Re: [PATCH] initrd: support erofs as initrd

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Christoph,

On 2025/4/7 16:57, hch@lst.de wrote:
> 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.

For unencoded (uncompressed) images, especially data is page-aligned,
FSDAX can be used directly to replace page cache, which is absolutely
fine now.

But for encoded I/Os, since it needs for somewhat data transformation,
currently BIOs (actually mainly bio_vecs, but bio_vecs are lack of
useful interfaces) is needed to get the on-disk (or original memory)
encoded data in a flexible way.

I hope BIO (or some other-like interfaces) can be eventually decoupled
from block devices, and instead form generic block-based interfaces
anyway, so that eveutually yes, we don't need to fake a block device
(like FSDAX support for brd) like the old initrd.  But I suspect if
any folk puts more development resource to work on this (I don't
have more resource from my current employer), so personally I guess
initrd could be workable for the first step for people to leverage
this feature in some way.

Thanks,
Gao Xiang