Ilya A. Volynets-Evenbakh wrote:
I think it's the other way around - if you exit from your linuxrc when
running from initramfs,
it'll continue on with standard boot sequence, while ending linuxrc in
initrd will panic the kernel
with "Trying to kill PID 1" error.
An initramfs exists for any 2.6 kernel. Sometimes it's just a very small
one. The way the kernel decides whether to execute it is to check for a
*/init* file in it. If it finds one, it runs it. If not, it does not.
As such, if you have a /linuxrc file in your initramfs, and it simply
exists at the end, all that will happen is that the kernel will not find
a /init and will skip running anything from the initramfs at all. In
that sense you might be said to be right that if your linuxrc exists,
the normal boot sequence will continue.
The history goes something like this:
Originally, the drivers for the root file system had to be compiled into
the kernel. This was very difficult, if not impossible, for distribution
kernels, as the list of potential drivers included, well, everything,
and the kernels were just too big. Then the idea of initrd came - we
compile the drivers only for the ram disk and the initrd file system
(typically, cramfs) into the kernel. The initrd loads the relevant
drivers into the kernel and quits, restoring the original boot sequence.
Then someone pointed out that, sometimes, an initrd is all you need. The
convention then arose to pass "boot=/dev/ram0" to the kernel, which
tells it that the initrd stage is the last one. It then became common to
put the real "boot=" option on the kernel command line, but then, from
initrd, to load the values for "/dev/ram0" into the proc entry for the
boot device from linuxrc, manually mount the real root file system, make
it the root file system using pivot_root, and exec init from there.
Then, for 2.6, people figured that if this is what you normally do
anyways, there is no point in carrying around the drivers for the ram
disk and cramfs (compiled into the kernel, which means it cannot be
unloaded). Instead, use the much lighter tmpfs file system, and use a
cpio archive for the actual files. The tmpfs file system was made
mandatory, whether initramfs was used or not, and so you couldn't move
it from the root (only mount on top of it). This means you cannot use
pivot_root, and switch_root was invented. While at it, the "legacy" boot
sequence was removed. Assuming, as Gilad seems to, that we leave initrd
is out of the discussion, your options are either run an initramfs and
perform the entire root mounting from the /init script there, or compile
the root file system drivers into the kernel and let the kernel mount
them using the boot= kernel option.
Shachar
--
Shachar Shemesh
Lingnu Open Source Consulting Ltd.
http://www.lingnu.com
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