[PATCH v2] fstests: btrfs: add a test case for failed compressed inline attempt

[Qu Wenruo <wqu@suse.com>]

[BUG]
There is a long existing behavior that if a compressed inline attempt
failed, the whole inode is marked incompressible.

Furthermore, the default threshold for compressed inline implies a
pretty high compression ratio (2K max inline vs 4K block size, thus 50%
compression ratio).

This makes btrfs to mark inodes incompressible prematurely.

[TEST]
The new test case is based on the previous btrfs/343 one, which will do:

- Create and mount the fs with compress,max_inline=4
  4 bytes will be too small for any header, thus all compressed inline
  writes should fail.

- Do the following operations:
  * Buffered write [0, blocksize / 2)
    Which can be inlined if using the default max_inline mount option
    and 4K fs block.
    But the new max_inline=4 will reject such compressed inline attempt.

    And such one block write should not trigger compression thus no
    compression ratio based check.

  * Sync
    For unpatched kernels, this will set the inode with NOCOMPRESS flag
    and reject all future compression on that inode.

  * Buffered write [1M, 2M)
    For unpatched kernels, the range will not be compressed due to the
    NOCOMPRESS flag.

- Unmount the fs

- Maker sure that:
  * The inode has no NOCOMPRESS flag
  * File extent at file offset 1M is being compressed

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Minor grammar error fixes
---
 tests/btrfs/344     | 53 +++++++++++++++++++++++++++++++++++++++++++++
 tests/btrfs/344.out |  2 ++
 2 files changed, 55 insertions(+)
 create mode 100755 tests/btrfs/344
 create mode 100644 tests/btrfs/344.out

diff --git a/tests/btrfs/344 b/tests/btrfs/344
new file mode 100755
index 00000000..4695104f
--- /dev/null
+++ b/tests/btrfs/344
@@ -0,0 +1,53 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 SUSE S.A.  All Rights Reserved.
+#
+# FS QA Test 344
+#
+# Check if a failed inline attempt for compression write will mark
+# the whole inode as incompressible
+#
+. ./common/preamble
+_begin_fstest auto quick compress
+
+_require_scratch
+_require_btrfs_command inspect-internal dump-tree
+
+_fixed_by_kernel_commit xxxxxxxxxxxx \
+	"btrfs: do not mark inode incompressible after inline attempt failed"
+
+_scratch_mkfs >>$seqres.full 2>&1
+
+# This inline limit is too small for any header.
+# Thus all compressed inline attempt should fail.
+_scratch_mount "-o compress,max_inline=4"
+
+blocksize=$(_get_file_block_size $SCRATCH_MNT)
+
+# The initial half block write can be compressed, but the compressed
+# size will not meet the 4 bytes limits, thus it will not be inlined.
+#
+# Normally such single block write itself should not mark the inode
+# incompressible, no matter if there is an inline attempt or not.
+#
+# The 1M write should be compressed.
+$XFS_IO_PROG -f -c "pwrite 0 $(( $blocksize / 2 ))" -c sync \
+		-c "pwrite 1m 1m" $SCRATCH_MNT/foobar >> $seqres.full
+ino=$(stat -c "%i" $SCRATCH_MNT/foobar)
+_scratch_unmount
+
+# Dump the fs tree into seqres.full for debug.
+$BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV >> $seqres.full
+
+# Check the NOCOMPRESS flag of the inode.
+$BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV |\
+grep -A 4 -e "item .* key ($ino INODE_ITEM 0)" | grep -q NOCOMPRESS
+[ $? -eq 0 ] && echo "inode $ino has NOCOMPRESS flag"
+
+# Check the file extent at file offset 1m.
+$BTRFS_UTIL_PROG inspect-internal dump-tree -t 5 $SCRATCH_DEV |\
+grep -A 4 -e "item .* key ($ino EXTENT_DATA 1048576)" | grep -q "compression 0"
+[ $? -eq 0 ] && echo "inode $ino file offset 1M is not compressed"
+
+echo "Silence is golden"
+_exit 0
diff --git a/tests/btrfs/344.out b/tests/btrfs/344.out
new file mode 100644
index 00000000..b950d5ae
--- /dev/null
+++ b/tests/btrfs/344.out
@@ -0,0 +1,2 @@
+QA output created by 344
+Silence is golden
-- 
2.51.2