Le 04/05/2015 16:48, Steve Litt a écrit :
Hi all,
I just documented the boot process, from Grub through init.
See http://troubleshooters.com/linux/diy/howboot.htm
Hope you like it.
SteveT
Dear Steve, I have a few comments, more or less important to
understand how things work:
It is pretty seldom, I think, that /bin and /sbin are symbolic
links to /usr/bin and /usr/sbin. I have never seen it in a major distro
up to now.
The root= argument to kernel is usefull also when using an
initramfs. In this case the the argument is ignored by the kernel but it
is available to the initramfs init program. Note that custom initramfs
may ignore it and have other ways to find the root partition.
The initramfs init does a few things before it pivot-root and
passes control to the ondisk init. It mounts /proc, /sys, /dev, creates
device files, mounts the disk root partition, then mounts proc, sys and
dev on this partition. On Debian, it mounts /run on tmpfs. When the
ondisk init starts, there are quite a few things ready.
Processes are created with a process-id which is incremented for
every new process, until it reaches the maximum and restarts from the
begining. If init has pid 1, it is because it is just the first one
created. There is no other process started by the kernel before init. If
the second init (after pivot-root) also has pid 1, it is because it is
just the same process, running a different program. I'm not sure what
happens if init crashes after other processes have been started, wether
the kernel panics or other processes continue without init - not a very
good situation.
The initramfs is a live OS and you can make it a friendly debugging
environment to help understand what is going on. As Laurent mentionned
some time ago, it is not even necessary to pivot-root; you could keep
/bin and /sbin in ramfs and just mount the rest. Of course upgrading
the files in /bin and /sbin would involve re-creating the whole initramfs.
A ramfs or tmpfs is not exactly like the old initrd. The Linux VFS
keeps data in buffers and filesystems periodically swap the data in
these buffers with hardware backup stores, that is disks. A ramdisk is
when the backup store is in some fixed location in ram. The ramfs is a
filesystem simply *without* a backing store - very nice idea which
emerged around 10 years ago I think, which shows that simplicity is not
given, it is an achievement. tmpfs is like ramfs, except it can expand
to the swapping area. I think that ramfs and tmpfs have been completely
merged a few years ago.
Hope this helps and there are no errors :-)
Didier
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