Repost :umount: /initrd: not mounted

[Andy Green]

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On Sunday 18 January 2004 22:19, Alexander Dalloz wrote:

> /initrd exists? chowned by root.root and chmod 755.

I know a little about what /initrd is for.  I don't know the answer to the 
original question tho, nor do I fully understand all the steps.  Also this is 
from memory :-)  But its basically right.

In /boot there is a compressed ext2 initrd filesystem to match the kernel 
version.  The kernel commandline in Redhat is set so that after the kernel 
has completed its startup action (the scrolling text even when using rhgb) it 
will unzip the compressed initrd filesystem and jump into it.

The kernel looks for an executable file called linuxrc in the initrd and runs 
it.  linuxrc has a #!/bin/nash or similar at the top in Redhat, so it runs 
nash, a kind of ultra-simplified bash/busybox kind of interpreter and goes 
through the steps listed in this linuxrc script.

One of the first jobs is to load up the jbd (Journalled Block Device) and ext3 
modules.  The kernel is compiled with ext3 as a module, without this step 
during the initrd phase the kernel could not handle an ext3 partition later.

Then it sets up some things /dev, mounts /proc, and then mounts the root 
filesystem readonly at a mountpoint which is IIRC /sysroot inside the initrd 
filesystem.  Then it does a pivot_root which makes /sysroot the new /, and 
the original initrd / is meant to end up at what used to be /sysroot/initrd 
but is now /initrd.  (Note for this reason if you delete the initrd 
mountpoint in your root filesystem, you won't be able to boot)

Then the boot process finishes with linuxrc and should start with /sbin/init 
using the root filesystem as / as set up by the pivot_root.  init then goes 
on to check / for errors, remounts it rw and then does the rhgb / [OK] part 
of the boot that we know and love.

One the pivot_root is done and the intended root filesystem is appearing at /, 
there is no need subsequently for the original initrd to appear any longer at 
/initrd, so I guess that is when it is unmounted.  If for some reason it is 
already unmounted, or the pivot_root somehow failed to deliver it to the root 
filesystem /initrd (while successfully doing the pivot of the initrd /sysroot 
to become the new root) then I guess that doesn't affect anything.  Would be 
interesting to know why tho!

- -Andy
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