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

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