Hi Christoph,
On 2025/4/7 16:57, hch@xxxxxx 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
