Possible issues with fsck of f2fs root

[Hagbard Celine <hagbardcelin@gmail.com>]

Hi, I lost the root filesystem on my previous install after a few
weeks of several power outages last winter. While trying to recover I
discovered that it seem fsck was never run properly during boot in the
lifetime of that install.
After getting the system installed again a while ago, I have been
trying to discern why.
So far I've found the following two possible issues:

ISSUE 1:
If I boot with kernel option "ro rootfstype=f2fs
rootflags=background_gc=on,heap,disable_ext_identify,discard,user_xattr,inline_xattr,inline_dentry,acl,inline_data,flush_merge,data_flush,extent_cache,whint_mode=fs-based,fsync_mode=strict"
I get the following halfway trough boot:

 * Checking local filesystems  ...
Info: Use default preen mode
Info: Mounted device!
Info: Check FS only due to RO
        Error: Failed to open the device!
 * Filesystems couldn't be fixed


                     [ !! ] * rc: Aborting!

If i from this state try to mount another partition:
# mount -o "ro,relatime,lazytime,background_gc=on,discard,heap,user_xattr,inline_xattr,acl,disable_ext_identify,inline_data,inline_dentry,flush_merge,extent_cache,data_flush,mode=adaptive,active_logs=6,whint_mode=fs-based,alloc_mode=default,fsync_mode=strict"
/dev/nvme0n1p7 /mnt/f2fstest/

I get the same error if I try to run fsck on it:
# fsck.f2fs /dev/nvme0n1p7
Info: Mounted device!
Info: Check FS only due to RO
        Error: Failed to open the device!

If I on the other had boot with kernel option "rw rootfstype=f2fs
rootflags=background_gc=on,heap,disable_ext_identify,discard,user_xattr,inline_xattr,inline_dentry,acl,inline_data,flush_merge,data_flush,extent_cache,whint_mode=fs-based,fsync_mode=strict
panic=30 scsi_mod.use_blk_mq=1"

The boot does not hang and if I try same test as before, mount test partition:
# mount -o "ro,relatime,lazytime,background_gc=on,discard,heap,user_xattr,inline_xattr,acl,disable_ext_identify,inline_data,inline_dentry,flush_merge,extent_cache,data_flush,mode=adaptive,active_logs=6,whint_mode=fs-based,alloc_mode=default,fsync_mode=strict"
/dev/nvme0n1p7 /mnt/f2fstest/

Run fsck:
# fsck.f2fs  -f /dev/nvme0n1p7
Info: Force to fix corruption
Info: Mounted device!
Info: Check FS only due to RO
Info: Segments per section = 1
Info: Sections per zone = 1
Info: sector size = 512
Info: total sectors = 134101647 (65479 MB)
Info: MKFS version
  "Linux version 5.0.5-gentoof2fsfix (root@40o2) (gcc version 8.2.0
(Gentoo 8.2.0-r6 p1.7)) #2 SMP PREEMPT Mon Apr 1 17:04:41 +01 2019"
Info: FSCK version
  from "Linux version 5.0.5-gentoo (root@40o2) (gcc version 8.2.0
(Gentoo 8.2.0-r6 p1.7)) #2 SMP PREEMPT Tue Apr 2 07:42:40 +01 2019"
    to "Linux version 5.0.5-gentoo (root@40o2) (gcc version 8.2.0
(Gentoo 8.2.0-r6 p1.7)) #2 SMP PREEMPT Tue Apr 2 07:42:40 +01 2019"
Info: superblock features = 0 :
Info: superblock encrypt level = 0, salt = 00000000000000000000000000000000
Info: total FS sectors = 134101640 (65479 MB)
Info: CKPT version = 70e1454a
Info: Checked valid nat_bits in checkpoint
Info: checkpoint state = 4c1 :  large_nat_bitmap nat_bits crc unmount

[FSCK] Unreachable nat entries                        [Ok..] [0x0]
[FSCK] SIT valid block bitmap checking                [Ok..]
[FSCK] Hard link checking for regular file            [Ok..] [0x70]
[FSCK] valid_block_count matching with CP             [Ok..] [0x1fe244]
[FSCK] valid_node_count matcing with CP (de lookup)   [Ok..] [0x6c487]
[FSCK] valid_node_count matcing with CP (nat lookup)  [Ok..] [0x6c487]
[FSCK] valid_inode_count matched with CP              [Ok..] [0x6c362]
[FSCK] free segment_count matched with CP             [Ok..] [0x6c44]
[FSCK] next block offset is free                      [Ok..]
[FSCK] fixing SIT types
[FSCK] other corrupted bugs                           [Ok..]

Done.

So a system booted with "rw" root can fsck an "ro" filesystem but a
system booted with root "ro" can not.


ISSUE 2:
Referring to the output from the fsck running against a "ro"
filesystem, especially this line:
Info: Check FS only due to RO

As far as i can tell this says that opposed to other filesystems
running fsck against a "ro" mounted f2fs partition will never fix any
errors.
So I tried running fsck against the same partition mounted "rw":
# mount -o remount,rw /mnt/f2fstest/
# fsck.f2fs  -f /dev/nvme0n1p7
Info: Force to fix corruption
Info: Mounted device!
        Error: Not available on mounted device!

I might be misunderstanding something, but all this tells me that
unless one make a custom initramfs that runs fsck before root is
mounted (something no distributions has, as far as I know), fsck will
never fix an f2fs formatted root partition during boot.
If this is by design and not a bug/unintended behavior, it should be
documented somewhere least more people will experience system crashes
like mine.

All tests above done with kernel 5.0.5 and f2fs-tools 1.12.0 with
"fsck.f2fs: allow to fsck readonly image w/ -f option"-patch by Chao
Yu.