326 lines
10 KiB
Plaintext
326 lines
10 KiB
Plaintext
================================================================
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Documentation for Kdump - The kexec-based Crash Dumping Solution
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================================================================
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This document includes overview, setup and installation, and analysis
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information.
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Overview
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========
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Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
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dump of the system kernel's memory needs to be taken (for example, when
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the system panics). The system kernel's memory image is preserved across
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the reboot and is accessible to the dump-capture kernel.
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You can use common Linux commands, such as cp and scp, to copy the
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memory image to a dump file on the local disk, or across the network to
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a remote system.
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Kdump and kexec are currently supported on the x86, x86_64, and ppc64
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architectures.
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When the system kernel boots, it reserves a small section of memory for
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the dump-capture kernel. This ensures that ongoing Direct Memory Access
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(DMA) from the system kernel does not corrupt the dump-capture kernel.
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The kexec -p command loads the dump-capture kernel into this reserved
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memory.
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On x86 machines, the first 640 KB of physical memory is needed to boot,
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regardless of where the kernel loads. Therefore, kexec backs up this
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region just before rebooting into the dump-capture kernel.
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All of the necessary information about the system kernel's core image is
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encoded in the ELF format, and stored in a reserved area of memory
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before a crash. The physical address of the start of the ELF header is
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passed to the dump-capture kernel through the elfcorehdr= boot
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parameter.
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With the dump-capture kernel, you can access the memory image, or "old
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memory," in two ways:
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- Through a /dev/oldmem device interface. A capture utility can read the
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device file and write out the memory in raw format. This is a raw dump
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of memory. Analysis and capture tools must be intelligent enough to
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determine where to look for the right information.
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- Through /proc/vmcore. This exports the dump as an ELF-format file that
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you can write out using file copy commands such as cp or scp. Further,
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you can use analysis tools such as the GNU Debugger (GDB) and the Crash
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tool to debug the dump file. This method ensures that the dump pages are
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correctly ordered.
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Setup and Installation
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======================
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Install kexec-tools and the Kdump patch
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---------------------------------------
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1) Login as the root user.
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2) Download the kexec-tools user-space package from the following URL:
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http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
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3) Unpack the tarball with the tar command, as follows:
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tar xvpzf kexec-tools-1.101.tar.gz
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4) Download the latest consolidated Kdump patch from the following URL:
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http://lse.sourceforge.net/kdump/
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(This location is being used until all the user-space Kdump patches
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are integrated with the kexec-tools package.)
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5) Change to the kexec-tools-1.101 directory, as follows:
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cd kexec-tools-1.101
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6) Apply the consolidated patch to the kexec-tools-1.101 source tree
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with the patch command, as follows. (Modify the path to the downloaded
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patch as necessary.)
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patch -p1 < /path-to-kdump-patch/kexec-tools-1.101-kdump.patch
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7) Configure the package, as follows:
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./configure
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8) Compile the package, as follows:
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make
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9) Install the package, as follows:
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make install
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Download and build the system and dump-capture kernels
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------------------------------------------------------
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Download the mainline (vanilla) kernel source code (2.6.13-rc1 or newer)
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from http://www.kernel.org. Two kernels must be built: a system kernel
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and a dump-capture kernel. Use the following steps to configure these
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kernels with the necessary kexec and Kdump features:
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System kernel
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-------------
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1) Enable "kexec system call" in "Processor type and features."
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CONFIG_KEXEC=y
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2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
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filesystems." This is usually enabled by default.
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CONFIG_SYSFS=y
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Note that "sysfs file system support" might not appear in the "Pseudo
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filesystems" menu if "Configure standard kernel features (for small
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systems)" is not enabled in "General Setup." In this case, check the
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.config file itself to ensure that sysfs is turned on, as follows:
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grep 'CONFIG_SYSFS' .config
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3) Enable "Compile the kernel with debug info" in "Kernel hacking."
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CONFIG_DEBUG_INFO=Y
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This causes the kernel to be built with debug symbols. The dump
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analysis tools require a vmlinux with debug symbols in order to read
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and analyze a dump file.
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4) Make and install the kernel and its modules. Update the boot loader
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(such as grub, yaboot, or lilo) configuration files as necessary.
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5) Boot the system kernel with the boot parameter "crashkernel=Y@X",
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where Y specifies how much memory to reserve for the dump-capture kernel
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and X specifies the beginning of this reserved memory. For example,
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"crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
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starting at physical address 0x01000000 for the dump-capture kernel.
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On x86 and x86_64, use "crashkernel=64M@16M".
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On ppc64, use "crashkernel=128M@32M".
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The dump-capture kernel
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-----------------------
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1) Under "General setup," append "-kdump" to the current string in
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"Local version."
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2) On x86, enable high memory support under "Processor type and
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features":
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CONFIG_HIGHMEM64G=y
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or
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CONFIG_HIGHMEM4G
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3) On x86 and x86_64, disable symmetric multi-processing support
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under "Processor type and features":
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CONFIG_SMP=n
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(If CONFIG_SMP=y, then specify maxcpus=1 on the kernel command line
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when loading the dump-capture kernel, see section "Load the Dump-capture
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Kernel".)
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4) On ppc64, disable NUMA support and enable EMBEDDED support:
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CONFIG_NUMA=n
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CONFIG_EMBEDDED=y
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CONFIG_EEH=N for the dump-capture kernel
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5) Enable "kernel crash dumps" support under "Processor type and
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features":
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CONFIG_CRASH_DUMP=y
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6) Use a suitable value for "Physical address where the kernel is
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loaded" (under "Processor type and features"). This only appears when
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"kernel crash dumps" is enabled. By default this value is 0x1000000
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(16MB). It should be the same as X in the "crashkernel=Y@X" boot
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parameter discussed above.
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On x86 and x86_64, use "CONFIG_PHYSICAL_START=0x1000000".
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On ppc64 the value is automatically set at 32MB when
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CONFIG_CRASH_DUMP is set.
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6) Optionally enable "/proc/vmcore support" under "Filesystems" ->
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"Pseudo filesystems".
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CONFIG_PROC_VMCORE=y
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(CONFIG_PROC_VMCORE is set by default when CONFIG_CRASH_DUMP is selected.)
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7) Make and install the kernel and its modules. DO NOT add this kernel
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to the boot loader configuration files.
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Load the Dump-capture Kernel
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============================
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After booting to the system kernel, load the dump-capture kernel using
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the following command:
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kexec -p <dump-capture-kernel> \
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--initrd=<initrd-for-dump-capture-kernel> --args-linux \
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--append="root=<root-dev> init 1 irqpoll"
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Notes on loading the dump-capture kernel:
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* <dump-capture-kernel> must be a vmlinux image (that is, an
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uncompressed ELF image). bzImage does not work at this time.
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* By default, the ELF headers are stored in ELF64 format to support
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systems with more than 4GB memory. The --elf32-core-headers option can
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be used to force the generation of ELF32 headers. This is necessary
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because GDB currently cannot open vmcore files with ELF64 headers on
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32-bit systems. ELF32 headers can be used on non-PAE systems (that is,
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less than 4GB of memory).
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* The "irqpoll" boot parameter reduces driver initialization failures
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due to shared interrupts in the dump-capture kernel.
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* You must specify <root-dev> in the format corresponding to the root
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device name in the output of mount command.
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* "init 1" boots the dump-capture kernel into single-user mode without
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networking. If you want networking, use "init 3."
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Kernel Panic
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============
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After successfully loading the dump-capture kernel as previously
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described, the system will reboot into the dump-capture kernel if a
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system crash is triggered. Trigger points are located in panic(),
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die(), die_nmi() and in the sysrq handler (ALT-SysRq-c).
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The following conditions will execute a crash trigger point:
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If a hard lockup is detected and "NMI watchdog" is configured, the system
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will boot into the dump-capture kernel ( die_nmi() ).
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If die() is called, and it happens to be a thread with pid 0 or 1, or die()
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is called inside interrupt context or die() is called and panic_on_oops is set,
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the system will boot into the dump-capture kernel.
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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.
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For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
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"echo c > /proc/sysrq-trigger or write a module to force the panic.
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Write Out the Dump File
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=======================
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After the dump-capture kernel is booted, write out the dump file with
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the following command:
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cp /proc/vmcore <dump-file>
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You can also access dumped memory as a /dev/oldmem device for a linear
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and raw view. To create the device, use the following command:
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mknod /dev/oldmem c 1 12
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Use the dd command with suitable options for count, bs, and skip to
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access specific portions of the dump.
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To see the entire memory, use the following command:
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dd if=/dev/oldmem of=oldmem.001
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Analysis
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========
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Before analyzing the dump image, you should reboot into a stable kernel.
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You can do limited analysis using GDB on the dump file copied out of
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/proc/vmcore. Use the debug vmlinux built with -g and run the following
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command:
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gdb vmlinux <dump-file>
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Stack trace for the task on processor 0, register display, and memory
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display work fine.
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Note: GDB cannot analyze core files generated in ELF64 format for x86.
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On systems with a maximum of 4GB of memory, you can generate
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ELF32-format headers using the --elf32-core-headers kernel option on the
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dump kernel.
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You can also use the Crash utility to analyze dump files in Kdump
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format. Crash is available on Dave Anderson's site at the following URL:
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http://people.redhat.com/~anderson/
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To Do
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=====
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1) Provide a kernel pages filtering mechanism, so core file size is not
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extreme on systems with huge memory banks.
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2) Relocatable kernel can help in maintaining multiple kernels for
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crash_dump, and the same kernel as the system kernel can be used to
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capture the dump.
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Contact
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=======
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Vivek Goyal (vgoyal@in.ibm.com)
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Maneesh Soni (maneesh@in.ibm.com)
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Trademark
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=========
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Linux is a trademark of Linus Torvalds in the United States, other
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countries, or both.
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