[PATCH v2] fstests: btrfs: verify the read behavior of compressed inline extent

Qu Wenruo
2024-01-27T20:44:43Z

[BUG]
There is a report about reading a zstd compressed inline file extent
would lead to either a VM_BUG_ON() crash, or lead to incorrect file
content.

[CAUSE]
The root cause is a incorrect memcpy_to_page() call, which uses
incorrect page offset, and can lead to either the VM_BUG_ON() as we may
write beyond the page boundary, or writes into the incorrect offset of
the page.

[TEST CASE]
The test case would:

- Mount with the specified compress algorithm
- Create a 4K file
- Verify the 4K file is all inlined and compressed
- Verify the content of the initial write
- Cycle mount to drop all the page cache
- Verify the content of the file again
- Unmount and fsck the fs

This workload would be applied to all supported compression algorithms.
And it can catch the problem correctly by triggering VM_BUG_ON(), as our
workload would result decompressed extent size to be 4K, and would
trigger the VM_BUG_ON() 100%.
And with the revert or the new fix, the test case can pass safely.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
tests/btrfs/310 | 83 +++++++++++++++++++++++++++++++++++++++++++++
tests/btrfs/310.out | 2 ++
2 files changed, 85 insertions(+)
create mode 100755 tests/btrfs/310
create mode 100644 tests/btrfs/310.out
---
Changelog:
v2:
- Add a comment on why a "sync" is needed
- Update the failure case comment
The specific design of the inline extent size is ensured to trigger
VM_BUG_ON(), thus remove the data corruption case.

diff --git a/tests/btrfs/310 b/tests/btrfs/310
new file mode 100755
index 00000000..507485a4
--- /dev/null
+++ b/tests/btrfs/310
@@ -0,0 +1,83 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2024 SUSE Linux Products GmbH. All Rights Reserved.
+#
+# FS QA Test 310
+#
+# Make sure reading on an compressed inline extent is behaving correctly
+#
+. ./common/preamble
+_begin_fstest auto quick compress
+
+# Import common functions.
+# . ./common/filter
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs btrfs
+_require_scratch
+
+# This test require inlined compressed extents creation, and all the writes
+# are designed for 4K sector size.
+_require_btrfs_inline_extents_creation
+_require_btrfs_support_sectorsize 4096
+
+_fixed_by_kernel_commit e01a83e12604 \
+ "Revert \"btrfs: zstd: fix and simplify the inline extent decompression\""
+
+# The correct md5 for the correct 4K file filled with "0xcd"
+md5sum_correct="5fed275e7617a806f94c173746a2a723"
+
+workload()
+{
+ local algo="$1"
+
+ echo "=== Testing compression algorithm ${algo} ===" >> $seqres.full
+ _scratch_mkfs >> $seqres.full
+ _scratch_mount -o compress=${algo}
+
+ _pwrite_byte 0xcd 0 4k "$SCRATCH_MNT/inline_file" > /dev/null
+ result=$(_md5_checksum "$SCRATCH_MNT/inline_file")
+ echo "after initial write, md5sum=${result}" >> $seqres.full
+ if [ "$result" != "$md5sum_correct" ]; then
+ _fail "initial write results incorrect content for \"$algo\""
+ fi
+ # Writeback data to get correct fiemap result, or we got FIEMAP_DEALLOC
+ # without compression/inline flags.
+ sync
+
+ $XFS_IO_PROG -c "fiemap -v" $SCRATCH_MNT/inline_file | tail -n 1 > $tmp.fiemap
+ cat $tmp.fiemap >> $seqres.full
+ # Make sure we got an inlined compressed file extent.
+ # 0x200 means inlined, 0x100 means not block aligned, 0x8 means encoded
+ # (compressed in this case), and 0x1 means the last extent.
+ if ! grep -q "0x309" $tmp.fiemap; then
+ rm -f -- $tmp.fiemap
+ _notrun "No compressed inline extent created, maybe subpage?"
+ fi
+ rm -f -- $tmp.fiemap
+
+ # Unmount to clear the page cache.
+ _scratch_cycle_mount
+
+ # For v6.8-rc1 without the revert or the newer fix, this would
+ # lead to VM_BUG_ON() thus crash
+ result=$(_md5_checksum "$SCRATCH_MNT/inline_file")
+ echo "after cycle mount, md5sum=${result}" >> $seqres.full
+ if [ "$result" != "$md5sum_correct" ]; then
+ _fail "read for compressed inline extent failed for \"$algo\""
+ fi
+ _scratch_unmount
+ _check_scratch_fs
+}
+
+algo_list=($(_btrfs_compression_algos))
+for algo in ${algo_list[@]}; do
+ workload $algo
+done
+
+echo "Silence is golden"
+
+status=0
+exit
diff --git a/tests/btrfs/310.out b/tests/btrfs/310.out
new file mode 100644
index 00000000..7b9eaf78
--- /dev/null
+++ b/tests/btrfs/310.out
@@ -0,0 +1,2 @@
+QA output created by 310
+Silence is golden
--
2.42.0