= How Is The Root File System Found? =

One of the important kernel boot parameters is "root=", which tells the kernel where to find the root filesystem. For instance,

{{{
root=/dev/hda1
}}}

This is commonly specified as what ''looks'' like a standard Unix pathname (as above). But standard Unix pathnames are interpreted according to currently-mounted filesystems. So how do you interpret the above root pathname, before you've even mounted any filesystems?

It took me a few hours to decipher the answer to this (the following applies at least as of the 2.6.11 kernel sources). First of all, at kernel initialization time, there is already an absolutely minimal filesystem registered, called "rootfs". The code that implements this filesystem can be found in {{{fs/ramfs/inode.c}}}, which also happens to contain the code for the "ramfs" filesystem. rootfs is basically identical to ramfs, except for the specification of the {{{MS_NOUSER}}} flag; this is interpreted by the routine {{{graft_tree}}} in {{{fs/namespace.c}}}, and I think it prevents userland processes doing their own mounts of rootfs.

The routine {{{init_mount_tree}}} (found in {{{fs/namespace.c}}}) is called at system startup time to mount an instance of rootfs, and make it the root namespace of the current process (remember that, under Linux, different processes can have different filesystem namespaces). This routine is called at the end of {{{mnt_init}}} (also in {{{fs/namespace.c}}}), as part of the following sequence:

{{{
sysfs_init(); /* causes sysfs to register itself--this is needed later for actually finding the root device */
init_rootfs(); /* causes rootfs to register itself */
init_mount_tree(); /* actually creates the initial filesystem namespace, with rootfs mounted at "/" */
}}}

{{{mnt_init}}} is called from {{{vfs_caches_init}}} in {{{fs/dcache.c}}}, which in turn is called from {{{start_kernel}}} in {{{init/main.c}}}.