exfat on bcache = failure

[Enoch Leung]

I am trying out exFAT, went with an existing bcache backing device and it fails. No cache is attached to the backing device.

Platform: Intel G4600 on H170, 4x4GB RAM
OS: Ubuntu 21.10
kernel: 5.13.0-22-lowlatency
exfatprogs: 1.1.0-1

Steps to reproduce:
1. apt-get install exfatprogs
2. mkfs.exfat [my device]
2.1 output of mkfs.exfat
------------------------------------------------------------
exfatprogs version : 1.1.0
Creating exFAT filesystem(/dev/bcache2, cluster size=131072)

Writing volume boot record: done
Writing backup volume boot record: done
Fat table creation: done
Allocation bitmap creation: done
Upcase table creation: done
Writing root directory entry: done
Synchronizing...

exFAT format complete!
------------------------------------------------------------
3. mount /dev/bcache2 /mnt
3.1 output of mount
------------------------------------------------------------
mount: /mnt: wrong fs type, bad option, bad superblock on /dev/bcache2, missing codepage or helper program, or other error.
------------------------------------------------------------
3.2 partial output of dmesg right after mount
------------------------------------------------------------
exFAT-fs (bcache2): Invalid exboot-signature(sector = 1): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 2): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 3): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 4): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 5): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 6): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 7): 0x4e414c00
exFAT-fs (bcache2): Invalid exboot-signature(sector = 8): 0x4e414c00
exFAT-fs (bcache2): Invalid boot checksum (boot checksum : 0x871fd31e, checksum : 0x3dbec625)
exFAT-fs (bcache2): invalid boot region
exFAT-fs (bcache2): failed to recognize exfat type
------------------------------------------------------------
4. check filesystem using "fsck.exfat -y /dev/bcache2"
4.1 fsck.exfat output
------------------------------------------------------------
exfatprogs version : 1.1.0
checksum of boot region is not correct. 0x871fd31e, but expected 0x3dbec625
boot region is corrupted. try to restore the region from backup. Fix (y/N)? y
checksum of boot region is not correct. 0x871fd31e, but expected 0x2261a277
backup boot region is also corrupted
------------------------------------------------------------

Additional info:
a) bcache-super-show /dev/sdm1
a.0) output of command above
------------------------------------------------------------
sb.magic ok
sb.first_sector 8 [match]
sb.csum 780177DD31F80579 [match]
sb.version 1 [backing device]

dev.label (empty)
dev.uuid 76c98519-37db-4cff-bf08-91d27fd94c80
dev.sectors_per_block 8
dev.sectors_per_bucket 32768
dev.data.first_sector 16
dev.data.cache_mode 0 [writethrough]
dev.data.cache_state 0 [detached]

cset.uuid 00000000-0000-0000-0000-000000000000
------------------------------------------------------------
b) echo 1 > /sys/block/bcache2/bcache/stop
c) wipefs -a /dev/sdm1
   /dev/sdm1: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
d) mkfs.exfat /dev/sdm1
e) mount /dev/sdm1 /mnt
f) mount
   /dev/sdm1 on /mnt type exfat (rw,relatime,fmask=0022,dmask=0022,iocharset=utf8,errors=remount-ro)