On Thu, 10 Mar 2016 01:24:44 -0700
Chris Murphy [off-list ref] wrote:

kernel 4.5.0 rc7
btrfs-progs-4.4.1-1

Summary: A new empty mixed block group "-M" filesystem shows up as
100% full with regular df (gnu coreutils 8.24). Pretty sure this is
not a regression, but no regression testing done.


https://bugzilla.kernel.org/show_bug.cgi?id=114281

Just guessing here, but this might be related?

  [PATCH] btrfs: statfs: report zero available if metadata are exhausted
  http://www.spinics.net/lists/linux-btrfs/msg48013.html

Maybe some maths there are wrong in case of a mixed blockgroup filesystem.

-- 
With respect,
Roman