Bug Report: Btrfs can't allocate space for delete

["Hongzhi, Song" <hongzhi.song@windriver.com>]

Hi friends,

*Description:*


     One LTP testcase, fs_fill.c, fails on btrfs with kernel error when 
unlink files on Btrfs device:

     "BTRFS warning (device loop0): could not allocate space for a 
delete; will truncate on mount".


     I found the loop block device formatted with btrfs roughly rangs 
from 460M to 560M will cause the error.

     256M and 1G all pass.


     The fs_fill.c source code:

[https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/fs/fs_fill/fs_fill.c]

     The fs_fill.c calls unlink which triggers the error.

[https://github.com/linux-test-project/ltp/blob/e3457e42c1b93f54bb81da746eba314fd34ad40e/testcases/kernel/fs/fs_fill/fs_fill.c#L55]

[https://github.com/linux-test-project/ltp/blob/e3457e42c1b93f54bb81da746eba314fd34ad40e/lib/safe_macros.c#L358]


*Error info:*

     The issue maybe not reproduced everytime but four fifths chance.

     fs_fill.c:53: INFO: Unlinking mntpoint/thread5/file0
     safe_macros.c:360: BROK: fs_fill.c:55: 
unlink(mntpoint/thread10/file0) failed: ENOSPC
     safe_macros.c:360: BROK: fs_fill.c:55: 
unlink(mntpoint/thread11/file0) failed: ENOSPC
     [62477.378848] BTRFS warning (device loop0): could not allocate 
space for a delete; will truncate on mount
     [62477.378905] BTRFS warning (device loop0): could not allocate 
space for a delete; will truncate on mount



*Kernel:*

     After v5.2-rc1, qemux86-64

     # make -j40 ARCH=x86_64 CROSS_COMPILE=x86-64-gcc
     use qemu to bootup kernel


*LTP:*

     master branch: I tested on 20190625
     Reproduce:

     // build Ltp
     # cd Ltp-source
     # ./build.sh

     // copy files to qemu
     # cp runltp testcases/kernel/fs/fs_fill/fs_fill to qemu

     // login to qemu:
     // adjust block device size to 512M
     # vi runltp
     in function: create_block()
         dd if=/dev/zero of=${TMP}/test.img bs=1024 count=262144
         --->
         dd if=/dev/zero of=${TMP}/test.img bs=1024 count=524288

     // execute testcase
     # runltp -f fs -s fs_fill


*Analysis:*

     One new kernel commit contained in v5.2-rc1 introduces the issue.

     commit c8eaeac7b734347c3afba7008b7af62f37b9c140
     Author: Josef Bacik <josef@toxicpanda.com>
     Date:   Wed Apr 10 15:56:10 2019 -0400

         btrfs: reserve delalloc metadata differently
         ...


Anyone's reply will be appreciated.

--Hongzhi