Re: [PATCH v2 2/3] init/do_cmounts.c: introduce 'user_root' for initramfs

From: Eric W. Biederman
Date: Tue May 25 2021 - 23:23:59 EST

Menglong Dong <menglong8.dong@xxxxxxxxx> writes:

> On Wed, May 26, 2021 at 2:50 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> ......
>> What is the flow where docker uses an initramfs?
>> Just thinking about this I am not being able to connect the dots.
>> The way I imagine the world is that an initramfs will be used either
>> when a linux system boots for the first time, or an initramfs would
>> come from the distribution you are running inside a container. In
>> neither case do I see docker being in a position to add functionality
>> to the initramfs as docker is not responsible for it.
>> Is docker doing something creating like running a container in a VM,
>> and running some directly out of the initramfs, and wanting that code
>> to exactly match the non-VM case?
>> If that is the case I think the easy solution would be to actually use
>> an actual ramdisk where pivot_root works.
> In fact, nowadays, initramfs is widely used by embedded devices in the
> production environment, which makes the whole system run in ram.
> That make sense. First, running in ram will speed up the system. The size
> of the system won't be too large for embedded devices, which makes this
> idea work. Second, this will reduce the I/O of disk devices, which can
> extend the life of the disk. Third, RAM is getting cheaper.
> So in this scene, Docker runs directly in initramfs.

That is the piece of the puzzle I was missing. An small system
with it's root in an initramfs.

>> I really don't see why it makes sense for docker to be a special
>> snowflake and require kernel features that no other distribution does.
>> It might make sense to create a completely empty filesystem underneath
>> an initramfs, and use that new rootfs as the unchanging root of the
>> mount tree, if it can be done with a trivial amount of code, and
>> generally make everything cleaner.
>> As this change sits it looks like a lot of code to handle a problem
>> in the implementation of docker. Which quite frankly will be a pain
>> to have to maintain if this is not a clean general feature that
>> other people can also use.
> I don't think that it's all for docker, pivot_root may be used by other
> users in the above scene. It may work to create an empty filesystem, as you
> mentioned above. But I don't think it's a good idea to make all users,
> who want to use pivot_root, do that. After all, it's not friendly to
> users.
> As for the code, it may look a lot, but it's not complex. Maybe a clean
> up for the code I add can make it better?

If we are going to do this something that is so small and clean it can
be done unconditionally always.

I will see if I can dig in and look at little more. I think there is
a reason Al Viro and H. Peter Anvin implemeted initramfs this way.
Perhaps it was just a desire to make pivot_root unnecessary.

Container filesystem setup does throw a bit of a wrench in the works as
unlike a initramfs where you can just delete everything there is not
a clean way to get rid of a root filesystem you don't need without

The net request as I understand it: Make the filesystem the initramfs
lives in be an ordinary filesystem so it can just be used as the systems
primary filesystem.

There might be technical reasons why that is a bad idea and userspace
would be requested to move everything into another ramfs manually (which
would have the same effect). But it is take a good look to see if it
can be accomplished cleanly.