Книга: Embedded Linux Primer: A Practical, Real-World Approach

6.4.5. The initrd Plumbing

6.4.5. The initrd Plumbing

As part of the Linux boot process, the kernel must locate and mount a root file system. Late in the boot process, the kernel decides what and where to mount in a function called prepare_namespace(). If initrd support is enabled in the kernel, as illustrated in Figure 6-1, and the kernel command line is so configured, the kernel decompresses the compressed initrd image from physical memory and eventually copies the contents of this file into a ramdisk device (/dev/ram). At this point, we have a proper file system on a kernel ramdisk. After the file system has been read into the ramdisk, the kernel effectively mounts this ramdisk device as its root file system. Finally, the kernel spawns a kernel thread to execute the linuxrc file on the initrd image.[54]

When the linuxrc script has completed execution, the kernel unmounts the initrd and proceeds with the final stages of system boot. If the real root device has a directory called /initrd, Linux mounts the initrd file system on this path (in this context, called a mount point). If this directory does not exist in the final root file system, the initrd image is simply discarded.

If the kernel command line contains a root= parameter specifying a ramdisk (root=/dev/ram0, for example), the previously described initrd behavior changes in two important ways. First, the processing of the linuxrc executable is skipped. Second, no attempt is made to mount another file system as root. This means that you can have a Linux system with initrd as the only root file system. This is useful for minimal system configurations in which the only root file system is the ramdisk. Placing /dev/ram0 on the kernel command line allows the full system initialization to complete with the initrd as the final root file system.

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