zfs filesystem skipped by df -h

On full pool when pool root filesystem references very few bytes,
the f_blocks returned to statvfs is 0 but should be at least 1.

Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Fixes #8253

module/zfs/zfs_vfsops.c

@@ -1448,7 +1448,7 @@ zfs_statvfs(struct dentry *dentry, struct kstatfs *statp)
 	 * the file system, but reported in terms of f_bsize - the
 	 * "preferred" size.
 	 */
-
+	refdbytes = P2ROUNDUP(refdbytes, statp->f_bsize);
 	statp->f_blocks = (refdbytes + availbytes) >> bshift;
 	statp->f_bfree = availbytes >> bshift;
 	statp->f_bavail = statp->f_bfree; /* no root reservation */

tests/runfiles/linux.run

@@ -656,7 +656,8 @@ tests = ['nestedfs_001_pos']
 tags = ['functional', 'nestedfs']
 
 [tests/functional/no_space]
-tests = ['enospc_001_pos', 'enospc_002_pos', 'enospc_003_pos']
+tests = ['enospc_001_pos', 'enospc_002_pos', 'enospc_003_pos',
+    'enospc_df']
 tags = ['functional', 'no_space']
 
 [tests/functional/nopwrite]

tests/zfs-tests/tests/functional/no_space/Makefile.am

@@ -4,7 +4,8 @@ dist_pkgdata_SCRIPTS = \
 	cleanup.ksh \
 	enospc_001_pos.ksh \
 	enospc_002_pos.ksh \
-	enospc_003_pos.ksh
+	enospc_003_pos.ksh \
+	enospc_df.ksh
 
 dist_pkgdata_DATA = \
 	enospc.cfg

tests/zfs-tests/tests/functional/no_space/enospc_df.ksh

@@ -0,0 +1,72 @@
+#!/bin/ksh -p
+#
+# CDDL HEADER START
+#
+# This file and its contents are supplied under the terms of the
+# Common Development and Distribution License ("CDDL"), version 1.0.
+# You may only use this file in accordance with the terms of version
+# 1.0 of the CDDL.
+#
+# A full copy of the text of the CDDL should have accompanied this
+# source.  A copy of the CDDL is also available via the Internet at
+# http://www.illumos.org/license/CDDL.
+#
+# CDDL HEADER END
+#
+
+#
+# Copyright (c) 2014, 2016 by Delphix. All rights reserved.
+# Copyright (c) 2019 by Datto Inc. All rights reserved.
+#
+
+. $STF_SUITE/include/libtest.shlib
+. $STF_SUITE/tests/functional/no_space/enospc.cfg
+
+#
+# DESCRIPTION:
+# After filling a filesystem, the df command produces the
+# expected result against the pool root filesystem.
+#
+
+verify_runnable "both"
+
+function cleanup
+{
+	log_must_busy zpool destroy -f $TESTPOOL
+}
+
+log_onexit cleanup
+
+log_assert "ENOSPC is returned when file system is full."
+
+default_setup_noexit $DISK_SMALL
+log_must zfs set compression=off $TESTPOOL/$TESTFS
+log_must zfs snapshot $TESTPOOL/$TESTFS@snap
+
+#
+# Completely fill the pool in order to ensure the commands below will more
+# reliably succeed or fail as a result of lack of space.  Care is taken to
+# force multiple transaction groups to ensure as many recently freed blocks
+# as possible are reallocated.
+#
+log_note "Writing files until ENOSPC."
+
+for i in $(seq 30); do
+	file_write -o create -f $TESTDIR/file.$i -b $BLOCKSZ \
+	    -c $NUM_WRITES -d $DATA
+	ret=$?
+	(( $ret != $ENOSPC )) && \
+	    log_fail "file.$i returned: $ret rather than ENOSPC."
+
+	log_must zpool sync -f
+done
+
+log_must zfs umount $TESTPOOL/$TESTFS
+log_must eval "df -h |grep $TESTPOOL"
+size=$(df -h /$TESTPOOL |grep $TESTPOOL | awk '{print $2}')
+used=$(df -h /$TESTPOOL |grep $TESTPOOL | awk '{print $3}')
+if [[ $size = "0" ]] || [[ $used = "0" ]]
+then
+	log_fail "df failed with zero filesystem bytes used"
+fi
+log_pass "df after ENOSPC works as expected."