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diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst new file mode 100644 index 0000000000..a748e7eb44 --- /dev/null +++ b/Documentation/admin-guide/kdump/kdump.rst @@ -0,0 +1,597 @@ +================================================================ +Documentation for Kdump - The kexec-based Crash Dumping Solution +================================================================ + +This document includes overview, setup, installation, and analysis +information. + +Overview +======== + +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 +memory. + +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: + +http://kernel.org/pub/linux/utils/kernel/kexec/kexec-tools.tar.gz + +This is a symlink to the latest version. + +The latest kexec-tools git tree is available at: + +- git://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git +- http://www.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git + +There is also a gitweb interface available at +http://www.kernel.org/git/?p=utils/kernel/kexec/kexec-tools.git + +More information about kexec-tools can be found at +http://horms.net/projects/kexec/ + +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:: + + ./configure + +6) Compile the package, as follows:: + + make + +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.":: + + CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y + + And both of them will select KEXEC_CORE:: + + CONFIG_KEXEC_CORE=y + + Subsequently, CRASH_CORE is selected by KEXEC_CORE:: + + CONFIG_CRASH_CORE=y + +2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo + filesystems." This is usually enabled by default:: + + CONFIG_SYSFS=y + + Note that "sysfs file system support" might not appear in the "Pseudo + filesystems" menu if "Configure standard kernel features (expert users)" + 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.":: + + CONFIG_DEBUG_INFO=Y + + 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 + features":: + + CONFIG_CRASH_DUMP=y + +2) Enable "/proc/vmcore support" under "Filesystems" -> "Pseudo filesystems":: + + CONFIG_PROC_VMCORE=y + + (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 + features":: + + CONFIG_HIGHMEM64G=y + + or:: + + CONFIG_HIGHMEM4G + +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 + features":: + + CONFIG_RELOCATABLE=y + +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 + kernel. + + 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 + CONFIG_PHYSICAL_START=0x1000000 + +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:: + + CONFIG_CRASH_DUMP=y + +2) Enable "Build a relocatable kernel" support:: + + CONFIG_RELOCATABLE=y + + 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:: + + crashkernel=256M@0 + + or:: + + crashkernel=256M + +Dump-capture kernel config options (Arch Dependent, arm) +---------------------------------------------------------- + +- To use a relocatable kernel, + Enable "AUTO_ZRELADDR" support under "Boot" options:: + + AUTO_ZRELADDR=y + +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:: + + crashkernel=256M@0 + + or:: + + crashkernel=256M + + 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 + wasted. + +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:: + + crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset] + range=start-[end] + + For example:: + + crashkernel=512M-2G:64M,2G-:128M + + 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:: + + crashkernel=0,low + +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 +loaded. + +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> + +or use scp to write out the dump file between hosts on a network, e.g:: + + scp /proc/vmcore remote_username@remote_ip:<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> + +Analysis +======== + +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 +command:: + + 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: + + https://github.com/crash-utility/crash + +Crash document can be found at: + https://crash-utility.github.io/ + +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. + +Contact +======= + +- kexec@lists.infradead.org + +GDB macros +========== + +.. include:: gdbmacros.txt + :literal: |