btrfs: test that defrag on small files does not hang or crashes

Test that defragging files with very small sizes works and does not
result in any crash, hang or corruption.

This is motivated by a regression introduced in kernel 5.16 where
attempting to defrag a file with a size of 1 byte would result in
the kernel code hitting an "infinite" loop (iterating from 0 to
(u64)-1 in increments of 256K, which in practice is an eternity).

The regression is fixed by a patch with the following subject:

  "btrfs: fix too long loop when defragging a 1 byte file"

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

2 files changed, 71 insertions, 0 deletions

diff --git a/tests/btrfs/256 b/tests/btrfs/256
new file mode 100755
index 0000000000..1360c2c202
--- /dev/null
+++ b/tests/btrfs/256
@@ -0,0 +1,68 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2022 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test No. 256
+#
+# Test that defragging files with very small sizes works and does not result in
+# any crash, hang or corruption.
+#
+# The regression is fixed by a patch with the following subject:
+#
+#   "btrfs: fix too long loop when defragging a 1 byte file"
+#
+. ./common/preamble
+_begin_fstest auto quick defrag
+
+_cleanup()
+{
+	cd /
+	rm -r -f $tmp.*
+}
+
+# Import common functions.
+. ./common/filter
+
+# real QA test starts here
+
+_supported_fs btrfs
+_require_scratch
+_require_fssum
+
+_scratch_mkfs >> $seqres.full 2>&1
+_scratch_mount
+
+checksums_file="$TEST_DIR/btrfs-test-$seq-checksums"
+block_size=$(_get_block_size "$SCRATCH_MNT")
+
+# Test file sizes of 0, 1, 512, 1K, 2K, 4K, 8K, 16K, 32K and 64K bytes.
+file_sizes=( 0 1 512 1024 2048 4096 8192 16384 32768 65536 )
+
+# Create the files and compute their checksums.
+for sz in ${file_sizes[@]}; do
+	byte=$(printf "0x%x" $((RANDOM % 255)))
+	$XFS_IO_PROG -f -c "pwrite -S $byte 0 $sz" "$SCRATCH_MNT/f_$sz" >> $seqres.full
+done
+
+# Compute the checksums.
+$FSSUM_PROG -A -f -w "$checksums_file" "$SCRATCH_MNT"
+
+# Now defrag each file.
+for sz in ${file_sizes[@]}; do
+	echo "Defragging file with $sz bytes..." >> $seqres.full
+	$BTRFS_UTIL_PROG filesystem defragment "$SCRATCH_MNT/f_$sz"
+done
+
+# Verify the checksums after the defrag operations.
+$FSSUM_PROG -r "$checksums_file" "$SCRATCH_MNT"
+
+# Unmount and mount again, this clears the page cache and we want to see that
+# no corruptions were persisted.
+_scratch_cycle_mount
+
+# Verify the checksums again.
+$FSSUM_PROG -r "$checksums_file" "$SCRATCH_MNT"
+
+# success, all done
+status=0
+exit
diff --git a/tests/btrfs/256.out b/tests/btrfs/256.out
new file mode 100644
index 0000000000..673affbb58
--- /dev/null
+++ b/tests/btrfs/256.out
@@ -0,0 +1,3 @@
+QA output created by 256
+OK
+OK