一.背景
文件系統安裝在sd卡的第一個分區中,使用的是ext4文件系統,linux內核版本為4.14
二.思考
在內核啟動之前，uboot給內核傳遞了參數root=/dev/mmcblk0p1,但是為何還會出現：
VFS: Cannot open root device "mmcblk0p1" or unknown-block(179,1): error -19
Please append a correct "root=" boot option; here are the available partitions:
三.扒一扒內核代碼
init/do_mouts.c
void __init mount_block_root(char *name, int flags)
{
... retry:
for (p = fs_names; *p; p += strlen(p)+1) {
int err = do_mount_root(name, p, flags, root_mount_data);
switch (err) {
case 0:
goto out;
case -EACCES:
case -EINVAL:
continue;
}
/*
* Allow the user to distinguish between failed sys_open
* and bad superblock on root device.
* and give them a list of the available devices
*/
#ifdef CONFIG_BLOCK
__bdevname(ROOT_DEV, b);
#endif
printk("VFS: Cannot open root device \"%s\" or %s: error %d\n",
root_device_name, b, err); 此處的打印信息被輸出
printk("Please append a correct \"root=\" boot option; here are the available partitions:\n");此處的打印信息被輸出
printk_all_partitions();
#ifdef CONFIG_DEBUG_BLOCK_EXT_DEVT
printk("DEBUG_BLOCK_EXT_DEVT is enabled, you need to specify "
"explicit textual name for \"root=\" boot option.\n");
#endif
panic("VFS: Unable to mount root fs on %s", b); 發生panic的地方在這里
...
}
static int __init do_mount_root(char *name, char *fs, int flags, void *data)
{
struct super_block *s;
int err = sys_mount(name, "/root", fs, flags, data); 此處返回的err是我們需要關心的,這里的err=-19,那么為何會等於-19呢？
if (err)
return err;
sys_chdir("/root");
s = current->fs->pwd.dentry->d_sb;
ROOT_DEV = s->s_dev;
printk(KERN_INFO
"VFS: Mounted root (%s filesystem)%s on device %u:%u.\n",
s->s_type->name,
sb_rdonly(s) ? " readonly" : "",
MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
return 0;
}
再分析sys_mount
fs/namespace.c
SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,char __user *, type, unsigned long, flags, void __user *, data)
{
...
ret = do_mount(kernel_dev, dir_name, kernel_type, flags, options);
...
}
long do_mount(const char *dev_name, const char __user *dir_name,const char *type_page, unsigned long flags, void *data_page)
{
...
retval = do_new_mount(&path, type_page, sb_flags, mnt_flags,dev_name, data_page);
...
}
/* * create a new mount for userspace and request it to be added into the
* namespace's tree
*/
static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
int mnt_flags, const char *name, void *data)
{
...
type = get_fs_type(fstype);
if (!type)
return -ENODEV;
...
}
-ENODEV=-19
從最后的get_fs_type可以得知:當前不支持某種文件系統，進而type等於非0，因此就會返回-19這個值
四.解決
當前sd卡上的第一個分區裝入的文件系統為ext4,然而內核此時並不支持此文件系統，因此加入CONFIG_EXT4_FS的配置項重新編譯內核即可