linux支持两种内存根文件系统：ramdisk和initramfs。
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Document/early-userspace/README
128 The kernel has currently 3 ways to mount the root filesystem:
129
130 a) all required device and filesystem drivers compiled into the kernel, no
131 initrd. init/main.c:init() will call prepare_namespace() to mount the
132 final root filesystem, based on the root= option and optional init= to run
133 some other init binary than listed at the end of init/main.c:init().
134
135 b) some device and filesystem drivers built as modules and stored in an
136 initrd. The initrd must contain a binary '/linuxrc' which is supposed to
137 load these driver modules. It is also possible to mount the final root
138 filesystem via linuxrc and use the pivot_root syscall. The initrd is
139 mounted and executed via prepare_namespace().
140
141 c) using initramfs. The call to prepare_namespace() must be skipped.
142 This means that a binary must do all the work. Said binary can be stored
143 into initramfs either via modifying usr/gen_init_cpio.c or via the new
144 initrd format, an cpio archive. It must be called "/init". This binary
145 is responsible to do all the things prepare_namespace() would do.
146
147 To maintain backwards compatibility, the /init binary will only run if it
148 comes via an initramfs cpio archive. If this is not the case,
149 init/main.c:init() will run prepare_namespace() to mount the final root
150 and exec one of the predefined init binaries.
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initramfs
initramfs可以直接嵌入linux内核中，同内核一起编译。它可以为压缩文件，也可为普通非压缩文件系统。
其位置通过CONFIG_INITRAMFS_SOURCE指定。
注：若指定了CONFIG_INITRAMFS_SOURCE，则系统不会再响应ramdisk，jffs2等根文件系统，
不响应CONFIG_CMDLINE，即使INITRAMFS_SOURCE指定的目录为空或非正确的文件系统。
initramfs文件系统启动时，会在根文件系统中执行第一个init程序，对uboot中穿过来的
init=/linuxrc不予理睬。
注：不管linuxrc，还是init，都是符号链接，指向/bin/busybox.