xfstest generic/015 run failed

Hi! I test on my host and find below issue, i want to know how can fix it?

root@ubuntu ~/xfstests# ./check generic/015

FSTYP         -- btrfs
PLATFORM      -- Linux/x86_64 ubuntu 4.12.0-rc6
MKFS_OPTIONS  -- /dev/sda2

MOUNT_OPTIONS -- /dev/sda2 /fs/scratch

generic/015 - output mismatch (see ~/xfstests/results//generic/015.out.bad)
--- tests/generic/015.out 2017-05-31 18:15:23.000000000 +0800
+++ ~/xfstests/results//generic/015.out.bad 2017-06-28 10:12:11.001630002 +0800
@@ -4,4 +4,5 @@
check free space:
delete fill:
check free space:
- !!! free space is in range
+ !!! free space has value of 17348
+free space is NOT in range 45591.48 .. 46512.52
...
(Run 'diff -u tests/generic/015.out ~/xfstests/results//generic/015.out.bad' to see the entire diff)
Ran: generic/015
Failures: generic/015
Failed 1 of 1 tests

I print _free to log and find after rm, the space do not free:

root@ubuntu ~/xfstests# cat log
/dev/sda2 btrfs 51200 16416 46052 27% /fs/scratch
46052
/dev/sda2 btrfs 51200 46144 17348 73% /fs/scratch
17348
/dev/sda2 btrfs 51200 46144 17348 73% /fs/scratch
17348

It makes me confused, and i am sure the file has removed and can not create size over 17348 file. Is it a bug or something wrong with my host?

Thanks
Dai Xiang

I don't observe this failure on rc6/rc7

The test purpose would need to be better defined, the file header says "check out-of-space behaviour". Not really helpful. The test script creates a 50MB filesystem, fills it, removes the file and compares what 'df' reports. The test result can change depending eg. on the mkfs options, if compression is enabled. So if the test fails, it does not tell us much. I would not worry about it.