admin-guide/kdump/kdump.rst

11 Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
12 dump of the system kernel's memory needs to be taken (for example, when
13 the system panics). The system kernel's memory image is preserved across
14 the reboot and is accessible to the dump-capture kernel.
23 When the system kernel boots, it reserves a small section of memory for
24 the dump-capture kernel. This ensures that ongoing Direct Memory Access
25 (DMA) from the system kernel does not corrupt the dump-capture kernel.
26 The kexec -p command loads the dump-capture kernel into this reserved
30 regardless of where the kernel loads. For simpler handling, the whole
31 low 1M is reserved to avoid any later kernel or device driver writing
33 by kdump kernel without extra handling.
36 regardless of where the kernel is loaded and to support 64K page size
40 with the region [0, crashkernel region size] and then the kdump kernel
41 runs in [0, crashkernel region size]. Therefore no relocatable kernel is
44 All of the necessary information about the system kernel's core image is
47 passed to the dump-capture kernel through the elfcorehdr= boot
51 With the dump-capture kernel, you can access the memory image through
55 options, e.g with '-d 31' it will only write out kernel data. Further,
70 http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz
76 - git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
77 - http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
80 http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git
110 1) Build a separate custom dump-capture kernel for capturing the
111    kernel core dump.
113 2) Or use the system kernel binary itself as dump-capture kernel and there is
114    no need to build a separate dump-capture kernel. This is possible
115    only with the architectures which support a relocatable kernel. As
117    relocatable kernel.
119 Building a relocatable kernel is advantageous from the point of view that
120 one does not have to build a second kernel for capturing the dump. But
121 at the same time one might want to build a custom dump capture kernel
127 System kernel config options
145    filesystems" menu if "Configure standard kernel features (expert users)"
151 3) Enable "Compile the kernel with debug info" in "Kernel hacking."::
155    This causes the kernel to be built with debug symbols. The dump
159 Dump-capture kernel config options (Arch Independent)
162 1) Enable "kernel crash dumps" support under "Processor type and
177 Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
189 2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
190    command line when loading the dump-capture kernel because one
191    CPU is enough for kdump kernel to dump vmcore on most of systems.
194    in kdump kernel.
198 3) A relocatable kernel is suggested to be built by default. If not yet,
199    enable "Build a relocatable kernel" support under "Processor type and
204 4) Use a suitable value for "Physical address where the kernel is
206    "kernel crash dumps" is enabled. A suitable value depends upon
207    whether kernel is relocatable or not.
209    If you are using a relocatable kernel use CONFIG_PHYSICAL_START=0x100000
210    This will compile the kernel for physical address 1MB, but given the fact
211    kernel is relocatable, it can be run from any physical address hence
213    kernel.
216    second kernel using boot parameter "crashkernel=Y@X". Here X is
217    start of memory region reserved for dump-capture kernel.
221 5) Make and install the kernel and its modules. DO NOT add this kernel
224 Dump-capture kernel config options (Arch Dependent, ppc64)
227 1) Enable "Build a kdump crash kernel" support under "Kernel" options::
231 2)   Enable "Build a relocatable kernel" support::
235    Make and install the kernel and its modules.
237 Dump-capture kernel config options (Arch Dependent, arm)
240 -   To use a relocatable kernel,
245 Dump-capture kernel config options (Arch Dependent, arm64)
248 - Please note that kvm of the dump-capture kernel will not be enabled
256    Here 'size' specifies how much memory to reserve for the dump-capture kernel
258    "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
259    starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
262    kernel at run time. This is done by specifying the base address as 0,
272    kernel will be aligned to a value (which is Arch dependent), so if the
281    the kernel command line to avoid a unbootable system after some memory has
307    When crashkernel=X,high is passed, kernel could allocate physical memory
311       1) Kernel will allocate at least 256M memory below 4G automatically
318 Boot into System Kernel
323 2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
326    start address 'X' is not necessary, kernel will search a suitable
336    kernel will automatically locate the crash kernel image within the
340    the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
342 Load the Dump-capture Kernel
345 After booting to the system kernel, dump-capture kernel needs to be
350 of dump-capture kernel. Following is the summary.
354 	- Use bzImage/vmlinuz if kernel is relocatable.
355 	- Use vmlinux if kernel is not relocatable.
374 to load dump-capture kernel::
376    kexec -p <dump-capture-kernel-vmlinux-image> \
377    --initrd=<initrd-for-dump-capture-kernel> --args-linux \
381 to load dump-capture kernel::
383    kexec -p <dump-capture-kernel-bzImage> \
384    --initrd=<initrd-for-dump-capture-kernel> \
388 to load dump-capture kernel::
390    kexec --type zImage -p <dump-capture-kernel-bzImage> \
391    --initrd=<initrd-for-dump-capture-kernel> \
392    --dtb=<dtb-for-dump-capture-kernel> \
396 to load dump-capture kernel::
398    kexec -p <dump-capture-kernel-Image> \
399    --initrd=<initrd-for-dump-capture-kernel> \
403 loading dump-capture kernel.
425 Notes on loading the dump-capture kernel:
437   due to shared interrupts in the dump-capture kernel.
442 * Boot parameter "1" boots the dump-capture kernel into single-user
445 * We generally don't have to bring up a SMP kernel just to capture the
447   kernel or specify maxcpus=1 option while loading dump-capture kernel.
451 * You should enable multi-cpu support in dump-capture kernel if you intend
455   SMP dump-capture kernel and specify maxcpus/nr_cpus options while loading it.
458   the elfcorehdr= kernel parameter, it is used by the kdump kernel as it
459   is done on all other architectures. If no elfcorehdr= kernel parameter is
460   specified, the s390x kdump kernel dynamically creates the header. The
464 * For s390x systems with many attached devices the "cio_ignore" kernel
465   parameter should be used for the kdump kernel in order to prevent allocation
466   of kernel memory for devices that are not relevant for kdump. The same
471 Kernel Panic
474 After successfully loading the dump-capture kernel as previously
475 described, the system will reboot into the dump-capture kernel if a
482 will boot into the dump-capture kernel ( die_nmi() ).
486 the system will boot into the dump-capture kernel.
489 and the system will boot into the dump-capture kernel.
497 After the dump-capture kernel is booted, write out the dump file with
514 Before analyzing the dump image, you should reboot into a stable kernel.
527 ELF32-format headers using the --elf32-core-headers kernel option on the
528 dump kernel.
541 The kernel parameter, panic_on_warn, calls panic() in all WARN() paths.  This
543 to specify this during runtime, /proc/sys/kernel/panic_on_warn can be set to 1
549 The kernel parameter panic_on_taint facilitates a conditional call to panic()