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:

"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