linux-kernel - Re: [PATCH] initrd: support erofs as initrd

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Message-ID: <20250407085751.GA27074@lst.de>
Date: Mon, 7 Apr 2025 10:57:51 +0200
From: "hch@....de" <hch@....de>
To: Julian Stecklina <julian.stecklina@...erus-technology.de>
Cc: "hch@....de" <hch@....de>,
	"hsiangkao@...ux.alibaba.com" <hsiangkao@...ux.alibaba.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"rafael@...nel.org" <rafael@...nel.org>,
	"viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] initrd: support erofs as initrd

On Fri, Mar 21, 2025 at 01:17:54PM +0000, Julian Stecklina wrote:
> Of course there are some solutions to using erofs images at boot now:
> https://github.com/containers/initoverlayfs
> 
> But this adds yet another step in the already complex boot process and feels
> like a hack. It would be nice to just use erofs images as initrd. The other
> building block to this is automatically sizing /dev/ram0:
> 
> https://lkml.org/lkml/2025/3/20/1296
> 
> I didn't pack both patches into one series, because I thought enabling erofs
> itself would be less controversial and is already useful on its own. The
> autosizing of /dev/ram is probably more involved than my RFC patch. I'm hoping
> for some input on how to do it right. :)

Booting from erofs seems perfectly fine to me.  Booting from erofs on
an initrd is not.  There is no reason to fake up a block device, just
have a version of erofs that directly points to pre-loaded kernel
memory instead.  This is a bit more work, but a lot more efficient
in that it removes the block path from the I/O stack, removes the boot
time copy and allows for much more flexible memory management.