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Documentation: kdump: update kdump guide
Some parts of the guide are aged, hence need be updated. 1) The backup area of the 1st 640K on X86_64 has been removed by below commits, update the description accordingly. commit7c321eb2b8("x86/kdump: Remove the backup region handling") commit6f599d8423("x86/kdump: Always reserve the low 1M when the crashkernel option is specified") 2) Sort out the descripiton of "crashkernel syntax" part. 3) And some other minor cleanups. Signed-off-by: Baoquan He <bhe@redhat.com> Acked-by: Dave Young <dyoung@redhat.com> Link: https://lore.kernel.org/r/20210609083218.GB591017@MiWiFi-R3L-srv [jc: added blank line to fix added build warning] Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Baoquan He
Jonathan Corbet
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@ -2,7 +2,7 @@
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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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This document includes overview, setup, installation, and analysis
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information.
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Overview
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@ -13,9 +13,9 @@ 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 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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You can use common commands, such as cp, scp or makedumpfile to copy
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the memory image to a dump file on the local disk, or across the network
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to a remote system.
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Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
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s390x, arm and arm64 architectures.
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@ -26,13 +26,15 @@ the dump-capture kernel. This ensures that ongoing Direct Memory Access
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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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On x86 machines, the first 640 KB of physical memory is needed for boot,
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regardless of where the kernel loads. For simpler handling, the whole
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low 1M is reserved to avoid any later kernel or device driver writing
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data into this area. Like this, the low 1M can be reused as system RAM
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by kdump kernel without extra handling.
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Similarly on PPC64 machines first 32KB of physical memory is needed for
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booting regardless of where the kernel is loaded and to support 64K page
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size kexec backs up the first 64KB memory.
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On PPC64 machines first 32KB of physical memory is needed for booting
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regardless of where the kernel is loaded and to support 64K page size
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kexec backs up the first 64KB memory.
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For s390x, when kdump is triggered, the crashkernel region is exchanged
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with the region [0, crashkernel region size] and then the kdump kernel
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@ -46,14 +48,14 @@ passed to the dump-capture kernel through the elfcorehdr= boot
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parameter. Optionally the size of the ELF header can also be passed
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when using the elfcorehdr=[size[KMG]@]offset[KMG] syntax.
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With the dump-capture kernel, you can access the memory image through
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/proc/vmcore. This exports the dump as an ELF-format file that you can
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write out using file copy commands such as cp or scp. Further, you can
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use analysis tools such as the GNU Debugger (GDB) and the Crash tool to
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debug the dump file. This method ensures that the dump pages are correctly
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ordered.
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write out using file copy commands such as cp or scp. You can also use
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makedumpfile utility to analyze and write out filtered contents with
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options, e.g with '-d 31' it will only write out kernel data. 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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@ -125,9 +127,18 @@ dump-capture kernels for enabling kdump support.
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System kernel config options
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----------------------------
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1) Enable "kexec system call" in "Processor type and features."::
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1) Enable "kexec system call" or "kexec file based system call" in
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"Processor type and features."::
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CONFIG_KEXEC=y
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CONFIG_KEXEC=y or CONFIG_KEXEC_FILE=y
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And both of them will select KEXEC_CORE::
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CONFIG_KEXEC_CORE=y
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Subsequently, CRASH_CORE is selected by KEXEC_CORE::
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CONFIG_CRASH_CORE=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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@ -175,17 +186,19 @@ Dump-capture kernel config options (Arch Dependent, i386 and x86_64)
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CONFIG_HIGHMEM4G
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2) On i386 and x86_64, disable symmetric multi-processing support
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under "Processor type and features"::
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2) With CONFIG_SMP=y, usually nr_cpus=1 need specified on the kernel
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command line when loading the dump-capture kernel because one
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CPU is enough for kdump kernel to dump vmcore on most of systems.
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CONFIG_SMP=n
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However, you can also specify nr_cpus=X to enable multiple processors
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in kdump kernel. In this case, "disable_cpu_apicid=" is needed to
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tell kdump kernel which cpu is 1st kernel's BSP. Please refer to
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admin-guide/kernel-parameters.txt for more details.
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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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With CONFIG_SMP=n, the above things are not related.
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3) If one wants to build and use a relocatable kernel,
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Enable "Build a relocatable kernel" support under "Processor type and
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3) A relocatable kernel is suggested to be built by default. If not yet,
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enable "Build a relocatable kernel" support under "Processor type and
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features"::
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CONFIG_RELOCATABLE=y
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@ -232,7 +245,7 @@ Dump-capture kernel config options (Arch Dependent, ia64)
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as a dump-capture kernel if desired.
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The crashkernel region can be automatically placed by the system
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kernel at run time. This is done by specifying the base address as 0,
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kernel at runtime. This is done by specifying the base address as 0,
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or omitting it all together::
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crashkernel=256M@0
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@ -241,10 +254,6 @@ Dump-capture kernel config options (Arch Dependent, ia64)
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crashkernel=256M
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If the start address is specified, note that the start address of the
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kernel will be aligned to 64Mb, so if the start address is not then
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any space below the alignment point will be wasted.
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Dump-capture kernel config options (Arch Dependent, arm)
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----------------------------------------------------------
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@ -260,46 +269,82 @@ Dump-capture kernel config options (Arch Dependent, arm64)
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on non-VHE systems even if it is configured. This is because the CPU
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will not be reset to EL2 on panic.
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Extended crashkernel syntax
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crashkernel syntax
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===========================
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1) crashkernel=size@offset
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While the "crashkernel=size[@offset]" syntax is sufficient for most
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configurations, sometimes it's handy to have the reserved memory dependent
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on the value of System RAM -- that's mostly for distributors that pre-setup
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the kernel command line to avoid a unbootable system after some memory has
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been removed from the machine.
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The syntax is::
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crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
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range=start-[end]
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For example::
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crashkernel=512M-2G:64M,2G-:128M
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This would mean:
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1) if the RAM is smaller than 512M, then don't reserve anything
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(this is the "rescue" case)
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2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
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3) if the RAM size is larger than 2G, then reserve 128M
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Boot into System Kernel
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=======================
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1) Update the boot loader (such as grub, yaboot, or lilo) configuration
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files as necessary.
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2) 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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Here 'size' specifies how much memory to reserve for the dump-capture kernel
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and 'offset' 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 (16MB) for the dump-capture kernel.
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On x86 and x86_64, use "crashkernel=64M@16M".
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The crashkernel region can be automatically placed by the system
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kernel at run time. This is done by specifying the base address as 0,
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or omitting it all together::
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crashkernel=256M@0
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or::
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crashkernel=256M
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If the start address is specified, note that the start address of the
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kernel will be aligned to a value (which is Arch dependent), so if the
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start address is not then any space below the alignment point will be
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wasted.
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2) range1:size1[,range2:size2,...][@offset]
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While the "crashkernel=size[@offset]" syntax is sufficient for most
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configurations, sometimes it's handy to have the reserved memory dependent
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on the value of System RAM -- that's mostly for distributors that pre-setup
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the kernel command line to avoid a unbootable system after some memory has
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been removed from the machine.
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The syntax is::
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crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
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range=start-[end]
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For example::
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crashkernel=512M-2G:64M,2G-:128M
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This would mean:
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1) if the RAM is smaller than 512M, then don't reserve anything
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(this is the "rescue" case)
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2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
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3) if the RAM size is larger than 2G, then reserve 128M
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3) crashkernel=size,high and crashkernel=size,low
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If memory above 4G is preferred, crashkernel=size,high can be used to
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fulfill that. With it, physical memory is allowed to be allocated from top,
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so could be above 4G if system has more than 4G RAM installed. Otherwise,
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memory region will be allocated below 4G if available.
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When crashkernel=X,high is passed, kernel could allocate physical memory
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region above 4G, low memory under 4G is needed in this case. There are
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three ways to get low memory:
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1) Kernel will allocate at least 256M memory below 4G automatically
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if crashkernel=Y,low is not specified.
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2) Let user specify low memory size instead.
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3) Specified value 0 will disable low memory allocation::
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crashkernel=0,low
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Boot into System Kernel
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-----------------------
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1) Update the boot loader (such as grub, yaboot, or lilo) configuration
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files as necessary.
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2) Boot the system kernel with the boot parameter "crashkernel=Y@X".
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On x86 and x86_64, use "crashkernel=Y[@X]". Most of the time, the
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start address 'X' is not necessary, kernel will search a suitable
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area. Unless an explicit start address is expected.
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On ppc64, use "crashkernel=128M@32M".
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@ -331,8 +376,8 @@ of dump-capture kernel. Following is the summary.
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For i386 and x86_64:
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- Use vmlinux if kernel is not relocatable.
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- Use bzImage/vmlinuz if kernel is relocatable.
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- Use vmlinux if kernel is not relocatable.
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For ppc64:
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@ -392,7 +437,7 @@ loading dump-capture kernel.
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For i386, x86_64 and ia64:
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"1 irqpoll maxcpus=1 reset_devices"
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"1 irqpoll nr_cpus=1 reset_devices"
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For ppc64:
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@ -400,7 +445,7 @@ For ppc64:
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For s390x:
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"1 maxcpus=1 cgroup_disable=memory"
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"1 nr_cpus=1 cgroup_disable=memory"
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For arm:
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@ -408,7 +453,7 @@ For arm:
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For arm64:
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"1 maxcpus=1 reset_devices"
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"1 nr_cpus=1 reset_devices"
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Notes on loading the dump-capture kernel:
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@ -488,6 +533,10 @@ the following command::
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cp /proc/vmcore <dump-file>
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You can also use makedumpfile utility to write out the dump file
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with specified options to filter out unwanted contents, e.g::
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makedumpfile -l --message-level 1 -d 31 /proc/vmcore <dump-file>
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Analysis
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========
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@ -535,8 +584,7 @@ This will cause a kdump to occur at the add_taint()->panic() call.
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Contact
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=======
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- Vivek Goyal (vgoyal@redhat.com)
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- Maneesh Soni (maneesh@in.ibm.com)
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- kexec@lists.infradead.org
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GDB macros
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==========
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