[PATCH] tests/generic: test writepage cached mapping validity

Brian Foster
2019-01-11T13:31:26Z

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>
---

Hi all,

This is a resend of an old post[1] that never quite made it upstream. It
wasn't a big deal at the time because we didn't really have a proper fix
for the problem. I'm resending now because there is a proposed fix[2].

I've verified that this still reproduces the problem and no longer fails
with the fix applied (in hundreds of iters). Note that reproduction may
require many iterations. It took me anywhere from 5 to 30 or so on the
box I tested, which I think is reasonable for the tradeoff of a fairly
quick test. There was some discussion on the original post around making
the test run longer for a more reliable reproducer, but I'm not sure how
valuable that is given this is a targeted regression test. Thoughts
appreciated.

Brian

[1] https://marc.info/?l=fstests&m=150902929900510&w=2
[2] https://marc.info/?l=linux-xfs&m=154721212321112&w=2

tests/generic/999 | 94 +++++++++++++++++++++++++++++++++++++++++++
tests/generic/999.out | 2 +
tests/generic/group | 1 +
3 files changed, 97 insertions(+)
create mode 100755 tests/generic/999
create mode 100644 tests/generic/999.out

diff --git a/tests/generic/999 b/tests/generic/999
new file mode 100755
index 00000000..9e56a1e0
--- /dev/null
+++ b/tests/generic/999
@@ -0,0 +1,94 @@
+#! /bin/bash
+# FS QA Test 999
+#
+# 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.
+#
+#-----------------------------------------------------------------------
+# Copyright (c) 2017 Red Hat, Inc. All Rights Reserved.
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+#-----------------------------------------------------------------------
+#
+
+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"
+
+_scratch_mkfs >> $seqres.full 2>&1 || _fail "mkfs failed"
+_scratch_mount || _fail "mount failed"
+
+file=$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 || _fail "mount failed"
+done
+
+echo Silence is golden
+
+# success, all done
+status=0
+exit
diff --git a/tests/generic/999.out b/tests/generic/999.out
new file mode 100644
index 00000000..3b276ca8
--- /dev/null
+++ b/tests/generic/999.out
@@ -0,0 +1,2 @@
+QA output created by 999
+Silence is golden
diff --git a/tests/generic/group b/tests/generic/group
index ea5aa7aa..ce165981 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -525,3 +525,4 @@
520 auto quick log
521 soak long_rw
522 soak long_rw
+999 auto quick
--
2.17.2