linux-kernel - Re: [PATCH] initramfs: Allow rootfs to use tmpfs instead of ramfs

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Message-ID: <44CCFF09.2000106@zytor.com>
Date:	Sun, 30 Jul 2006 11:48:41 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Al Boldi <a1426z@...ab.com>, Rob Landley <rob@...dley.net>,
	linux-kernel@...r.kernel.org, torvalds@...l.org, stable@...nel.org,
	akpm@...l.org, chrisw@...s-sol.org, grim@...ead.cc
Subject: Re: [PATCH] initramfs:  Allow rootfs to use tmpfs instead of ramfs

Hugh Dickins wrote:
> On Sun, 30 Jul 2006, Al Boldi wrote:
>> Replugs rootfs to use tmpfs instead of ramfs as a Kconfig option.
> 
> Why?  Looking further down we see what you should have explained here:
>> + This option switches rootfs so that it uses tmpfs rather than ramfs
>> + for its file storage.  This makes rootfs swappable so having a large
>> + initrd or initramfs image won't eat up valuable RAM.
> 
> Now, I'm far from an expert on initramfs and early userspace, but my
> understanding is that the "init" of a (properly designed) initramfs
> would pretty much "rm -rf" everything in the initramfs before passing
> control to the final "init".  So (almost?) no valuable RAM is eaten
> up, nor the less valuable swap if you do extend this to tmpfs (unless
> something gets left open, which I think should not be the case).
> 
> So I'm inclined to say that this patch is simply unnecessary.  But
> if people who know better think it's a good thing, I've no objection
> (though I've not tried it): the Kconfiggery looks more likely to
> provoke argument than the tmpfs/ramfs mods.
> 

Well...

There is some justification: embedded people would like to load 
inittmpfs and then continue running.

The main issue -- which I am not sure what effect this patch has -- is 
that we would really like to move initramfs initialization even earlier 
in the kernel, so that it can include firmware loading for built-in 
device drivers, for example.

Thus, if this patch makes it harder to push initramfs initialization 
earlier, it's probably a bad thing.  If not, the author of the patch 
really needs to explain why it works and why it doesn't add new 
dependencies to the initialization order.

Saying "this is a trivial patch" and pushing it on the -stable tree 
doesn't inspire too much confidence, as initialization is subtle.

	-hpa
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