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

Brian Foster
2019-01-14T15:34:37Z

On Mon, Jan 14, 2019 at 05:30:36PM +0800, Eryu Guan wrote:
> On Fri, Jan 11, 2019 at 08:31:24AM -0500, Brian Foster wrote:
> > 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].
>
> Thanks for the resending!
>
> >
> > 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.
>
> It took me around 5 iterations to hit the corruption, I think it's fine.
>
> But a couple of things changed over the years :)
>

Indeed, these changes all sound good. I'll include them in v2, thanks!

Brian

> >
> > 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.
> ^^^^ 2019?
> > +#
> > +# 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
> > +#-----------------------------------------------------------------------
>
> And please change this to SPDX-License-Identifier.
>
> > +#
> > +
> > +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 >> $seqres.full 2>&1 || _fail "mkfs failed"
> > +_scratch_mount || _fail "mount failed"
>
> _scratch_mount will _fail the test on failure now :)
>
> > +
> > +file=$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"
>
> And _scratch_cycle_mount will exit the test on failure as well.
>
> Thanks,
> Eryu
>
> > +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
> >