linux 3.10的kdump配置的小坑

之前在2.6系列linux内核中，当发现某个模块不要在保留内核中加载的时候，可以通过blacklist参数将其在/etc/kdump.conf中屏蔽

blacklist <list of kernel modules>

最近发现某个sas驱动存在问题，所以打算也这么屏蔽，结果，出错了：

[root@localhost ~]# service kdump restart
Redirecting to /bin/systemctl restart kdump.service
Job for kdump.service failed because the control process exited with error code. See "systemctl status kdump.service" and "journalctl -xe" for details.
[root@localhost ~]# systemctl status kdump.service
* kdump.service - Crash recovery kernel arming
   Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Tue -- :: UTC; 10s ago
  Process:  ExecStop=/usr/bin/kdumpctl stop (code=exited, status=/SUCCESS)
  Process:  ExecStart=/usr/bin/kdumpctl start (code=exited, status=/FAILURE)
 Main PID:  (code=exited, status=/FAILURE)
 
Nov  :: localhost.localdomain kdumpctl[]: Deprecated kdump config option: blacklist. Refer to kdump.conf manpage for alternatives.
Nov  :: localhost.localdomain kdumpctl[]: Starting kdump: [FAILED]
Nov  :: localhost.localdomain systemd[]: kdump.service: main process exited, code=exited, status=/FAILURE
Nov  :: localhost.localdomain systemd[]: Failed to start Crash recovery kernel arming.
Nov  :: localhost.localdomain systemd[]: Unit kdump.service entered failed state.
Nov  :: localhost.localdomain systemd[]: kdump.service failed.
[root@localhost ~]# journalctl -xe
Nov  :: localhost.localdomain kdumpctl[]: kexec: unloaded kdump kernel
Nov  :: localhost.localdomain kdumpctl[]: Stopping kdump: [OK]
Nov  :: localhost.localdomain kdumpctl[]: Deprecated kdump config option: blacklist. Refer to kdump.conf manpage for alternatives.
Nov  :: localhost.localdomain kdumpctl[]: Starting kdump: [FAILED]
Nov  :: localhost.localdomain systemd[]: kdump.service: main process exited, code=exited, status=/FAILURE
Nov  :: localhost.localdomain systemd[]: Failed to start Crash recovery kernel arming.
-- Subject: Unit kdump.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit kdump.service has failed.
--
-- The result is failed.
Nov  :: localhost.localdomain systemd[]: Unit kdump.service entered failed state.
Nov  :: localhost.localdomain systemd[]: kdump.service failed.
Nov  :: localhost.localdomain polkitd[]: Unregistered Authentication Agent for unix-process:: (system bus name :1.5128, object path /org/freedesktop/PolicyKit1/AuthenticationAgent
[root@localhost ~]# 

发现blacklist是过时的用法了，然后参照提示：

man kdump.conf 看到如下打印：

blacklist option was recently being used to prevent loading modules in initramfs. General terminology for blacklist has been that module is present in initramfs but it is not actu-
ally loaded in kernel. Hence retaining blacklist option creates more confusing behavior. It has been deprecated.
 
Instead, use rd.driver.blacklist option on second kernel to blacklist a certain module. One can edit /etc/sysconfig/kdump.conf and edit KDUMP_COMMANDLINE_APPEND to pass kernel com-
mand line options. Refer to dracut.cmdline man page for more details on module blacklist option.

好吧，按照最新的要求，打算修改/etc/sysconfig/kdump.conf，发现这个文件不存在，当然配置文件路径不是关键，/etc/kdump.conf里面配置也行，

我按照manpage的提示，修改文件名是/etc/sysconfig/kdump，然后修改KDUMP_COMMANDLINE_APPEND这行命令，具体的格式参考：

man  dracut.cmdline

  rd.driver.blacklist=<drivername>[,<drivername>,...]
           do not load kernel module <drivername>. This parameter can be specified multiple times.
 
       rd.driver.pre=<drivername>[,<drivername>,...]
           force loading kernel module <drivername>. This parameter can be specified multiple times.
 
       rd.driver.post=<drivername>[,<drivername>,...]
           force loading kernel module <drivername> after all automatic loading modules have been loaded. This parameter can be specified multiple times.

另外需要注意的是，当修改了配置，就要重启kdump服务，而这个时候，由于修改了blacklist，会导致重启的时候比较慢，因为在涉及到配置文件变动时，如生成路径修改或blacklist内容增加，都需要重新生成kdump的RAM文件，不然其在发生问题时还是使用老的img RAM文件，这类文件在/boot下以kdump.img结尾的文件就是：

[root@localhost ~]# ls -l /boot/*kdump*
-rw------- 1 root root 16878919 Nov 29 01:02 /boot/initramfs-3.10.0-693.5.2.el7.x86_64kdump.img
-rw------- 1 root root 35261890 Nov 27 07:04 /boot/initramfs-3.10.0caq1.0kdump.img
-rw------- 1 root root 36508192 Nov 24 06:21 /boot/initramfs-3.10.0kdump.img
[root@localhost ~]# 

最后需要注意的就是，当配置的保留内核在加载驱动或者运行的时候，遇到panic，这个时候就再也没有内核去接管它了，只能在屏幕上打印，或者接串口查看。之前遇到过保留内存不够的

情况下，保留内核自己出现oom了，导致无法收集到crash，查看当前的保留内存可以使用：

[root@localhost ~]# cat /sys/kernel/kexec_crash_size
536870912

查看保留内核是否加载，可以使用：
[root@localhost ~]# cat /sys/kernel/kexec_crash_loaded
1