linux-kernel - Re: [RFC PATCH] rootfs: Use tmpfs for rootfs even if root= is given

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Date:   Wed, 1 Nov 2023 09:48:55 -0500
From:   Rob Landley <rob@...dley.net>
To:     Stefan Berger <stefanb@...ux.ibm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Milton D. Miller II" <mdmii@...look.com>,
        Jeff Layton <jlayton@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Jim Cromie <jim.cromie@...il.com>,
        Sam Ravnborg <sam@...nborg.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>, Mimi Zohar <zohar@...ux.ibm.com>
Subject: Re: [RFC PATCH] rootfs: Use tmpfs for rootfs even if root= is given

On 11/1/23 07:11, Stefan Berger wrote:
> On 11/1/23 07:35, Rob Landley wrote:
>> On 10/31/23 11:56, Greg Kroah-Hartman wrote:
>>> On Tue, Oct 31, 2023 at 11:44:17AM -0400, Stefan Berger wrote:
>>>> rootfs currently does not use tmpfs if the root= boot option is passed
>>>> even though the documentation about rootfs (added in 6e19eded3684) in
>>>> Documentation/filesystems/ramfs-rootfs-initramfs.rst states:
>>>>
>>>>    If CONFIG_TMPFS is enabled, rootfs will use tmpfs instead of ramfs by
>>>>    default.  To force ramfs, add "rootfstype=ramfs" to the kernel command
>>>>    line.
>>>
>>> At this point in time, is there even any difference between ramfs and
>>> tmpfs anymore?  Why would you want to choose one over the other here?
>> 
>> I submitted a patch to fix this to the list multiple times, which got ignored as
>> always. Most recently here:
>> 
>> https://lore.kernel.org/lkml/8244c75f-445e-b15b-9dbf-266e7ca666e2@landley.net/
> 
> I just tried it with your patch and the machine I am using this with 
> (OpenBMC) uses the boot command line 'console=ttyS4,115200n8 
> root=/dev/ram rw'. When I append rootfstype=tmpfs to this boot command 
> line then it starts actually using tmpfs. So I think this would work for me.
> I can add my Tested-by tag to the patch if this helps to get it merged. 
> Ideally it would also propagate back with a Fixes tag...

Regarding the "why ramfs" question, one bug report I periodically get is people
using initramfs.cpio.gz that works on ramfs but fails to extract when they try
tmpfs, and the reason is tmpfs defaults to size=50% of memory and their archive
extracts to more than that. Since ramfs hasn't got a limit, it extracted and ran
fine (generally a dedicated init app doing its IoT thing) and so far they've
always gone back to ramfs as their fix.

I vaguely recall I had some todo item to let them supply arguments so they could
specify their own size= for initmpfs (ramfs doesn't take any so it hadn't been
wired up last time I looked), but somewhere between
https://lkml.org/lkml/2016/6/22/686 and
https://lkml.iu.edu/hypermail/linux/kernel/2302.2/05597.html still being out of
tree 7 years later I kind of lost interest...

Rob