generic: test writepage cached mapping validity

XFS has a bug where page writeback can end up sending data to the
wrong location due to a stale, cached file mapping. Add a test to
trigger this problem by racing background writeback with a
truncate/rewrite of the final page of the file.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/524

@@ -0,0 +1,80 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2017-2019 Red Hat, Inc.  All Rights Reserved.
+#
+# FS QA Test No. 524
+#
+# Test XFS page writeback code for races with the cached file mapping. XFS
+# caches the file -> block mapping for a full extent once it is initially looked
+# up. The cached mapping is used for all subsequent pages in the same writeback
+# cycle that cover the associated extent. Under certain conditions, it is
+# possible for concurrent operations on the file to invalidate the cached
+# mapping without the knowledge of writeback. Writeback ends up sending I/O to a
+# partly stale mapping and potentially leaving delalloc blocks in the current
+# mapping unconverted.
+
+seq=`basename $0`
+seqres=$RESULT_DIR/$seq
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+_cleanup()
+{
+	cd /
+	rm -f $tmp.*
+}
+
+# get standard environment, filters and checks
+. ./common/rc
+
+# remove previous $seqres.full before test
+rm -f $seqres.full
+
+# real QA test starts here
+
+# Modify as appropriate.
+_supported_fs generic
+_supported_os Linux
+_require_scratch
+_require_test_program "feature"
+_require_xfs_io_command "sync_range"
+
+_scratch_mkfs >> $seqres.full 2>&1 || _fail "mkfs failed"
+_scratch_mount
+
+file=$SCRATCH_MNT/file
+filesize=$((1024 * 1024 * 32))
+pagesize=`src/feature -s`
+truncsize=$((filesize - pagesize))
+
+for i in $(seq 0 15); do
+	# Truncate the file and fsync to persist the final size on-disk. This is
+	# required so the subsequent truncate will not wait on writeback.
+	$XFS_IO_PROG -fc "truncate 0" $file
+	$XFS_IO_PROG -c "truncate $filesize" -c fsync $file
+
+	# create a small enough delalloc extent to likely be contiguous
+	$XFS_IO_PROG -c "pwrite 0 $filesize" $file >> $seqres.full 2>&1
+
+	# Start writeback and a racing truncate and rewrite of the final page.
+	$XFS_IO_PROG -c "sync_range -w 0 0" $file &
+	sync_pid=$!
+	$XFS_IO_PROG -c "truncate $truncsize" \
+		     -c "pwrite $truncsize $pagesize" $file >> $seqres.full 2>&1
+
+	# If the test fails, the most likely outcome is an sb_fdblocks mismatch
+	# and/or an associated delalloc assert failure on inode reclaim. Cycle
+	# the mount to trigger detection.
+	wait $sync_pid
+	_scratch_cycle_mount
+done
+
+echo Silence is golden
+
+# success, all done
+status=0
+exit

tests/generic/524.out

@@ -0,0 +1,2 @@
+QA output created by 524
+Silence is golden

tests/generic/group

@@ -526,3 +526,4 @@
 521 soak long_rw
 522 soak long_rw
 523 auto quick attr
+524 auto quick