[PATCH] btrfs: test defrag --nocomp converts comp extents to uncomp

[Dhruva G <goledhruva@gmail.com>]

"btrfs filesystem defragment --nocomp" rewrites already compressed file
data as uncompressed data. The kernel flag was added by commit
009b2056cb25 ("btrfs: defrag: add flag to force no-compression") in
v6.17, and the userspace option shipped in btrfs-progs v6.16. There is
no test for it.

Add a test that writes compressible data on a filesystem mounted with
compression enabled, runs defragment with --nocomp, and verifies that no
extent remains compressed while the file contents are unchanged across a
remount. The filesystem stays mounted with compress=zlib for the whole
test, which also covers that the defrag ioctl takes precedence over mount
options.

Regular defrag deliberately leaves maximum-sized compressed extents alone,
as asserted by btrfs/259. A --nocomp implementation that silently behaves
like regular defrag therefore requires a dedicated test.

Check support in two stages: first require btrfs-progs to advertise
--nocomp, then invoke it on a 10-byte file to verify that the kernel
accepts the flag. Kernels with the unknown-flag validation from commit
173431b274a9 return EOPNOTSUPP when NOCOMPRESS is unavailable. Kernels
predating that validation may silently ignore the flag and fail the
semantic checks instead of skipping.

Signed-off-by: Dhruva Gole <goledhruva@gmail.com>
---

Logs/ compatibility matrix:
https://gist.github.com/DhruvaG2000/62101a37f5e53e41883b4b3181250724

---

 tests/btrfs/354     | 92 +++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/354.out |  2 +
 2 files changed, 94 insertions(+)
 create mode 100755 tests/btrfs/354
 create mode 100644 tests/btrfs/354.out

diff --git a/tests/btrfs/354 b/tests/btrfs/354
new file mode 100755
index 00000000..fae8f47e
--- /dev/null
+++ b/tests/btrfs/354
@@ -0,0 +1,92 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Meta Platforms, Inc.  All Rights Reserved.
+#
+# FS QA Test 354
+#
+# Make sure "btrfs filesystem defragment --nocomp" rewrites compressed extents
+# as uncompressed ones, even though the filesystem is mounted with compression
+# enabled, and without changing the file contents.
+#
+# Note that regular defrag deliberately skips compressed extents that are
+# already at their maximum size (see btrfs/259), so a broken --nocomp that
+# simply does nothing would go unnoticed without this test.
+#
+. ./common/preamble
+_begin_fstest auto quick defrag compress
+
+# real QA test starts here
+
+_require_scratch
+_require_btrfs_command inspect-internal dump-tree
+_require_btrfs_command filesystem defragment --nocomp
+
+_wants_kernel_commit 009b2056cb25 \
+	"btrfs: defrag: add flag to force no-compression"
+
+# Same helper as btrfs/260: btrfs specific extent attributes are only
+# available from dump-tree, not from fiemap.
+#
+# NOTE: At the moment this only works if the file is on a filesystem on top of
+#       the scratch device and the file is in the default subvolume (tree id 5).
+check_file_extent()
+{
+	local file="$1"
+	local offset="$2"
+	local expected="$3"
+	local ino=$(stat -c "%i" "$file")
+
+	echo "=== file extent at file '$file' offset $offset ===" >> $seqres.full
+	$BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV |\
+		grep -A4 "($ino EXTENT_DATA $offset)" > $tmp.output
+	cat $tmp.output >> $seqres.full
+	grep -q "$expected" $tmp.output ||\
+		echo "file \"$file\" offset $offset doesn't have expected string" \
+		     "\"$expected\""
+}
+
+_scratch_mkfs >> $seqres.full 2>&1 || _fail "mkfs failed"
+_scratch_mount -o compress=zlib
+
+# Probe kernel support separately from the userspace check above. Kernels
+# without the flag reject it with EOPNOTSUPP when unknown-flag validation is
+# available.
+$XFS_IO_PROG -f -c "pwrite 0 10" "$SCRATCH_MNT/probe" >> $seqres.full
+$BTRFS_UTIL_PROG filesystem defragment --nocomp "$SCRATCH_MNT/probe" \
+	>> $seqres.full 2>&1 || _notrun "defrag --nocomp not supported"
+
+# Default xfs_io pattern is highly compressible, so every extent of this file
+# ends up compressed and at the 128K maximum compressed extent size.
+$XFS_IO_PROG -f -c "pwrite 0 1m" "$SCRATCH_MNT/foo" >> $seqres.full
+$BTRFS_UTIL_PROG filesystem sync $SCRATCH_MNT
+
+# Should be zlib compressed
+check_file_extent "$SCRATCH_MNT/foo" 0 "compression 1"
+
+csum_before=$(_md5_checksum "$SCRATCH_MNT/foo")
+
+$BTRFS_UTIL_PROG filesystem defragment --nocomp "$SCRATCH_MNT/foo" \
+	>> $seqres.full 2>&1
+
+# Need to commit the transaction or dump-tree won't grab the new
+# metadata on-disk.
+$BTRFS_UTIL_PROG filesystem sync $SCRATCH_MNT
+
+# Should no longer be compressed, despite the compress mount option
+check_file_extent "$SCRATCH_MNT/foo" 0 "compression 0"
+
+# No extent of this inode may remain compressed
+ino=$(stat -c "%i" "$SCRATCH_MNT/foo")
+$BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV |
+	grep -A4 "($ino EXTENT_DATA " | grep -q "compression [^0]" &&
+	echo "file still has compressed extents after --nocomp defrag"
+
+# Contents must survive the rewrite, read back from disk and not page cache
+_scratch_cycle_mount "compress=zlib"
+csum_after=$(_md5_checksum "$SCRATCH_MNT/foo")
+[ "$csum_before" = "$csum_after" ] || echo "file content changed"
+
+echo "Silence is golden"
+
+# success, all done
+_exit 0
diff --git a/tests/btrfs/354.out b/tests/btrfs/354.out
new file mode 100644
index 00000000..8bc7ecf6
--- /dev/null
+++ b/tests/btrfs/354.out
@@ -0,0 +1,2 @@
+QA output created by 354
+Silence is golden
-- 
2.43.0