From: Hariprasad Nellitheertha <hari@in.ibm.com> This patch contains the documentation for the kexec based crash dump tool. Signed off by Hariprasad Nellitheertha <hari@in.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> --- 25-akpm/Documentation/kdump.txt | 113 ++++++++++++++++++++++++++++++++++++++++ 1 files changed, 113 insertions(+) diff -puN /dev/null Documentation/kdump.txt --- /dev/null Thu Apr 11 07:25:15 2002 +++ 25-akpm/Documentation/kdump.txt Tue Sep 21 14:36:20 2004 @@ -0,0 +1,113 @@ +Documentation for kdump - the kexec based crash dumping solution +================================================================ + +DESIGN +====== + +We use kexec to reboot to a second kernel whenever a dump needs to be taken. +This second kernel is booted with with very little memory (configurable +at compile time). 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. The first 640k of physical memory is +needed irrespective of where the kernel loads at. Hence, this region is +backed up before reboot. + +In the second kernel, "old memory" can be accessed in two ways. The +first one is through a device interface. We can create a /dev/oldmem or +whatever and write out the memory in raw format. 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 some minimal debugging on the dump file. Both these +methods ensure that there is correct ordering of the dump pages (corresponding +to the first 640k that has been relocated). + +SETUP +===== + +1) Obtain the appropriate -mm tree patch and apply it on to the vanilla + kernel tree. + +2) In order to enable the kernel to boot from a non-default location, the + following patches (by Eric Biederman) needs to be applied. + + http://www.xmission.com/~ebiederm/files/kexec/2.6.8.1-kexec3/ + broken-out/highbzImage.i386.patch + http://www.xmission.com/~ebiederm/files/kexec/2.6.8.1-kexec3/ + broken-out/vmlinux-lds.i386.patch + +3) Two kernels need to be built in order to get this feature working. + + For the first kernel, choose the default values for the following options. + + a) Physical address where the kernel expects to be loaded + b) kexec system call + c) kernel crash dumps + + All the options are under "Processor type and features" + + For the second kernel, change (a) to 16MB. If you want to choose another + value here, ensure "location from where the crash dumping kernel will boot + (MB)" under (c) reflects the same value. + + Also ensure you have CONFIG_HIGHMEM on. + +4) Boot into the first kernel. You are now ready to try out kexec based crash + dumps. + +5) Load the second kernel to be booted using + + kexec -l <second-kernel> --args-linux --append="root=<root-dev> dump + init 1 memmap=exactmap memmap=640k@0 memmap=32M@16M" + + Note that <second-kernel> has to be a vmlinux image. bzImage will not + work, as of now. + +6) Enable kexec based dumping by + + echo 1 > /proc/kexec-dump + + If this is not set, the system will not do a kexec reboot in the event + of a panic. + +7) System reboots into the second kernel when a panic occurs. + You could write a module to call panic, for testing purposes. + +8) Write out the dump file using + + cp /proc/vmcore <dump-file> + +You can also access the dump as a device for a linear/raw view. To do this, +you will need the kd-oldmem-<version>.patch built into the kernel. 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. + +ANALYSIS +======== + +You can run 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. + +TODO +==== + +1) Provide a kernel-pages only view for the dump. This could possibly turn up + as /proc/vmcore-kern. +2) Provide register contents of all processors (similar to what multi-threaded + core dumps does). +3) Modify "crash" to make it recognize this dump. +4) Make the i386 kernel boot from any location so we can run the second kernel + from the reserved location instead of the current approach. + +CONTACT +======= + +Hariprasad Nellitheertha - hari at in dot ibm dot com _