Re: [PATCH] initrd: support erofs as initrd

[Julian Stecklina <julian.stecklina@cyberus-technology.de>]

On Fri, 2025-03-21 at 13:27 +0800, Gao Xiang wrote:
> Hi Christoph,
> 
> On 2025/3/21 13:01, Christoph Hellwig wrote:
> > We've been trying to kill off initrd in favor of initramfs for about
> > two decades.  I don't think adding new file system support to it is
> > helpful.
> > 
> 
> Disclaimer: I don't know the background of this effort so
> more background might be helpful.

So erofs came up in an effort to improve the experience for users of NixOS on
smaller systems. We use erofs a lot and some people in the community just
consider it a "better" cpio at this point. A great property is that the contents
stays compressed in memory and there is no need to unpack anything at boot.
Others like that the rootfs is read-only by default. In short: erofs is a great
fit.

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. :)

> 
> Two years ago, I once thought if using EROFS + FSDAX to directly
> use the initrd image from bootloaders to avoid the original initrd
> double caching issue (which is what initramfs was proposed to
> resolve) and initramfs unnecessary tmpfs unpack overhead:
> https://lore.kernel.org/r/ZXgNQ85PdUKrQU1j@infradead.org
> 
> Also EROFS supports xattrs so the following potential work (which
> the cpio format doesn't support) is no longer needed although I
> don't have any interest to follow either):
> https://lore.kernel.org/r/20190523121803.21638-1-roberto.sassu@huawei.com

Thanks for the pointers!

Julian