM size exceeds the 4 GB limit and if not, uses ELF32.
So, on non-PAE systems, ELF32 is always used.
The --elf32-core-headers option can be used to force the generation of ELF32
headers. This is necessary because GDB currently cannot open vmcore files
with ELF64 headers on 32-bit systems.
* The "irqpoll" boot parameter reduces driver initialization failures
due to shared interrupts in the dump-capture kernel.
* You must specify <root-dev> in the format corresponding to the root
device name in the output of mount command.
* Boot parameter "1" boots the dump-capture kernel into single-user
mode without networking. If you want networking, use "3".
* We generally don't have to bring up a SMP kernel just to capture the
dump. Hence generally it is useful either to build a UP dump-capture
kernel or specify maxcpus=1 option while loading dump-capture kernel.
Note, though maxcpus always works, you had better replace it with
nr_cpus to save memory if supported by the current ARCH, such as x86.
* You should enable multi-cpu support in dump-capture kernel if you intend
to use multi-thread programs with it, such as parallel dump feature of
makedumpfile. Otherwise, the multi-thread program may have a great
performance degradation. To enable multi-cpu support, you should bring up an
SMP dump-capture kernel and specify maxcpus/nr_cpus, disable_cpu_apicid=[X]
options while loading it.
* For s390x there are two kdump modes: If a ELF header is specified with
the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
is done on all other architectures. If no elfcorehdr= kernel parameter is
specified, the s390x kdump kernel dynamically creates the header. The
second mode has the advantage that for CPU and memory hotplug, kdump has
not to be reloaded with kexec_load().
* For s390x systems with many attached devices the "cio_ignore" kernel
parameter should be used for the kdump kernel in order to prevent allocation
of kernel memory for devices that are not relevant for kdump. The same
applies to systems that use SCSI/FCP devices. In that case the
"allow_lun_scan" zfcp module parameter should be set to zero before
setting FCP devices online.
Kernel Panic
After successfully loading the dump-capture kernel as previously
described, the system will reboot into the dump-capture kernel if a
system crash is triggered. Trigger points are located in panic(),
die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
The following conditions will execute a crash trigger point:
If a hard lockup is detected and "NMI watchdog" is configured, the system
will boot into the dump-capture kernel ( die_nmi() ).
If die() is called, and it happens to be a thread with pid 0 or 1, or die()
is called inside interrupt context or die() is called and panic_on_oops is set,
the system will boot into the dump-capture kernel.
On powerpc systems when a soft-reset is generated, die() is called by all cpus
and the system will boot into the dump-capture kernel.
For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
"echo c > /proc/sysrq-trigger" or write a module to force the panic.
Write Out the Dump File
After the dump-capture kernel is booted, write out the dump file with
the following command::
cp /proc/vmcore <dump-file>
You can also use makedumpfile utility to write out the dump file
with specified options to filter out unwanted contents, e.g::
makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
Before analyzing the dump image, you should reboot into a stable kernel.
You can do limited analysis using GDB on the dump file copied out of
/proc/vmcore. Use the debug vmlinux built with -g and run the following
gdb vmlinux <dump-file>
Stack trace for the task on processor 0, register display, and memory
display work fine.
Note: GDB cannot analyze core files generated in ELF64 format for x86.
On systems with a maximum of 4GB of memory, you can generate
ELF32-format headers using the --elf32-core-headers kernel option on the
dump kernel.
You can also use the Crash utility to analyze dump files in Kdump
format. Crash is available at the following URL:
Crash document can be found at:
Trigger Kdump on WARN()
The kernel parameter, panic_on_warn, calls panic() in all WARN() paths. This
will cause a kdump to occur at the panic() call. In cases where a user wants
to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
to achieve the same behaviour.
Trigger Kdump on add_taint()
The kernel parameter panic_on_taint facilitates a conditional call to panic()
from within add_taint() whenever the value set in this bitmask matches with the
bit flag being set by add_taint().
This will cause a kdump to occur at the add_taint()->panic() call.
GDB macros
.. include:: gdbmacros.txt