fstest: regression test for ext4 crash consistency bug

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein <[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
1 write 0x137dd thru 0x21445 (0xdc69 bytes)
2 falloc from 0xb531 to 0x16ade (0xb5ad bytes)
3 collapse from 0x1c000 to 0x20000, (0x4000 bytes)
4 write 0x3e5ec thru 0x3ffff (0x1a14 bytes)
5 zero from 0x20fac to 0x27d48, (0x6d9c bytes)
6 mapwrite 0x216ad thru 0x23dfb (0x274f bytes)
All 7 operations completed A-OK!
_check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is inconsistent *** fsck.ext4 output ***
fsck from util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 12, end of extent exceeds allowed value

          (logical block 33, physical block 33441, len 7)

Clear? no
Inode 12, i_blocks is 184, should be 128. Fix? no

Note that the inconsistency is "applied" by journal replay during mount. fsck -nf before mount does not report any errors.

I did not intend for this test to be merged as is, but rather to be used by ext4 developers to analyze the problem and then re-write the test with more comments and less arbitrary offset/length values.

P.S.: crash consistency tests also reliably reproduce a btrfs fsck error.
a detailed report with I/O recording was sent to Josef.
P.S.2: crash consistency tests report file data checksum errors on xfs
after fsync+crash, but I still need to prove the reliability of these reports.

[1] https://github.com/amir73il/xfstests/commits/dm-log-writes

 tests/generic/501     | 80 +++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/generic/501.out |  2 ++
 tests/generic/group   |  1 +
 3 files changed, 83 insertions(+)
 create mode 100755 tests/generic/501
 create mode 100644 tests/generic/501.out

tests/generic/501 new file +80 -0

new file mode 100755
index 0000000..ccb513d
@@ -0,0 +1,80 @@
+#! /bin/bash
+# FS QA Test No. 501
+#
+# This test is motivated by a bug found in ext4 during random crash
+# consistency tests.
+#
+#-----------------------------------------------------------------------
+# Copyright (C) 2017 CTERA Networks. All Rights Reserved.
+# Author: Amir Goldstein <[email protected]>
+#
+# 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!
+
+_cleanup()
+{
+	_cleanup_flakey
+	cd /
+	rm -f $tmp.*
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/filter
+. ./common/dmflakey
+
+# real QA test starts here
+_supported_fs generic
+_supported_os Linux
+_require_scratch
+_require_dm_target flakey
+_require_metadata_journaling $SCRATCH_DEV
+
+rm -f $seqres.full
+
+_scratch_mkfs >> $seqres.full 2>&1
+
+_init_flakey
+_mount_flakey
+
+fsxops=$tmp.fsxops
+cat <<EOF > $fsxops
+write 0x137dd 0xdc69 0x0
+fallocate 0xb531 0xb5ad 0x21446
+collapse_range 0x1c000 0x4000 0x21446
+write 0x3e5ec 0x1a14 0x21446
+zero_range 0x20fac 0x6d9c 0x40000 keep_size
+mapwrite 0x216ad 0x274f 0x40000
+EOF
+run_check $here/ltp/fsx -d --replay-ops $fsxops $SCRATCH_MNT/testfile
+
+_flakey_drop_and_remount
+_unmount_flakey
+_cleanup_flakey
+_check_scratch_fs
+
+echo "Silence is golden"
+
+status=0
+exit

tests/generic/501.out new file +2 -0

new file mode 100644
index 0000000..00133b6
@@ -0,0 +1,2 @@
+QA output created by 501
+Silence is golden

tests/generic/group +1 -0

index 2396b72..bb870f2 100644
@@ -454,3 +454,4 @@
 449 auto quick acl enospc
 450 auto quick rw
 500 auto log replay
+501 auto quick metadata

Quoted 39 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein <[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
1 write 0x137dd thru 0x21445 (0xdc69 bytes)
2 falloc from 0xb531 to 0x16ade (0xb5ad bytes)
3 collapse from 0x1c000 to 0x20000, (0x4000 bytes)
4 write 0x3e5ec thru 0x3ffff (0x1a14 bytes)
5 zero from 0x20fac to 0x27d48, (0x6d9c bytes)
6 mapwrite 0x216ad thru 0x23dfb (0x274f bytes)
All 7 operations completed A-OK!
_check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is inconsistent *** fsck.ext4 output ***
fsck from util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 12, end of extent exceeds allowed value

Quoted 3 lines

          (logical block 33, physical block 33441, len 7)

Clear? no
Inode 12, i_blocks is 184, should be 128. Fix? no

Hi Amir,

I always get the following output when running your xfstests test case 501.

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no
---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

Quoted 43 lines · Xiao Yang

Quoted 34 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein<[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

   ./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
   1 write 0x137dd thru    0x21445 (0xdc69 bytes)
   2 falloc        from 0xb531 to 0x16ade (0xb5ad bytes)
   3 collapse      from 0x1c000 to 0x20000, (0x4000 bytes)
   4 write 0x3e5ec thru    0x3ffff (0x1a14 bytes)
   5 zero  from 0x20fac to 0x27d48, (0x6d9c bytes)
   6 mapwrite      0x216ad thru    0x23dfb (0x274f bytes)
   All 7 operations completed A-OK!
   _check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is inconsistent

Quoted 8 lines

   *** fsck.ext4 output ***
   fsck from util-linux 2.27.1
   e2fsck 1.42.13 (17-May-2015)
   Pass 1: Checking inodes, blocks, and sizes
   Inode 12, end of extent exceeds allowed value
           (logical block 33, physical block 33441, len 7)
   Clear? no
   Inode 12, i_blocks is 184, should be 128.  Fix? no

Quoted 3 lines

Hi Amir,

I always get the following output when running your xfstests test case 501.

Now merged as test generic/456

Quoted 7 lines

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no
---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

I can't say I am doing anything special, but I can say that I get the same output as you did when running the test inside kvm-xfstests. Actually, I could not reproduce ANY of the the crash consistency bugs inside kvm-xfstests. Must be something to do with different timing of IO with KVM+virtio disks??

When running on my laptop (Ubuntu 16.04 with latest kernel) on a 10G SSD volume, I always get the error reported above. I just re-verified with latest stable e2fsprogs (1.43.6).

Amir.
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To unsubscribe from this list: send the line "unsubscribe fstests" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html

.

Quoted 74 lines · Amir Goldstein

Quoted 48 lines · Xiao Yang

Quoted 42 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein<[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops
on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

   ./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
   1 write 0x137dd thru    0x21445 (0xdc69 bytes)
   2 falloc        from 0xb531 to 0x16ade (0xb5ad bytes)
   3 collapse      from 0x1c000 to 0x20000, (0x4000 bytes)
   4 write 0x3e5ec thru    0x3ffff (0x1a14 bytes)
   5 zero  from 0x20fac to 0x27d48, (0x6d9c bytes)
   6 mapwrite      0x216ad thru    0x23dfb (0x274f bytes)
   All 7 operations completed A-OK!
   _check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is

inconsistent
*** fsck.ext4 output ***
fsck from util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 12, end of extent exceeds allowed value

Quoted 3 lines

           (logical block 33, physical block 33441, len 7)

Clear? no
Inode 12, i_blocks is 184, should be 128. Fix? no

Quoted 4 lines

Hi Amir,

I always get the following output when running your xfstests test case 501.

Now merged as test generic/456

Quoted 9 lines

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no

---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

I can't say I am doing anything special, but I can say that I get the same output as you did when running the test inside kvm-xfstests. Actually, I could not reproduce ANY of the the crash consistency bugs inside kvm-xfstests. Must be something to do with different timing of IO with KVM+virtio disks??

Quoted 3 lines

When running on my laptop (Ubuntu 16.04 with latest kernel) on a 10G SSD volume, I always get the error reported above. I just re-verified with latest stable e2fsprogs (1.43.6).

Hi Amir,

I tested generic/456 with KVM+virtio disks and SATA volumes on some kernels

(including
v3.10.0, the latest kernel), but i still got the same output as i reported.

Could you determine whether the two different outputs are caused by the same bug
or not ?

Quoted 43 lines · Xiao Yang

Quoted 39 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein <[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
1 write 0x137dd thru 0x21445 (0xdc69 bytes)
2 falloc from 0xb531 to 0x16ade (0xb5ad bytes)
3 collapse from 0x1c000 to 0x20000, (0x4000 bytes)
4 write 0x3e5ec thru 0x3ffff (0x1a14 bytes)
5 zero from 0x20fac to 0x27d48, (0x6d9c bytes)
6 mapwrite 0x216ad thru 0x23dfb (0x274f bytes)
All 7 operations completed A-OK!
_check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is inconsistent *** fsck.ext4 output ***
fsck from util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 12, end of extent exceeds allowed value

Quoted 3 lines

          (logical block 33, physical block 33441, len 7)

Clear? no
Inode 12, i_blocks is 184, should be 128. Fix? no

Quoted 3 lines

Hi Amir,

I always get the following output when running your xfstests test case 501.

Now merged as test generic/456

Quoted 7 lines

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no
---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

I can't say I am doing anything special, but I can say that I get the same output as you did when running the test inside kvm-xfstests. Actually, I could not reproduce ANY of the the crash consistency bugs inside kvm-xfstests. Must be something to do with different timing of IO with KVM+virtio disks??

When running on my laptop (Ubuntu 16.04 with latest kernel) on a 10G SSD volume, I always get the error reported above. I just re-verified with latest stable e2fsprogs (1.43.6).

Amir.

Hi Xiao,

I am a student at the University of Texas at Austin. Some researchers in the computer science department at UT, myself included, have recently been working to develop a file system crash consistency test harness called CrashMonkey [1][2]. I have been working on the CrashMonkey project since it was started late last year. With CrashMonkey we have also been able to reproduce the incorrect i_size error you noted but we have not been able to reproduce the other output that Amir found. CrashMonkey works by logging and replaying operations for a workload, so it should not be sensitive to differences in timing that could be caused by things like KVM+virtio. I also did a few experiments with Amir's new xfstests test 456 (both with and without KVM and virtio) and I was unable to reproduce the output noted in the xfstest. I have not spent a lot of time looking into the cause of the bug that Amir found and it is rather unfortunate that I was unable to reproduce it with either xfstests or CrashMonkey.

At any rate, CrashMonkey is still under development, so it does have some caveats. First, we are running with a fixed random seed in our default RandomPermuter (used to generate crash states) to aid development. Second, the branch with the reproduction of this ext4 regression bug in CrashMonkey [3] will yield a few false positives due to the way CrashMonkey works and how fsx runs. These false positives are due to CrashMonkey generating crash states where the directories for files used for the test have not be fsync-ed in the file system. The top of the README in the CrashMonkey branch with this bug reproduction outlines how we determined these were false positives

[1] https://github.com/utsaslab/crashmonkey
[2] https://www.usenix.org/conference/hotstorage17/program/presentation/martinez
[3] https://github.com/utsaslab/crashmonkey/tree/ext4_regression_bug

Quoted 62 lines · Amir Goldstein

Quoted 43 lines · Xiao Yang

Quoted 34 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein<[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

   ./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
   1 write 0x137dd thru    0x21445 (0xdc69 bytes)
   2 falloc        from 0xb531 to 0x16ade (0xb5ad bytes)
   3 collapse      from 0x1c000 to 0x20000, (0x4000 bytes)
   4 write 0x3e5ec thru    0x3ffff (0x1a14 bytes)
   5 zero  from 0x20fac to 0x27d48, (0x6d9c bytes)
   6 mapwrite      0x216ad thru    0x23dfb (0x274f bytes)
   All 7 operations completed A-OK!
   _check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is inconsistent

Quoted 8 lines

   *** fsck.ext4 output ***
   fsck from util-linux 2.27.1
   e2fsck 1.42.13 (17-May-2015)
   Pass 1: Checking inodes, blocks, and sizes
   Inode 12, end of extent exceeds allowed value
           (logical block 33, physical block 33441, len 7)
   Clear? no
   Inode 12, i_blocks is 184, should be 128.  Fix? no

Quoted 3 lines

Hi Amir,

I always get the following output when running your xfstests test case 501.

Now merged as test generic/456

Quoted 7 lines

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no
---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

I can't say I am doing anything special, but I can say that I get the same output as you did when running the test inside kvm-xfstests. Actually, I could not reproduce ANY of the the crash consistency bugs inside kvm-xfstests. Must be something to do with different timing of IO with KVM+virtio disks??

Quoted 5 lines

When running on my laptop (Ubuntu 16.04 with latest kernel) on a 10G SSD volume, I always get the error reported above. I just re-verified with latest stable e2fsprogs (1.43.6).

Amir.

.

Quoted 18 lines · Ashlie Martinez

Hi Xiao,

I am a student at the University of Texas at Austin. Some researchers in the computer science department at UT, myself included, have recently been working to develop a file system crash consistency test harness called CrashMonkey [1][2]. I have been working on the CrashMonkey project since it was started late last year. With CrashMonkey we have also been able to reproduce the incorrect i_size error you noted but we have not been able to reproduce the other output that Amir found. CrashMonkey works by logging and replaying operations for a workload, so it should not be sensitive to differences in timing that could be caused by things like KVM+virtio. I also did a few experiments with Amir's new xfstests test 456 (both with and without KVM and virtio) and I was unable to reproduce the output noted in the xfstest. I have not spent a lot of time looking into the cause of the bug that Amir found and it is rather unfortunate that I was unable to reproduce it with either xfstests or CrashMonkey.

Hi Ashlie,

Thanks for your detailed comments.

1) Do you think the output that Amir noted in xfstests is a false positive?

2) About the output that both i and you reproduced, did you look into it and find its root cause?

Thanks,
Xiao Yang

Quoted 103 lines

At any rate, CrashMonkey is still under development, so it does have some caveats. First, we are running with a fixed random seed in our default RandomPermuter (used to generate crash states) to aid development. Second, the branch with the reproduction of this ext4 regression bug in CrashMonkey [3] will yield a few false positives due to the way CrashMonkey works and how fsx runs. These false positives are due to CrashMonkey generating crash states where the directories for files used for the test have not be fsync-ed in the file system. The top of the README in the CrashMonkey branch with this bug reproduction outlines how we determined these were false positives

[1] https://github.com/utsaslab/crashmonkey
[2]
https://www.usenix.org/conference/hotstorage17/program/presentation/martinez
[3] https://github.com/utsaslab/crashmonkey/tree/ext4_regression_bug

Quoted 74 lines · Amir Goldstein

Quoted 48 lines · Xiao Yang

Quoted 42 lines · Amir Goldstein

This test is motivated by a bug found in ext4 during random crash consistency tests.

This test uses device mapper flakey target to demonstrate the bug found using device mapper log-writes target.

Signed-off-by Amir Goldstein<[email protected]>

---

Ted,

While working on crash consistency xfstests [1], I stubmled on what appeared to be an ext4 crash consistency bug.

The tests I used rely on the log-writes dm target code written by Josef Bacik, which had little exposure to the wide community as far as I know. I wanted to prove to myself that the found inconsistency was not due to a test bug, so I bisected the failed test to the minimal operations that trigger the failure and wrote a small independent test to reproduce the issue using dm flakey target.

The following fsck error is reliably reproduced by replaying some fsx ops
on overlapping file regions, then emulating a crash, followed by mount, umount and fsck -nf:

   ./ltp/fsx -d --replay-ops /tmp/8995.fsxops /mnt/scratch/testfile
   1 write 0x137dd thru    0x21445 (0xdc69 bytes)
   2 falloc        from 0xb531 to 0x16ade (0xb5ad bytes)
   3 collapse      from 0x1c000 to 0x20000, (0x4000 bytes)
   4 write 0x3e5ec thru    0x3ffff (0x1a14 bytes)
   5 zero  from 0x20fac to 0x27d48, (0x6d9c bytes)
   6 mapwrite      0x216ad thru    0x23dfb (0x274f bytes)
   All 7 operations completed A-OK!
   _check_generic_filesystem: filesystem on /dev/mapper/ssd-scratch is

inconsistent
*** fsck.ext4 output ***
fsck from util-linux 2.27.1
e2fsck 1.42.13 (17-May-2015)
Pass 1: Checking inodes, blocks, and sizes
Inode 12, end of extent exceeds allowed value

Quoted 3 lines

           (logical block 33, physical block 33441, len 7)

Clear? no
Inode 12, i_blocks is 184, should be 128. Fix? no

Quoted 4 lines

Hi Amir,

I always get the following output when running your xfstests test case 501.

Now merged as test generic/456

Quoted 9 lines

---------------------------------------------------------------------------
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes Inode 12, i_size is 147456, should be 163840. Fix? no

---------------------------------------------------------------------------

Could you tell me how to get the expected output as you reported?

I can't say I am doing anything special, but I can say that I get the same output as you did when running the test inside kvm-xfstests. Actually, I could not reproduce ANY of the the crash consistency bugs inside kvm-xfstests. Must be something to do with different timing of IO with KVM+virtio disks??

Quoted 5 lines

When running on my laptop (Ubuntu 16.04 with latest kernel) on a 10G SSD volume, I always get the error reported above. I just re-verified with latest stable e2fsprogs (1.43.6).

Amir.

.

Quoted 22 lines · Xiao Yang

Quoted 18 lines · Ashlie Martinez

Hi Xiao,

I am a student at the University of Texas at Austin. Some researchers in the computer science department at UT, myself included, have recently been working to develop a file system crash consistency test harness called CrashMonkey [1][2]. I have been working on the CrashMonkey project since it was started late last year. With CrashMonkey we have also been able to reproduce the incorrect i_size error you noted but we have not been able to reproduce the other output that Amir found. CrashMonkey works by logging and replaying operations for a workload, so it should not be sensitive to differences in timing that could be caused by things like KVM+virtio. I also did a few experiments with Amir's new xfstests test 456 (both with and without KVM and virtio) and I was unable to reproduce the output noted in the xfstest. I have not spent a lot of time looking into the cause of the bug that Amir found and it is rather unfortunate that I was unable to reproduce it with either xfstests or CrashMonkey.

Hi Ashlie,

Quoted 3 lines

Thanks for your detailed comments.

1) Do you think the output that Amir noted in xfstests is a false positive?

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

One thing I have identified as being a
potential source of false positives is code in the kernel to remap the block number sent to device drives to something relative to the start of a block device, not the start of a partition. I'm thinking this could cause trouble if 1. a partition higher than the first partition is monitored by dm-log-writes, 2. the block numbers are recorded verbatim in dm-log-writes, and 3. the dm-log-writes log is replayed on a different device with different partitions (or possibly the same device, but a different partition).

I know some other undergrad
students at UT on the CrashMonkey team are looking into this right now, but I have not had time to test this myself. The offending code in the kernel is in the completely misnamed generic_make_request_checks function which has given the CrashMonkey team trouble in the past. Links to that function and the remapping function it calls are below.

http://elixir.free-electrons.com/linux/latest/source/block/blk-core.c#L2041
http://elixir.free-electrons.com/linux/latest/source/block/blk-core.c#L1902

Quoted 1 line

Quoted 2 lines

2) About the output that both i and you reproduced, did you look into it and find its root cause?

I have not looked to find the root cause of the issue that we both reliably reproduce.

Quoted 27 lines · Ashlie Martinez

Quoted 25 lines · Xiao Yang

Quoted 18 lines · Ashlie Martinez

Hi Xiao,

I am a student at the University of Texas at Austin. Some researchers in the computer science department at UT, myself included, have recently been working to develop a file system crash consistency test harness called CrashMonkey [1][2]. I have been working on the CrashMonkey project since it was started late last year. With CrashMonkey we have also been able to reproduce the incorrect i_size error you noted but we have not been able to reproduce the other output that Amir found. CrashMonkey works by logging and replaying operations for a workload, so it should not be sensitive to differences in timing that could be caused by things like KVM+virtio. I also did a few experiments with Amir's new xfstests test 456 (both with and without KVM and virtio) and I was unable to reproduce the output noted in the xfstest. I have not spent a lot of time looking into the cause of the bug that Amir found and it is rather unfortunate that I was unable to reproduce it with either xfstests or CrashMonkey.

Hi Ashlie,

Thanks for your detailed comments.

1) Do you think the output that Amir noted in xfstests is a false positive?

Quoted 3 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

There is no use in speculating why the problem doesn't happen on your systems. If the issue reproduces on A system (2 systems actually that I tested) that it is a problem.

Attached is an e2image dump of the inconsistent file system following test generic/456 on my system. I would have attached the image sooner, but since on my system problem reproduces 100% on the time, I did not think that was a need. Without an ext4 developer looking into this image, I don't really see the point in further discussion.

I would be interesting to get more test samples from people running the test on other systems. If only some people see the error
"end of extent exceeds allowed value"
it would be interesting to find the commonality between those setups.

Quoted 9 lines

One thing I have identified as being a
potential source of false positives is code in the kernel to remap the block number sent to device drives to something relative to the start of a block device, not the start of a partition. I'm thinking this could cause trouble if 1. a partition higher than the first partition is monitored by dm-log-writes, 2. the block numbers are recorded verbatim in dm-log-writes, and 3. the dm-log-writes log is replayed on a different device with different partitions (or possibly the same device, but a different partition).

You do realize that the test generic/456 is not using dm-log-writes at all. I intentionally took it out of the equation and used the even dumber dm-flakey target to demonstrate the crash inconsistency.

Quoted 13 lines

I know some other undergrad
students at UT on the CrashMonkey team are looking into this right now, but I have not had time to test this myself. The offending code in the kernel is in the completely misnamed generic_make_request_checks function which has given the CrashMonkey team trouble in the past. Links to that function and the remapping function it calls are below.

http://elixir.free-electrons.com/linux/latest/source/block/blk-core.c#L2041
http://elixir.free-electrons.com/linux/latest/source/block/blk-core.c#L1902

Quoted 1 line

Quoted 6 lines

Quoted 2 lines

2) About the output that both i and you reproduced, did you look into it and find its root cause?

I have not looked to find the root cause of the issue that we both reliably reproduce.

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

Amir.

Quoted 8 lines

Quoted 3 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

Quoted 8 lines · Ashlie Martinez

Quoted 8 lines

Quoted 3 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

[1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o <[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a crash

If there are pending writes subject to delayed allocation, then i_size will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either

with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be updated, and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2) call is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically < 30 seconds, in practice this condition will rarely happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o <[email protected]>

Reported-by Amir Goldstein <[email protected]>

Cc [email protected]

fs/ext4/extents.c +4 -2

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file, loff_t offset,
        }
 
        if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-            offset + len > i_size_read(inode)) {
+           (offset + len > i_size_read(inode) ||
+            offset + len > EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);
                if (ret)

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
        }
 
        if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-            offset + len > i_size_read(inode)) {
+           (offset + len > i_size_read(inode) ||
+            offset + len > EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);
                if (ret)

Quoted 1 line

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either

According to patch without keep size flag?

BTW looking at the patch, does truncate racing with fallocate that would increase size beyond truncated size works correctly?

Quoted 8 lines · Ashlie Martinez

Quoted 8 lines

Quoted 3 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

[1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o <[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a crash

If there are pending writes subject to delayed allocation, then i_size will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be updated, and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2) call is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically < 30 seconds, in practice this condition will rarely happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o <[email protected]>

Reported-by Amir Goldstein <[email protected]>

Cc [email protected]

fs/ext4/extents.c +4 -2

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file, loff_t offset,
        }
 
        if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-            offset + len > i_size_read(inode)) {
+           (offset + len > i_size_read(inode) ||
+            offset + len > EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);
                if (ret)

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
        }
 
        if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-            offset + len > i_size_read(inode)) {
+           (offset + len > i_size_read(inode) ||
+            offset + len > EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);
                if (ret)

Quoted 7 lines · Ashlie Martinez

Quoted 7 lines

Quoted 3 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

[1] https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o<[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a crash

If there are pending writes subject to delayed allocation, then i_size will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be updated, and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2) call is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically< 30 seconds, in practice this condition will rarely happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o<[email protected]>

Reported-by Amir Goldstein<[email protected]>

Cc [email protected]

fs/ext4/extents.c +4 -3

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file, loff_t offset,
  	}
 
  	if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-	     offset + len>  i_size_read(inode)) {
+	    (offset + len>  i_size_read(inode) ||
+	     offset + len>  EXT4_I(inode)->i_disksize)) {
  		new_size = offset + len;
  		ret = inode_newsize_ok(inode, new_size);
  		if (ret)

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
  	}
 
  	if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-	     offset + len>  i_size_read(inode)) {
+	    (offset + len>  i_size_read(inode) ||
+	     offset + len>  EXT4_I(inode)->i_disksize)) {
  		new_size = offset + len;
  		ret = inode_newsize_ok(inode, new_size);
  		if (ret)
--

To unsubscribe from this list: send the line "unsubscribe fstests" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html

.

Quoted 14 lines · Theodore Ts'o

Quoted 9 lines · Ashlie Martinez

Quoted 9 lines

Quoted 4 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

Quoted 8 lines

[1]
https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

From what I've surmised from the test case, it replays the workload

Quoted 28 lines

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

Quoted 6 lines

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o<[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a
crash

Hi Theodore,

After applying your patch, generic/456 always passes on my system which just triggers the output[2].
So i could believe this two different outputs[1][2] are triggered by different environments, but they
are caused by the same bug which your patch fixes. Is this right?

[1] Inode 12, end of extent exceeds allowed value(logical block 33, physical block 33441, len 7)Clear? no

      Inode 12, i_blocks is 184, should be 128. Fix? no

[2] Inode 12, i_size is 147456, should be 163840. Fix? no

Sorry, i am not familiar with ext4.

Thanks,
Xiao Yang

Quoted 69 lines

If there are pending writes subject to delayed allocation, then
i_size
will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be
updated,
and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2)
call
is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically< 30 seconds, in practice this condition will rarely
happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o<[email protected]>

Reported-by Amir Goldstein<[email protected]>

Cc [email protected]

fs/ext4/extents.c +2 -1

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file,
 loff_t offset,
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)

fs/ext4/extents.c +2 -1

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode,
 loff_t offset, loff_t len)
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)
--
To unsubscribe from this list: send the line "unsubscribe fstests" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html

.

Quoted 15 lines · Xiao Yang

Quoted 14 lines · Theodore Ts'o

Quoted 9 lines · Ashlie Martinez

Quoted 9 lines

Quoted 4 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

Quoted 8 lines

Quoted 8 lines

[1]
https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

Amir,

I was actually looking into why only some machines can reproduce the output from generic/456 and I found that I was a little unsure about exactly how the test itself was operating. I'm not an expert on how device mapper works, so I was wondering if you could help shed some light on it.

From what I've surmised from the test case, it replays the workload you specified with fsx and then immediately inserts the dm_flakey table and device in place of the hard disk *while requests are still in flight* (see nolockfs flag in dmsetup which is used by dm_flakey calls). The flakey device drops all requests sent to it. The flakey device is then unmounted (to force all pending writes out), the dm table changed once again back to the original backing block device, and then mounted again.

Is this summary of the test case correct? If it is, then perhaps the reason why it is so hard for some people to reproduce the exact bug output you saw is because there does not appear to be anything in the test case that specifies the timing for when dm_flakey should begin dropping requests. Since the flakey device could be inserted any time after fsx completes, but writes are still pending, it seems like the test creates a race condition between inserting dm_flakey and a kernel thread waking up to persist writes. This race condition would likely be affected by things like having > 1 core on the VM or hardware (so that dm_flakey could be inserted at the same time a kernel thread on another core is trying to persist writes) the test is running on, system load, dirty writeback times, etc.

Do you have any thoughts on the above?

Quoted 125 lines

Quoted 29 lines

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

Quoted 6 lines

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o<[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a
crash

Hi Theodore,

After applying your patch, generic/456 always passes on my system which just triggers the output[2].
So i could believe this two different outputs[1][2] are triggered by different environments, but they
are caused by the same bug which your patch fixes. Is this right?

[1] Inode 12, end of extent exceeds allowed value(logical block 33, physical block 33441, len 7)Clear? no

      Inode 12, i_blocks is 184, should be 128. Fix? no

[2] Inode 12, i_size is 147456, should be 163840. Fix? no

Sorry, i am not familiar with ext4.

Thanks,
Xiao Yang

Quoted 69 lines

If there are pending writes subject to delayed allocation, then
i_size
will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be
updated,
and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2)
call
is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically< 30 seconds, in practice this condition will rarely
happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o<[email protected]>

Reported-by Amir Goldstein<[email protected]>

Cc [email protected]

fs/ext4/extents.c +2 -1

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file,
 loff_t offset,
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)

fs/ext4/extents.c +2 -1

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode,
 loff_t offset, loff_t len)
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)
--
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.

Quoted 55 lines · Ashlie Martinez

Quoted 15 lines · Xiao Yang

Quoted 14 lines · Theodore Ts'o

Quoted 9 lines · Ashlie Martinez

Quoted 9 lines

Quoted 4 lines

It almost seems that way, though to be honest, I don't think I know enough about 1. the setup used by Amir and 2. all the internal working of KVM+virtio to say for sure.

I believe you misread my email.
The problem was NOT reproduced on KVM+virtio.
The problem *is* reproduced on a 10G LVM volume over SSD on Ubuntu 16.04 with latest kernel and latest e2fsprogs.

I was able to reproduce it using both kvm-xfstests[1] and gce-xfstests[2].

Quoted 8 lines

Quoted 8 lines

[1]
https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-xfstests.md
[2] https://thunk.org/gce-xfstests

It did turn out to be timing related, and it requires that a journal commit take place after fsstress runs, but it can *not* be triggered by a sync/fsync (as this would cause the delayed allocation writes to be forced out to disk, and that makes the problem go away).

Amir,

I was actually looking into why only some machines can reproduce the output from generic/456 and I found that I was a little unsure about exactly how the test itself was operating. I'm not an expert on how device mapper works, so I was wondering if you could help shed some light on it.

From what I've surmised from the test case, it replays the workload you specified with fsx and then immediately inserts the dm_flakey table and device in place of the hard disk *while requests are still in flight* (see nolockfs flag in dmsetup which is used by dm_flakey calls). The flakey device drops all requests sent to it. The flakey device is then unmounted (to force all pending writes out), the dm table changed once again back to the original backing block device, and then mounted again.

Is this summary of the test case correct? If it is, then perhaps the reason why it is so hard for some people to reproduce the exact bug output you saw is because there does not appear to be anything in the test case that specifies the timing for when dm_flakey should begin dropping requests. Since the flakey device could be inserted any time after fsx completes, but writes are still pending, it seems like the test creates a race condition between inserting dm_flakey and a kernel thread waking up to persist writes. This race condition would likely be affected by things like having > 1 core on the VM or hardware (so that dm_flakey could be inserted at the same time a kernel thread on another core is trying to persist writes) the test is running on, system load, dirty writeback times, etc.

Quoted 1 line

Do you have any thoughts on the above?

Sounds about right.
Having more cores to the VM was my first guess as the reason. Note that all the bugs I found so far depended on some timing between processes or kernel threads, so crash monkey trying all io completion combinations would not have helped. Maybe it would help to find other bugs though.

Quoted 125 lines

Quoted 125 lines

Quoted 29 lines

I initially tried using xfs_io as a replacement for fsstress (since it is more flexible and would allow me to more easily run experiments), but it turns out xfs_io was too fast/efficient, and so using xfs_io to execute the same system calls (verified by strace) would not replicate the problem.

Quoted 2 lines

Quoted 2 lines

Now you have a broken file system image and the exact set of operations that led to it. That's should be a pretty big lead for investigation.

It was indeed a big lead for investigation (thanks, Amir!), but it still took me several hours before I was finally able to figure out the problem. The patch and the commit description should explain what was going on.

I'll leave it to Ashlie and Vijay to investigate how to improve Crash Monkey so it can better find problems like this automatically. Since you now have a clear reproducer (you can use generic/456 and run it on gce-xfstests, using is a standard cloud VM configuration) and an explanation of the bug and the four-line fix, I suspect this might be good grist for follow-on research after your Hot Storage '17 workshop paper. :-)

Best regards,

- Ted

Quoted 6 lines

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o<[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a
crash

Hi Theodore,

After applying your patch, generic/456 always passes on my system which just triggers the output[2].
So i could believe this two different outputs[1][2] are triggered by different environments, but they
are caused by the same bug which your patch fixes. Is this right?

[1] Inode 12, end of extent exceeds allowed value(logical block 33, physical block 33441, len 7)Clear? no

      Inode 12, i_blocks is 184, should be 128. Fix? no

[2] Inode 12, i_size is 147456, should be 163840. Fix? no

Sorry, i am not familiar with ext4.

Thanks,
Xiao Yang

Quoted 69 lines

If there are pending writes subject to delayed allocation, then
i_size
will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be
updated,
and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2)
call
is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically< 30 seconds, in practice this condition will rarely
happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o<[email protected]>

Reported-by Amir Goldstein<[email protected]>

Cc [email protected]

fs/ext4/extents.c +2 -1

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file,
 loff_t offset,
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)

fs/ext4/extents.c +2 -1

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode,
 loff_t offset, loff_t len)
        }
 
        if (!(mode&  FALLOC_FL_KEEP_SIZE)&&
-            offset + len>  i_size_read(inode)) {
+           (offset + len>  i_size_read(inode) ||
+            offset + len>  EXT4_I(inode)->i_disksize)) {
                new_size = offset + len;
                ret = inode_newsize_ok(inode, new_size);

if (ret)
--
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.

Amir,

I know this is a bit late, but I've spent some time working through the disk image that you provided (so that I could determine how/if I could modify CrashMonkey to catch errors like this) and I don't think I understand what state the disk image reflects.

After digging around
the journal of the disk image you provided, I found that the first 10 journal blocks are used, with the journal superblock being placed in the very first block of the journal. The journal superblock says that

the first journal transaction ID that should be in the journal is transaction ID 4. However, dumping the other journal blocks, I found that the next block is a descriptor block for transaction ID 2. The rest of the journal blocks are data blocks for that transaction plus a transaction commit block. This seems a little odd considering that the journal refers to a 4th transaction, which I have not been able to find (I quickly dumped the first 50 blocks in debugfs and found the rest to contain only zeros).

With this in mind, I looked back at the xfstests code for controlling the dm_flakey device. What I realized is the nolockfs flag is provided both when it switches from the real device to the flakey device that drops writes and when it switches from the flakey device back to the real device. I know there is a call to umount once the flakey device that drops writes is inserted, but do you think it is possible that the flakey device is swapped back to the real device before all the writes forced out by umount have made it to the flakey device?

Unfortunately I still don't have a local machine that is capable of reproducing your test results and I have not made any gce test appliance images to test this yet, so I'm not sure if this is a valid theory.

The disk image SHOULD reflect a state on a disk after the power was cut in the middle of mounted fs. Then power came back on, filesystem was mounted, journal recovered, then filesystem was cleanly unmounted. At this stage, I don't expect there should be anything interesting in the journal.

I believe umount call should be blocked until all writes have been flushed out to flakey device.

Ted explained that the bug related to very specific timing of flusher thread vs. fallocate thread.
I was under the impression that CrashMonkey can only reorder writes between recorded FLUSH requests, so I am not really sure how you intent to modify CrashMonkey to catch this bug.

Quoted 6 lines · Amir Goldstein

The disk image SHOULD reflect a state on a disk after the power was cut in the middle of mounted fs. Then power came back on, filesystem was mounted, journal recovered, then filesystem was cleanly unmounted. At this stage, I don't expect there should be anything interesting in the journal.

I suspect what Ashlie was hoping for was a file system image *before* the file system was remounted and the journal replayed (and then truncated). That would allow for an analysis of image right after the simulated power cut, so it could be seen what was in the journal.

The only way to get that is to modify the test so that it aborts before the file system is remounted. I did some investigations where I ran commands (such as "debugfs -R "logdump -ac /dev/vdc") before the file system was remounted to gather debugging information. That's how I tracked down the problem. Unfortunately I never bothered to grab full file system snapshot, so I can't give Ashlie what she's hoping for.

Quoted 2 lines

I believe umount call should be blocked until all writes have been flushed out to flakey device.

That is correct.

Quoted 5 lines

Ted explained that the bug related to very specific timing of flusher thread vs. fallocate thread.
I was under the impression that CrashMonkey can only reorder writes between recorded FLUSH requests, so I am not really sure how you intent to modify CrashMonkey to catch this bug.

The real issue is that what CrashMonkey is testing is given a test trace with N CACHE FLUSH operations, given a random X such that:

       1 <= X < N

If of the writes before the Xth CACHE FLUSH are completed, and a random set of writes between the Xth and (X+1)th CACHE FLUSH are completed, is the file system still consistent after a journal replay.

That's a fine thing to test, although you can probably do that more efficiently by simply looking at all of the metadata writes between the Xth and X+1th CACHE FLUSH. Those writes must be effective no-ops after the journal is replayed up to the Xth cache flush. Which is to say, the writes must either (a) be to a data block, or (b) the contents of the writes must match either (a) the most recent journal entry for that block (up to the Xth cache flush), or (b) the current state of the disk.

So if you are willing to assume knowledge of what is stored in the journal and how ext4 works, it should be possible to implement CrashMonkey much more effectively.

The problem that this bug exposed is different sort of problem. To find this bug, given the I/O stream, you can simply examine the file system state after each journal commit. (e.g., after each CACHE FLUSH). And just make sure the file system state is consistent. There is no need to include some random set of writes from the last commit epoch.

The sort of searching of the test space new CrashMonkey' would have to test can't be done just by looking at the io traces. Instead, a workload consists of a series of micro-transactions (jbd2 handles) which are assigned to a set of journal transactions. Normally, which handles get assigned to a given transaction is based on timing (we close a transaction every 5 seconds), or based on the size of the transaction (we limit the number of blocks in a transaction), or based file system operations --- e.g., a fsync() will cause a transaction to close.

This CrashMoney' would have to explore a different set of transaction boundaries (e.g., which handles are assigned to a transaction before a transaction closes), and whether the file system is consistent at each transaction boundary given a each possible assignment of handles to transactions.

It's doable, but it would have to be done by logging the data passed to the jbd2 logging layer, and checking file system consistency at each handle boundary.

- Ted

Hi Ted,

This was what we were suspecting as well. If the bug being exposed or not only depends on the number of transactions in the journal before the test workload starts, that’s something we could easily capture in CrashMonkey. It would be one more parameter to tweak, and we would potentially have to get multiple traces of the test workload with the journal being X percent full.

It does expand our already-large search space, but our first order of business is making sure CrashMonkey can reproduce every crash-consistency bug reported in recent times (mostly by Amir :) ). So for now we were just analyzing the bug and trying to understand it, but it looks like the post-recovery image is not very useful for this.

Quoted 23 lines · Theodore Ts'o

Quoted 7 lines · Amir Goldstein

The disk image SHOULD reflect a state on a disk after the power was cut in the middle of mounted fs. Then power came back on, filesystem was mounted, journal recovered, then filesystem was cleanly unmounted. At this stage, I don't expect there should be anything interesting in the
journal.

I suspect what Ashlie was hoping for was a file system image *before* the file system was remounted and the journal replayed (and then truncated). That would allow for an analysis of image right after the simulated power cut, so it could be seen what was in the journal.

The only way to get that is to modify the test so that it aborts before the file system is remounted. I did some investigations where I ran commands (such as "debugfs -R "logdump -ac /dev/vdc") before the file system was remounted to gather debugging information. That's how I tracked down the problem. Unfortunately I never bothered to grab full file system snapshot, so I can't give Ashlie what she's hoping for.

It does expand our already-large search space, but our first order of business is making sure CrashMonkey can reproduce every crash-consistency bug reported in recent times (mostly by Amir :) ). So for now we were just analyzing the bug and trying to understand it, but it looks like the post-recovery image is not very useful for this.

Quoted 5 lines · Vijay Chidambaram

It does expand our already-large search space, but our first order of business is making sure CrashMonkey can reproduce every crash-consistency bug reported in recent times (mostly by Amir :) ). So for now we were just analyzing the bug and trying to understand it, but it looks like the post-recovery image is not very useful for this.

Right, the post-recovery (after the journal replayed) is not very useful. Unfortunately, the pre-recovery (after the power cut, but before the journal replay) I suspect won't be terribly interesting either. It will show that the corruption is baked into the journal --- which is to say, the problem wasn't in whether the calls to the jbd2 layer were correct --- but rather, that one of the file system mutations in a specific jbd2 handle's "micro-transaction" left the file system is an inconsistent state.

Not a terrible inconsistency, and it would be quickly papered over in a follow-up handle --- but one where if the handle which left the file system in an inconsistent state, and the handle which cleaned it up were in different transactions, and the power cut happened after the first transaction, the file system be left in a state where e2fsck would complain.

So if you have the I/O trace where the handles in question were assigned to the right (wrong) set of transactions, then yes, you'll see the problem, just as the xfstest will see the problem.

But if you want to improve the CrashMonkey's search of the problem space, it will require higher-level logging, because this is really a different sort of bug. CrashMonkey will find (a) bugs in jbd2, and
(b) bugs in how the jbd2 layer is called. This bug is really a bug in ext4 implementation, because it is in *how* the file system was mutated that temporarily left it in an inconsistent state, and that's a different thing from (a) or (b). Which is great --- it's arguably additional research work that can be segregated into a different "Minimum Publishable Unit". :-)

- Ted

commit 3912e7b44cf77e9452d4d0cb6c1da9c7043bb7f1

Author: Theodore Ts'o <[email protected]>
Date:   Fri Oct 6 23:09:55 2017 -0400

ext4: fix interaction between i_size, fallocate, and delalloc after a crash

If there are pending writes subject to delayed allocation, then i_size will show size after the writes have completed, while i_disksize contains the value of i_size on the disk (since the writes have not been persisted to disk).

If fallocate(2) is called with the FALLOC_FL_KEEP_SIZE flag, either with or without the FALLOC_FL_ZERO_RANGE flag set, and the new size after the fallocate(2) is between i_size and i_disksize, then after a crash, if a journal commit has resulted in the changes made by the fallocate() call to be persisted after a crash, but the delayed allocation write has not resolved itself, i_size would not be updated, and this would cause the following e2fsck complaint:

Inode 12, end of extent exceeds allowed value
(logical block 33, physical block 33441, len 7)

This can only take place on a sparse file, where the fallocate(2) call is allocating blocks in a range which is before a pending delayed allocation write which is extending i_size. Since this situation is quite rare, and the window in which the crash must take place is typically < 30 seconds, in practice this condition will rarely happen.

Nevertheless, it can be triggered in testing, and in particular by xfstests generic/456.

Signed-off-by Theodore Ts'o <[email protected]>

Reported-by Amir Goldstein <[email protected]>

Cc [email protected]

fs/ext4/extents.c +4 -2

index 97f0fd06728d..07bca11749d4 100644
@@ -4794,7 +4794,8 @@ static long ext4_zero_range(struct file *file, loff_t offset,
 	}
 
 	if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-	     offset + len > i_size_read(inode)) {
+	    (offset + len > i_size_read(inode) ||
+	     offset + len > EXT4_I(inode)->i_disksize)) {
 		new_size = offset + len;
 		ret = inode_newsize_ok(inode, new_size);
 		if (ret)

@@ -4965,7 +4966,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	}
 
 	if (!(mode & FALLOC_FL_KEEP_SIZE) &&
-	     offset + len > i_size_read(inode)) {
+	    (offset + len > i_size_read(inode) ||
+	     offset + len > EXT4_I(inode)->i_disksize)) {
 		new_size = offset + len;
 		ret = inode_newsize_ok(inode, new_size);
 		if (ret)