libext4 does not supports metadata checksum feature

[Meandres]

As a foreword, thank you for adding support for ext(2/3/4) to OSv !

I was playing around following the documentation and I encountered the following error: failed to mount ext, error = Not supported.
After turning on several debugging output, I realized that the metadata_checksum feature of ext4 seems to be unsupported by the library, as seen in the following:

OSv v0.57.0-509-gd85aa856
[ext4] libext loaded!
[ext4] [ext4] Trying to open device: [vblk1]
[ext4] Trying to mount ext4 on device: [vblk1] with size:134217728
[ext4] block_init done
[ext4] blockdev read 1024 bytes at offset 1024 to 0x50007f3f4000 with error:0
ext4_fs: l: 219   [info]  sblock features_incompatible:
ext4_fs: l: 144   filetype
ext4_fs: l: 152   extents
ext4_fs: l: 154   64bit
ext4_fs: l: 158   flex_bg
ext4_fs: l: 164   meta_csum
ext4_fs: l: 222   [info]  sblock features_compatible:
ext4_fs: l: 177   has_journal
ext4_fs: l: 179   ext_attr
ext4_fs: l: 181   resize_inode
ext4_fs: l: 183   dir_index
ext4_fs: l: 225   [info]  sblock features_read_only:
ext4_fs: l: 189   sparse_super
ext4_fs: l: 191   large_file
ext4_fs: l: 195   huge_file
ext4_fs: l: 199   dir_nlink
ext4_fs: l: 201   extra_isize
ext4_fs: l: 207   metadata_csum
ext4_fs: l: 232   [error] sblock has unsupported features incompatible:
ext4_fs: l: 164   meta_csum
failed to mount ext, error = Not supported
... rest of the output ...

I worked around it by disabling it when creating the filesystem and took the liberty to propose a PR to add this to the doc: #1378

[wkozaczuk]

Hi, thanks for the PR.

I have researched this problem, and I believe it is more complex than what you have reported.

In general, the metadata_csum feature is supported by lwext4, which is a library used by the OSv libext module behind the scenes - please see metadata_csum under the [info] sblock features_read_only: line in your printout. However, some aspects of it are not supported, like the block group's checksum indicated by the line ext4_fs: l: 164 meta_csum (use crc32c for bg comment in the code). You can read more about it here - https://www.kernel.org/doc/html/latest/filesystems/ext4/group_descr.html. You can find more details of what lwext4 supports and does not, here and there.

It seems like the specific default features of ext4 depend on a Linux distribution. In my case, I used Fedora and I did not encounter this error. The dumpe2fs shows following for an ext4 disk created in my case:

Filesystem features:     has_journal ext_attr resize_inode dir_index filetype needs_recovery\
 extent 64bit flex_bg sparse_super large_file huge_file dir_nlink extra_isize metadata_csum

What does it show for you?

[wkozaczuk]

BTW, there is more documentation about the ext support here - https://github.com/cloudius-systems/osv/wiki/Filesystems#ext. As this wiki illustrates, right now it is also possible to build an OSv image with ext as a root filesystem. The wiki also mentions that there is no page cache integration. It may be an ideal subject for a future bachelor's thesis, if I may suggest ;-)

[Meandres]

Right, I use NixOS, which pulls directly util-linux (that contains mkfs) from the kernel mirror (https://github.com/NixOS/nixpkgs/blob/nixos-25.05/pkgs/by-name/ut/util-linux/package.nix#L42) instead of the Fedora fork (https://src.fedoraproject.org/rpms/util-linux). I guess this is the issue.
Thanks for the additional documentation.