kk Blog —— 通用基础

1、修改启动级别错误配置的方法

如果你不小心把级别设成0或6，用下边的方法补救：
1. 启动系统后，在倒计时结束前按任意键，进入grub引导界面
2. 在grub引导界面的输入‘e’，进入下一级，高亮第二行，再输入‘e’
3. 在最后输入 1[单用户模式] 按回车回到上一级（单用户模式启动不会去读取/etc/inittab） 4. 输入‘b’重启系统

也可以改其他启动参数 进行紧急引导最简单的方法是：
把 init=/bin/bash 或 init=/bin/sh 选项传递给内核

2、磁盘满导致无法进入系统

删除一些文件后重启
没法删除 说文件系统是只读的 ，重新mount 你的磁盘系统为可读写的。

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mount -o remount,rw  /

3、忘记密码

passwd user

kexec-tools-1.102pre-154.el5 会直接进入 dump.img，于是便看到单CPU在跑，内存只有crashkernel中大小的情况。但是指定了ext /dev/sd* 后就能正常。其他问题就是它通过网络时只会走eth0，不然就失败。

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e2fsck 1.38 (30-Jun-2005)
fsck.ext3: while determining whether /dev/sda2 is mounted.
/: recovering journal
/: clean, 100877/4653056 files, 1236284/4648809 blocks
Mounting root filesystem.
Trying mount -t ext4 /dev/sda2 /sysroot
Trying mount -t ext3 /dev/sda2 /sysroot
Using ext3 on root filesystem
Switching to new root and running init.
^MINIT: version 2.86 booting^M
	        Welcome to  CentOS release 5.8 (Final)
	        Press 'I' to enter interactive startup.
Cannot access the Hardware Clock via any known method.
Use the --debug option to see the details of our search for an access method.
Setting clock  (utc): Tue Aug  5 19:18:49 PDT 2014 [  OK  ]^M
Starting udev: [  OK  ]^M

ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Client/en/os/SRPMS/kexec-tools-1.102pre-154.el5.src.rpm

vim mkdumprd

https://bugzilla.redhat.com/show_bug.cgi?id=788678

Hmm, after a second thought, did you put the block device mounted on /var into your /etc/kdump.conf? Something like:

ext3 /dev/sdbX #the device mounted on /var path crash #relative path inside /var

? Please share your kdump.conf if possible.

Thanks!

–

The point is to not touch the default kdump.conf, and mkdumprd should just work, like it does in RHEL6.

If I do put the ext3 and path directives into kdump.conf, then of course things work fine, but it shouldn’t be needed for the stock case where you just want to dump to /var/crash on your local filesystem.

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Yeah… I saw how RHEL6 handles this, will try to backport it to RHEL5. Thanks!

–

Created attachment 594144 detailsProposed Patch v3

Ok, let’s just remove the UUID converting code.

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Yep - it works now!

–

… almost. I’m pretty sure that the RHEL6 default mkdumprd uses makedumpfile by default so it isn’t just using “cp” to create the vmcore file.

The currently-patched version appears to just use “cp” instead.

–

Yeah, this is expected, because we don’t have a chance to change the default core_collector to makedumpfile on RHEL5, so “cp” is still the default one. :)

Thanks for testing!

printk 的输出都是保存在log_buf里的

How to read it the ring buffer? Here is a beautiful illustration from IBM Developerworks dmesg would be your first resort! How does dmesg accomplish its task? By a call to syslog()! How does syslog do its job? Through the system call interface which in turn call do_syslog(). do_syslog() does the finishing act like this

// 模仿kernel/printk.c中do_syslog，在module 中读printk输出的最后4k

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char **log_buf;
int *log_buf_len, *log_start, *log_end;

int MASK;
int end, len;
unsigned i, j, limit, count;
char c;
int ret;


log_buf = (char**) 0xffffffff803270e0;
log_buf_len = (int*)0xffffffff803270d8;
log_start = (int*) 0xffffffff804eabd0;
log_end = (int*) 0xffffffff804eabe0;

i = 0;
end = *log_end;
len = *log_buf_len;
MASK = len - 1;

count = L;
if (count > len) count = len;
limit = end;

for (i=0;i<count;i++) {
        j = limit-1-i;
        c = *(*log_buf + (j&MASK));
        if (c == '\0') c = '\n';
        buf[count-1-i] = c;
}

// 输出

file = filp_open(file_path, O_RDWR|O_CREAT|O_APPEND, 0777);
if(IS_ERR(file)) {
        printk("Open file %s failed..\n", file_path);
        return 0;
}
old_fs = get_fs();
set_fs(get_ds());
ret = file->f_op->write(file, buf, i, &file->f_pos);
file->f_op->fsync(file, file->f_dentry, 0);
set_fs(old_fs);
filp_close(file, NULL);