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

Brian Foster
2017-10-26T14:48:17Z

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>
---

Here's a new version of the writepages test I previously posted as RFC.
This variant does not require an artificial delay to reproduce, so I've
dropped the need for the error injection tag.

I have been playing a bit with the file size and iteration count of the
test. I started with something that ran a decent bit longer (~2m) as was
necessary to reproduce on my dev/debug vm, but recently trimmed the file
size and iteration count to something that runs much quicker (~10s) and
reproduces nearly 100% of the time on my actual test hardware. The
tradeoff is the reproducibility is much lower on my debug vm (~20-25%
perhaps). The test still does reproduce when run over 10-15 iters, so I
opted for the quicker test.

In all, I am a bit curious about whether this reproduces reliably on
others' test setups. If not, does tweaking the size/iterations improve
the reproducibility?

Brian

v1:
- New test algorithm that does not require artificial delay.
- Created as generic test.
rfc: https://marc.info/?l=linux-xfs&m=150886719725497&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 0000000..9e56a1e
--- /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 0000000..3b276ca
--- /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 fbe0a7f..89342da 100644
--- a/tests/generic/group
+++ b/tests/generic/group
@@ -468,3 +468,4 @@
463 auto quick clone dangerous
464 auto rw
465 auto rw quick aio
+999 auto quick
--
2.9.5