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-Documentation for kdump - the kexec-based crash dumping solution
+Documentation for Kdump - The kexec-based Crash Dumping Solution
+This document includes overview, setup and installation, and analysis
-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=.
+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.
-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.
+You can use common Linux commands, such as cp and scp, to copy the
+memory image to a dump file on the local disk, or across the network to
+a remote system.
-In the second kernel, "old memory" can be accessed in two ways.
+Kdump and kexec are currently supported on the x86, x86_64, and ppc64
-- 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.
+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
-- 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
+On x86 machines, the first 640 KB of physical memory is needed to boot,
+regardless of where the kernel loads. Therefore, kexec backs up this
+region just before rebooting into the dump-capture kernel.
+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
+With the dump-capture kernel, you can access the memory image, or "old
+memory," in two ways:
+- Through a /dev/oldmem device interface. A capture utility can read the
+ device file and write out the memory in raw format. This is a raw dump
+ of memory. Analysis and capture tools must be intelligent enough to
+ determine where to look for the right information.
+- 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. 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 and the Kdump patch
+1) Login as the root user.
+2) Download the kexec-tools user-space package from the following URL:
+3) Unpack the tarball with the tar command, as follows:
+ tar xvpzf kexec-tools-1.101.tar.gz
+4) Download the latest consolidated Kdump patch from the following URL:
+ (This location is being used until all the user-space Kdump patches
+ are integrated with the kexec-tools package.)
+5) Change to the kexec-tools-1.101 directory, as follows:
+ cd kexec-tools-1.101
+6) Apply the consolidated patch to the kexec-tools-1.101 source tree
+ with the patch command, as follows. (Modify the path to the downloaded
+ patch as necessary.)
+ patch -p1 < /path-to-kdump-patch/kexec-tools-1.101-kdump.patch
+7) Configure the package, as follows:
+8) Compile the package, as follows:
+9) Install the package, as follows:
+ make install
+Download and build the system and dump-capture kernels
+Download the mainline (vanilla) kernel source code (2.6.13-rc1 or newer)
+from http://www.kernel.org. Two kernels must be built: a system kernel
+and a dump-capture kernel. Use the following steps to configure these
+kernels with the necessary kexec and Kdump features:
+1) Enable "kexec system call" in "Processor type and features."
+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.
+4) Make and install the kernel and its modules. Update the boot loader
+ (such as grub, yaboot, or lilo) configuration files as necessary.
+5) Boot the system kernel with the boot parameter "crashkernel=Y@X",
+ where Y specifies how much memory to reserve for the dump-capture kernel
+ and X 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 for the dump-capture kernel.
+ On x86 and x86_64, use "crashkernel=64M@16M".
+ On ppc64, use "crashkernel=128M@32M".
+The dump-capture kernel
-1) Download the upstream kexec-tools userspace package from
- 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 or regular kernel:
- a) Enable "kexec system call" feature (in Processor type and features).
- b) Enable "sysfs file system support" (in Pseudo filesystems).
- 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 or dump capture kernel:
- a) For i386 architecture enable Highmem support
- b) Enable "kernel crash dumps" feature (under "Processor type and features")
- 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.
- d) Enable "/proc/vmcore support" (Optional, under "Pseudo filesystems").
-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 maxcpus=1"
- 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
- 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
-5) Once the second kernel has booted, write out the dump file using
+1) Under "General setup," append "-kdump" to the current string in
+ "Local version."
+2) On x86, enable high memory support under "Processor type and
+3) On x86 and x86_64, disable symmetric multi-processing support
+ under "Processor type and features":
+ (If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
+ when loading the dump-capture kernel, see section "Load the Dump-capture
+4) On ppc64, disable NUMA support and enable EMBEDDED support:
+ CONFIG_EEH=N for the dump-capture kernel
+5) Enable "kernel crash dumps" support under "Processor type and
+6) 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. By default this value is 0x1000000
+ (16MB). It should be the same as X in the "crashkernel=Y@X" boot
+ parameter discussed above.
+ On x86 and x86_64, use "CONFIG_PHYSICAL_START=0x1000000".
+ On ppc64 the value is automatically set at 32MB when
+ CONFIG_CRASH_DUMP is set.
+6) Optionally enable "/proc/vmcore support" under "Filesystems" ->
+ "Pseudo filesystems".
+ (CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
+7) Make and install the kernel and its modules. DO NOT add this kernel
+ to the boot loader configuration files.
+Load the Dump-capture Kernel
+After booting to the system kernel, load the dump-capture kernel using
+the following command:
+ kexec -p <dump-capture-kernel> \
+ --initrd=<initrd-for-dump-capture-kernel> --args-linux \
+ --append="root=<root-dev> init 1 irqpoll"
+Notes on loading the dump-capture kernel:
+* <dump-capture-kernel> must be a vmlinux image (that is, an
+ uncompressed ELF image). bzImage does not work at this time.
+* By default, the ELF headers are stored in ELF64 format to support
+ systems with more than 4GB memory. 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. ELF32 headers can be used on non-PAE systems (that is,
+ less than 4GB of memory).
+* 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.
+* "init 1" boots the dump-capture kernel into single-user mode without
+ networking. If you want networking, use "init 3."
+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 powererpc systems when a soft-reset is generated, die() is called by all cpus and the system 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>
- Dump memory can also be accessed as a /dev/oldmem device for a linear/raw
- view. To create the device, type:
+You can also access dumped memory as a /dev/oldmem device for a linear
+and raw view. To create the device, use the following command:
- mknod /dev/oldmem c 1 12
+ mknod /dev/oldmem c 1 12
- Use "dd" with suitable options for count, bs and skip to access specific
- portions of the dump.
+Use the dd command with suitable options for count, bs, and skip to
+access specific portions of the dump.
- Entire memory: dd if=/dev/oldmem of=oldmem.001
+To see the entire memory, use the following command:
+ dd if=/dev/oldmem of=oldmem.001
-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>
+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, memory display
+Stack trace for the task on processor 0, register display, and memory
+display work fine.
-Note: gdb cannot analyse core files generated in ELF64 format for i386.
+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
-Latest "crash" (crash-4.0-2.18) as available on Dave Anderson's site
-http://people.redhat.com/~anderson/ works well with kdump format.
+You can also use the Crash utility to analyze dump files in Kdump
+format. Crash is available on Dave Anderson's site at the following URL:
-1) Provide a kernel pages filtering mechanism so that core file size is not
- insane on systems having huge memory banks.
-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.
+1) Provide a kernel pages filtering mechanism, so core file size is not
+ extreme on systems with huge memory banks.
+2) Relocatable kernel can help in maintaining multiple kernels for
+ crash_dump, and the same kernel as the system kernel can be used to
+ capture the dump.
Vivek Goyal (email@example.com)
Maneesh Soni (firstname.lastname@example.org)
+Linux is a trademark of Linus Torvalds in the United States, other
+countries, or both.