[RFC PATCH] xfs: regression test for writes racing with reclaim writeback

From: "Darrick J. Wong" <djwong@kernel.org>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	fstests <fstests@vger.kernel.org>
Subject: [RFC PATCH] xfs: regression test for writes racing with reclaim writeback
Date: Tue, 29 Nov 2022 13:55:47 -0800	[thread overview]
Message-ID: <Y4Z/45QkWgn23oQw@magnolia> (raw)
In-Reply-To: <Y4VeuqfVBU4/x9aB@magnolia>

From: Darrick J. Wong <djwong@kernel.org>

This test uses the new write delay debug knobs to set up a slow-moving
write racing with writeback of an unwritten block at the end of the
file range targetted by the slow write.  The test succeeds if the file
does not end up corrupt and if the ftrace log captures a message about
the revalidation occurring.

NOTE: I'm not convinced that madvise actually causes the page to be
removed from the pagecache, which means that this is effectively a
functional test for the invalidation, not a corruption reproducer.
On the other hand, the functional test can be done quickly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
This test has been adapted to use the tracepoints defined in the final
version of this patch.
---
 tests/xfs/925     |  127 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/xfs/925.out |    2 +
 2 files changed, 129 insertions(+)
 create mode 100755 tests/xfs/925
 create mode 100644 tests/xfs/925.out

diff --git a/tests/xfs/925 b/tests/xfs/925
new file mode 100755
index 0000000000..6503377c0f
--- /dev/null
+++ b/tests/xfs/925
@@ -0,0 +1,127 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2022 Oracle.  All Rights Reserved.
+#
+# FS QA Test 925
+#
+# This is a regression test for a data corruption bug that existed in iomap's
+# buffered write routines.
+#
+. ./common/preamble
+_begin_fstest auto quick rw
+
+# Import common functions.
+. ./common/inject
+. ./common/tracing
+
+# real QA test starts here
+_cleanup()
+{
+	_ftrace_cleanup
+	cd /
+	rm -r -f $tmp.* $sentryfile $tracefile
+}
+
+# Modify as appropriate.
+_supported_fs xfs
+_require_ftrace
+_require_error_injection
+_require_scratch
+
+_scratch_mkfs >> $seqres.full
+_scratch_mount >> $seqres.full
+
+# This is a pagecache test, so try to disable fsdax mode.
+$XFS_IO_PROG -c 'chattr -x' $SCRATCH_MNT &> $seqres.full
+_require_pagecache_access $SCRATCH_MNT
+
+knob="$(_find_xfs_mountdev_errortag_knob $SCRATCH_DEV "write_delay_ms")"
+test -w "$knob" || _notrun "Kernel does not have write_delay_ms error injector"
+
+blocks=10
+blksz=$(get_page_size)
+filesz=$((blocks * blksz))
+dirty_offset=$(( filesz - 1 ))
+write_len=$(( ( (blocks - 1) * blksz) + 1 ))
+
+# Create a large file with a large unwritten range.
+$XFS_IO_PROG -f -c "falloc 0 $filesz" $SCRATCH_MNT/file >> $seqres.full
+
+# Write the same data to file.compare as we're about to do to file.  Do this
+# before slowing down writeback to avoid unnecessary delay.
+_pwrite_byte 0x58 $dirty_offset 1 $SCRATCH_MNT/file.compare >> $seqres.full
+_pwrite_byte 0x57 0 $write_len $SCRATCH_MNT/file.compare >> $seqres.full
+
+# Reinitialize the page cache for this file.
+_scratch_cycle_mount
+
+# Dirty the last page in the range and immediately set the write delay so that
+# any subsequent writes have to wait.
+$XFS_IO_PROG -c "pwrite -S 0x58 $dirty_offset 1" $SCRATCH_MNT/file >> $seqres.full
+write_delay=500
+echo $write_delay > $knob
+curval="$(cat $knob)"
+test "$curval" -eq $write_delay || echo "expected write_delay_ms == $write_delay"
+
+_ftrace_record_events 'xfs_iomap_invalid'
+
+# Start a sentry to look for evidence of the XFS_ERRORTAG_REPORT logging.  If
+# we see that, we know we've forced writeback to revalidate a mapping.  The
+# test has been successful, so turn off the delay.
+sentryfile=$TEST_DIR/$seq.sentry
+tracefile=$TEST_DIR/$seq.ftrace
+wait_for_errortag() {
+	while [ -e "$sentryfile" ]; do
+		_ftrace_dump | grep iomap_invalid >> "$tracefile"
+		if grep -q iomap_invalid "$tracefile"; then
+			echo 0 > $knob
+			_ftrace_ignore_events
+			break;
+		fi
+		sleep 0.5
+	done
+}
+touch $sentryfile
+wait_for_errortag &
+
+# Start thread 1 + writeback above
+($XFS_IO_PROG -c "pwrite -S 0x57 -b $write_len 0 $write_len" \
+	-c 'bmap -celpv' -c 'bmap -elpv' \
+	$SCRATCH_MNT/file >> $seqres.full; rm -f $sentryfile) &
+sleep 1
+
+# Start thread 2 to simulate reclaim writeback via sync_file_range and fadvise
+# to drop the page cache.
+#	-c "fadvise -d $dirty_offset 1" \
+dirty_pageoff=$((filesz - blksz))
+$XFS_IO_PROG -c "sync_range -a -w $dirty_pageoff $blksz" \
+	-c "mmap -r 0 $filesz" \
+	-c "madvise -d 0 $filesz" \
+	-c 'bmap -celpv' -c 'bmap -elpv' \
+	$SCRATCH_MNT/file >> $seqres.full
+wait
+rm -f $sentryfile
+
+cat "$tracefile" >> $seqres.full
+grep -q iomap_invalid "$tracefile"
+saw_invalidation=$?
+
+# Flush everything to disk.  If the bug manifests, then after the cycle,
+# file should have stale 0x58 in block 0 because we silently dropped a write.
+_scratch_cycle_mount
+
+if ! cmp -s $SCRATCH_MNT/file $SCRATCH_MNT/file.compare; then
+	echo file and file.compare do not match
+	$XFS_IO_PROG -c 'bmap -celpv' -c 'bmap -elpv' $SCRATCH_MNT/file >> $seqres.full
+	echo file.compare
+	od -tx1 -Ad -c $SCRATCH_MNT/file.compare
+	echo file
+	od -tx1 -Ad -c $SCRATCH_MNT/file
+elif [ $saw_invalidation -ne 0 ]; then
+	# The files matched, but nothing got logged about the revalidation?
+	echo "Expected to hear about write iomap invalidation?"
+fi
+
+echo Silence is golden
+status=0
+exit
diff --git a/tests/xfs/925.out b/tests/xfs/925.out
new file mode 100644
index 0000000000..95088ce8a5
--- /dev/null
+++ b/tests/xfs/925.out
@@ -0,0 +1,2 @@
+QA output created by 925
+Silence is golden
