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How to use kdump to debug kernel crashes
From FedoraProject
Kdump is a kernel crash dumping mechanism and is very reliable because the
crash dump is captured from the context of a freshly booted kernel and not
from the context of the crashed kernel. Kdump uses kexec to boot into
a second kernel whenever system crashes. This second kernel, often called
the crash kernel, boots with very little memory and captures the dump image.
The first kernel reserves a section of memory that the second kernel uses
to boot. Kexec enables booting the capture kernel without going through the
BIOS, so contents of the first kernel's memory are preserved, which is
essentially the kernel crash dump.
First, install the kexec-tools, crash and kernel-debuginfo packages. Use following command line to install the packages.
dnf install --enablerepo=fedora-debuginfo --enablerepo=updates-debuginfo kexec-tools crash kernel-debuginfo
NOTE: The `crash` and `kernel-debuginfo` packages are only required if you are planning on looking at the resulting kernel vmcore yourself.
Most often this is the case, however if you are setting up kdump on a machine simply to capture a vmcore that will be analyzed by someone else or on a different machine, you can skip those packages.
Next, edit /boot/grub/grub.conf or /boot/grub2/grub.cfg and add the "crashkernel=128M" command line option.
An example command line might look like this (for GRUB Legacy, "linux" is replaced by "kernel"):
linux /vmlinuz-3.1.4-1.fc16.x86_64 ro root=/dev/VolGroup00/LogVol00 rhgb LANG=en_US.UTF-8 crashkernel=128M
Next, consider editing the kdump configuration file /etc/kdump.conf.
This will allow you to write the dump over the network or to some other location on the local system, rather than to the default location of /var/crash.
For additional information, consult the mkdumprd man page and the comments in /etc/kdump.conf.
Next, reboot your system
Finally, active the kdump system service
systemctl start kdump.service
Considerations:
Above shown parameter reserves 128MB of physical memory. This reserved memory is used to preload and run the capture kernel.
Init scripts take care of pre-loading the capture kernel at system boot time.
It is recommended to either set up a serial console or switch to run level 3 (init 3) for testing purposes. The reason being that kdump does not reset the console if you are in X or
framebuffer mode, and no message might be visible on console after system crash. You may also see screen corruption in graphics mode during capture.
Capturing a crash dump can take a long time, especially if the system has a lot of memory. Be patient. The system will reboot after the dump is captured.
Normally kernel panic() will trigger booting into capture kernel but for
testing purposes one can simulate the trigger in one of the following
then trigger a panic through /proc interface
echo 1 & /proc/sys/kernel/sysrq
echo c & /proc/sysrq-trigger
Trigger by inserting a module which calls panic().
The system will boot into the capture kernel.
A kernel dump will be automatically saved in /var/crash/&dumpdir& and the system will boot back into the regular kernel.
The name of the dump directory will depend on date and time of crash. For example, /var/crash/-17:02/vmcore.
Once the system has returned from recovering the crash, you may wish to analyse the kernel dump file using the crash tool.
First, locate the recent vmcore dump file:
find /var/crash -type f -mtime -1
One you have located a vmcore dump file, call crash:
crash /var/crash/-10\:36/vmcore /usr/lib/debug/lib/modules/`uname -r`/vmlinux
Missing debuginfo?Cannot find any files under /usr/lib/debug?
Make sure you have the kernel-debuginfo package installed.
For more information on using the crash tool, see .
The versions of kexec-tools and crash can be very reliant on the version of kernel running.
On Fedora, from time-to-time the package versions can get out of sync and can lead to partially working crash dumps.
This may manifest as warning messages from crash such as
page excluded: kernel virtual address: ffff.........9d28
type: &...&
If you want to know specifically what versions are supported, you can examine the srpm for the version of kexec-tools you are running, in particular makedumpfile.h will have something like
#define OLDEST_VERSION
KERNEL_VERSION(2, 6, 15)/* linux-2.6.15 */
#define LATEST_VERSION
KERNEL_VERSION(4, 5, 3)/* linux-4.5.3 */
If you run makedumpfile against an unsupported kernel version it will probably still mostly work.
It will output an error message to the console, but it can be easy to miss in the kexec output.
If the dump is behaving unexpectedly you can modify kdump.conf to not filter any pages (perhaps except zero-filled pages with -d1) and only use it to compress the vmcore with -c.
This might result in a more useful vmcore.
If that fails, you could take makedumpfile out of the picture entirely by change the core_collector in kdump.conf to scp, which will simply copy /proc/vmcore to a permanent location.
If having further issues, you may also try building the latest crash tool from source.
If you are at the point of debugging kernel crash dumps you can probably figure it out :)
You might want to try something like:
$ sudo dnf builddep crash # quick way to get the right libraries
$ git clone /crash-utility/crash.git
$ cd crash
$ make lzo # don't forget the lzo if you're using compressed dumps
Kernel Source (Documentation/kdump/kdump.txt).
Using crash -
Copyright & 2017 Red Hat, Inc. and others. For comments or queries, .
Red Hat, Red Hat Enterprise Linux, the Shadowman logo, and JBoss are trademarks or registered trademarks of
Red Hat, Inc. or its subsidiaries in the United States and other countries.
Linux& is the registered trademark of Linus Torvalds in the U.S. and other countries.
The Fedora Project is maintained and driven by the community and sponsored by Red Hat. This is a community
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2(X^2+1/X^2)-3(X+1/X)-1=0如何解这是答案设u=x+1/x 2(u^2-2)-3u-1=0 2u^2-3u-5=0 (2u-5)(u+1)=0 u=-1或u=5/2 当u=-1时,x^2+x+1=0 (x≠0) Δ＜0,无实根 当u=5/2时,2x^2-5x+2=0 (x≠0) x=1/2或x=2可我不明白当u=-1时,x^2+x+1=0 (x≠0) Δ＜0,无实根 当u=5/2时,2x^2-5x+2=0 (x≠0) x=1/2或x=2这四步是什么意思x^2+x+1=0与2x^2-5x+2=0怎么来的?-1为什么要舍?还有一个问题计算1/（1*3）+1/(2*4)+1/(3*5)+…………+1/(9*11)答案是1/n(n+2)=1/2*(1/n-1/(n+2))所以1/1*3+1/2*4+1/3*5+……+1/9*11=1/2*[1-1/3+1/2-1/4+1/3-1/5+...+1/9-1/11]=1/2*(1+1/2-1/10-1/11)=1/2*144/110=36/55但为什么1/（1*2）+1/(2*3)+1/(3*4)+…………+1/(99*100)前面不乘以1/2???不是1/n(n+2)=1/2*(1/n-1/(n+2))吗？？
宅喵是神517
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这是换元法,先解u再解x,所以解出u后要验算x存不存在,这四步做的就是这件事当u=-1时,∵u=x+1/x,∴-1=x+1/x,两边乘x,就有x^2+x+1=0,然后解二次方程,发现误解,所以舍掉了 通分就知道了1/n-1/(n+2)通分后分子为2,要使原式不变,式子前面要乘系数1/2但1/n-1/(n+1)通分后分子为1,所以不用乘系数的
-1=x+1/x两边乘x是x^2+x+x才对啊？
左边是-1*x=-x
右边是(x+1/x)*x=x^2+1
∴-x=x^2+1，移项后为x^2+x+1=0
其他类似问题
当u=-1时 X+1/x=-1
两边同乘X 得 x^2+x+1=0
再通过Δ法求解 当u=5/2时 同理
1=x+1/x两边乘x是x^2+x+x才对啊？
1/x乘上X就变成1了呀
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