Why '-c DEV' option for switch_root?

[Rob Landley]

On 12/08/13 20:09, ChenQi wrote:
> On 12/06/2013 06:28 PM, Laurent Bercot wrote:
>>
>>> I don't know if I understand it right.
>>> Does it mean that as long as we don't do some strange redirections of
>>> standard IO in our initramfs, we don't need to add this option when
>>> switching root? So in most cases we don't need to reopen the stdio,
>>> right?
>>
>>  Unless your real root's /dev/console is the same file as your
>> initramfs's /dev/console, you probably do.
>>
>>  When your system boots, process 1 (and all its offspring, if there's
>> no redirection) has fds 0, 1 and 2 pointing to your initramfs's
>> /dev/console. That device will remain open as long as you don't close
>> those fds. switch_root needs to reopen 0, 1 and 2 to your new
>> /dev/console
>> to be able to really clean up your initramfs.
>>
>
> Got it.

It's actually probably obsolete these days, now that we have initmpfs 
and can mount --move the initrmpfs into the new root filesystem. My code 
did it because the klibc code was doing it, but I vaguely recall the 
initial reason was not relying on a node that would no longer be 
accessible and thus provide confusing information in /proc/1/fd, or some 
such. (Really, you'd have to ask the klibc guys why.)

> Thanks,
> Chen Qi
>
>> (The best solution is to stop using initramfs and boot directly on your
>> real root filesystem, using a tmpfs to perform early operations that need
>> writable space, and using pivot_root if you really need to change root
>> filesystems. initramfs is a trap.)

That's actively bad advice.

The most recent kernel has my initmpfs patches, meaning initramfs can 
now be a tmpfs instead of ramfs.

This means it has size limits (50% by default, changeable with -o 
remount, so "cat /dev/zero > /blah" won't fill up memory and freeze the 
box). It means it reports available space so trying to use something 
like "rpm" that checks available space before proceeding should work out 
of one of those. And it means that it should show up in a sanely written 
df (which filters out 0 block filesystems so it doesn't show /proc and 
/sys and stuff). Plus the usual "memory in this can swap to swap 
partitions if you've got 'em" that tmpfs provides.

Of course some people special case initramfs in their code because they 
don't understand how the system works, and thus when you use initramfs 
as your rootfs their code doesn't see it even when it's not overmounted. 
But that's because those people wrote broken code.

Rob