On Sat, Jul 31, 2021 at 03:42:40PM +0800, Qu Wenruo wrote:

[HICCUP]
There is a bug report that mkfs.btrfs -R free-space-tree still makes
kernel to try to cleanup the v1 space cache:

  # mkfs.btrfs -R free-space-tree -f /dev/test/scratch1
  # mount /dev/test/scratch1 /mnt/btrfs
  # dmesg | grep cleaning
  BTRFS info (device dm-6): cleaning free space cache v1

[CAUSE]
By default, mkfs.btrfs will set super cache generation to (u64)-1, which
will inform kernel that the v1 space cache is invalid, needs to
regenerate it.

But for free space cache tree, kernel will set super cache generation to
0, to indicate v1 space cache is not in use.

This means, even we enabled free space tree with all the RO compatible
bits and new tree, as long as super cache generation is not 0, kernel
still consider the fs has some invalid v1 space cache, and will try to
remove them.

[FIX]
This is not a big deal, but to make the "-R free-space-tree" to really
work as kernel, we also need to set super cache generation to 0.

Reported-by: Chris Murphy <redacted>
Link: https://lore.kernel.org/linux-btrfs/CAJCQCtSvgzyOnxtrqQZZirSycEHp+g0eDH5c+Kw9mW=PgxuXmw@mail.gmail.com/ (local)
Signed-off-by: Qu Wenruo <redacted>

Added to devel, thanks.