linux-kernel - Re: [PATCH] use initmpfs even if there's root= cmdline

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Date:	Wed, 18 Dec 2013 11:51:30 -0600
From:	Rob Landley <rob@...dley.net>
To:	Dave Young <dyoung@...hat.com>, akpm@...ux-foundation.org,
	gregkh@...uxfoundation.org, grant.likely@...retlab.ca,
	sebastian.capella@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] use initmpfs even if there's root= cmdline

On 12/12/13 20:38, Dave Young wrote:
> On 12/12/13 at 05:25pm, Dave Young wrote:
>>
>> Current code use ramfs instead of tmpfs for stub when root= defined.
>>
>> But for real use case with initramfs, usually there's like cmdline like
>> root=UUID=$UUID the root dev is the real device. For that case we have
>> no way to use initmpfs, thus this patch removes the limitation so tmpfs
>> can benefit more people.

The reason I did that was if you specify a root= then you don't want to 
_stay_ on rootfs. You specify root= so either the kernel does 
switch_root for you, or so rootfs does a swich_root at the end.

The point of initmpfs is that when rootfs _is_ the "real" root device, 
it can benefit from being tmpfs. When you're just goign to switch to a 
different root device, tmpfs doesn't make much difference.

> Discussed with Vivek Goyal about the kdump use case, I missed one thing that
> tmpfs has default size limit though we can tune it.
>
> So I will think more about it, will address this later, please ignore this
> patch.

I have a vague todo item of feeding rootflags= through to initmpfs, but 
that's really intended to specify flags for root=. There isn't really an 
existing command line option to specify initramfs flags because ramfs 
doesn't care.

It was one of those "only parse rootflags= for initmpfs when there's no 
root=" vs "create a new rdrootflags= ala rdinit= even though that's a 
subtly wrong name these days..." and it went on the todo list because 
neither approach was obviously superior.

Happy to take suggestions and whip up a patch if this is inconveniencing 
somebody. :)

> Thanks
> Dave

Rob
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