+++ This bug was initially created as a clone of Bug #1665968 +++

Description of problem:

df will report negative IUsed for a directory which has project quotas enabled.

df for the prjquota dir:
Filesystem     Inodes   IUsed  IFree IUse% Mounted on
/dev/loop7         53 -524229 524282     - /mnt

Version-Release number of selected component (if applicable):
4.18.0-32.el8.x86_64

How reproducible:
Always

Steps to Reproduce:

1. Create a repro.sh file with following contents and make it executable. 

#!/bin/bash
truncate -s1G foo.img 
mkfs.xfs -K -f foo.img > /dev/null 
mount foo.img /mnt/ -o prjquota 
mkdir -p /mnt/test 
xfs_quota -x -c 'project -s -p /mnt/test 42' /mnt
echo df before setting up limits
echo df for mnt
df -i /mnt/
echo df for prjquota dir
df -i /mnt/test
xfs_quota -x -c 'limit -p isoft=53 test-bug' /mnt
echo inode soft limits set for /mnt/test
xfs_quota -x -c report /mnt
echo df after setup for mnt: 
     df -i /mnt
echo df for the prjquota dir: 
     df -i /mnt/test/

echo unmounting
umount /mnt/

2. Execute the script. DF will show negative value

Filesystem     Inodes   IUsed  IFree IUse% Mounted on
/dev/loop7         53 -524229 524282     - /mnt
unmounting

Actual results:

Filesystem     Inodes   IUsed  IFree IUse% Mounted on
/dev/loop7         53 -524229 524282     - /mnt

Expected results:

Filesystem     Inodes   IUsed  IFree IUse% Mounted on
/dev/loop7         53     0      53     - /mnt


Additional info:

This bug has been fixed in commit the following commit 

$ git show de7243057e7cefa923fa5f467c0f1ec24eef41d2
commit de7243057e7cefa923fa5f467c0f1ec24eef41d2
Author: Ye Yin <dbyin>
Date:   Fri Nov 23 09:55:18 2018 -0800

    fs/xfs: fix f_ffree value for statfs when project quota is set
    
    When project is set, we should use inode limit minus the used count
    
    Signed-off-by: Ye Yin <dbyin>
    Reviewed-by: Darrick J. Wong <darrick.wong>
    Signed-off-by: Darrick J. Wong <darrick.wong>

diff --git a/fs/xfs/xfs_qm_bhv.c b/fs/xfs/xfs_qm_bhv.c
index 73a1d77ec187..3091e4bc04ef 100644
--- a/fs/xfs/xfs_qm_bhv.c
+++ b/fs/xfs/xfs_qm_bhv.c
@@ -40,7 +40,7 @@ xfs_fill_statvfs_from_dquot(
                statp->f_files = limit;
                statp->f_ffree =
                        (statp->f_files > dqp->q_res_icount) ?
-                        (statp->f_ffree - dqp->q_res_icount) : 0;
+                        (statp->f_files - dqp->q_res_icount) : 0;
        }
 }
 

After applying the patch, I am not able to reproduce the issue.