generic: do operations which doesn't take data space on full fs

This test will test if we can still do the following operations when a
full is full:
- buffered write into unpopulated preallocated extent
- clone the untouched preallocated extent
- fsync
- no data loss if power loss happens after above fsync
Above operations should not fail, as they takes no extra data space.

Xfs passes the test, while btrfs fails at fsync and has data loss.
The fix for btrfs is:
"btrfs: Flush before reflinking any extent to prevent NOCOW write falling
 back to CoW without data reservation"

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/546

@@ -0,0 +1,84 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2019 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 546
+#
+# Test when a fs is full we can still:
+# - Do buffered write into a unpopulated preallocated extent
+# - Clone the untouched part of that preallocated extent
+# - Fsync
+# - No data loss even power loss happens after fsync
+# All operations above should not fail.
+#
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	_cleanup_flakey
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+. ./common/reflink
+. ./common/dmflakey
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs generic
+_supported_os Linux
+_require_xfs_io_command "falloc"
+_require_scratch_reflink
+_require_dm_target flakey
+
+_scratch_mkfs_sized $((512 * 1024 * 1024)) >> $seqres.full 2>&1
+_require_metadata_journaling $SCRATCH_DEV
+_init_flakey
+_mount_flakey
+
+# Create preallocated extent where we can write into
+$XFS_IO_PROG -f -c 'falloc 8k 64m' "$SCRATCH_MNT/foobar" >> $seqres.full
+
+# Use up all data space, to test later write-into-preallocate behavior
+_pwrite_byte 0x00 0 512m "$SCRATCH_MNT/padding" >> $seqres.full 2>&1
+
+# Sync to ensure that padding file reach disk so that at log recovery we
+# still have no data space
+sync
+
+# This should not fail
+_pwrite_byte 0xcd 1m 16m "$SCRATCH_MNT/foobar" >> $seqres.full
+
+# Do reflink here, we shouldn't use extra data space, thus it should not fail
+$XFS_IO_PROG -c "reflink ${SCRATCH_MNT}/foobar 8k 0 4k" "$SCRATCH_MNT/foobar" \
+	>> $seqres.full
+
+# Checksum before power loss
+echo md5 before $(_md5_checksum "$SCRATCH_MNT/foobar")
+
+# Fsync to check if writeback is ok
+$XFS_IO_PROG -c 'fsync'  "$SCRATCH_MNT/foobar"
+
+# Now emulate power loss
+_flakey_drop_and_remount
+
+# Checksum after power loss
+echo md5 after $(_md5_checksum "$SCRATCH_MNT/foobar")
+
+# success, all done
+status=0
+exit

tests/generic/546.out

@@ -0,0 +1,3 @@
+QA output created by 546
+md5 before cdccc121ae95c72fdbe53d8e343ef5ee
+md5 after cdccc121ae95c72fdbe53d8e343ef5ee

tests/generic/group

@@ -548,3 +548,4 @@
 543 auto quick clone
 544 auto quick clone
 545 auto quick cap
+546 auto quick clone enospc log