Re: [PATCH] generic/311: Disable dmesg check

Chandan Rajendra
2015-07-21T10:33:25Z

On Tuesday 21 Jul 2015 08:12:20 Dave Chinner wrote:
> On Mon, Jul 20, 2015 at 08:55:32AM -0400, Josef Bacik wrote:
> > On 07/19/2015 07:54 PM, Dave Chinner wrote:
> > >On Fri, Jul 17, 2015 at 05:10:50PM +0530, Chandan Rajendra wrote:
> > >>On Friday 17 Jul 2015 06:16:02 Brian Foster wrote:
> > >>>On Fri, Jul 17, 2015 at 12:56:43AM -0400, Chandan Rajendra wrote:
> > >>>>When running generic/311 on Btrfs' subpagesize-blocksize patchset (on
> > >>>>ppc64
> > >>>>with 4k sectorsize and 16k node/leaf size) I noticed the following
> > >>>>call
> > >>>>trace,
> > >>>>
> > >>>>BTRFS (device dm-0): parent transid verify failed on 29720576 wanted
> > >>>>160
> > >>>>found 158 BTRFS (device dm-0): parent transid verify failed on
> > >>>>29720576
> > >>>>wanted 160 found 158 BTRFS: Transaction aborted (error -5)
> > >>>>
> > >>>>WARNING: at /root/repos/linux/fs/btrfs/super.c:260
> > >>>>Modules linked in:
> > >>>>CPU: 3 PID: 30769 Comm: umount Tainted: G W L
> > >>>>4.0.0-rc5-11671-g8b82e73e #63 task: c000000079aaddb0 ti:
> > >>>>c000000079a48000
> > >>>>task.ti: c000000079a48000 NIP: c000000000499aa0 LR: c000000000499a9c
> > >>>>CTR:
> > >>>>c000000000779630
> > >>>>REGS: c000000079a4b480 TRAP: 0700 Tainted: G W L
> > >>>>(4.0.0-rc5-11671-g8b82e73e) MSR: 8000000100029032 <SF,EE,ME,IR,DR,RI>
> > >>>>CR: 28008828 XER: 20000000 CFAR: c000000000a23914 SOFTE: 1
> > >>>>GPR00: c000000000499a9c c000000079a4b700 c00000000103bdf8
> > >>>>0000000000000025
> > >>>>GPR04: 0000000000000001 0000000000000502 c00000000107e918
> > >>>>0000000000000cda
> > >>>>GPR08: 0000000000000007 0000000000000007 0000000000000001
> > >>>>c0000000010f5044
> > >>>>GPR12: 0000000028008822 c00000000fdc0d80 0000000020000000
> > >>>>0000000010152e00
> > >>>>GPR16: 0000010002979380 0000000010140724 0000000000000000
> > >>>>0000000000000000
> > >>>>GPR20: ffffffffffffffff 0000000000000000 0000000000000000
> > >>>>0000000000000000
> > >>>>GPR24: c0000000151f61a8 0000000000000000 c000000055e5e800
> > >>>>c000000000aac270
> > >>>>GPR28: 00000000000004a4 fffffffffffffffb c000000055e5e800
> > >>>>c0000000679204d0
> > >>>>NIP [c000000000499aa0] .__btrfs_abort_transaction+0x180/0x190
> > >>>>LR [c000000000499a9c] .__btrfs_abort_transaction+0x17c/0x190
> > >>>>Call Trace:
> > >>>>[c000000079a4b700] [c000000000499a9c]
> > >>>>.__btrfs_abort_transaction+0x17c/0x190 (unreliable) [c000000079a4b7a0]
> > >>>>[c000000000541678] .__btrfs_run_delayed_items+0xe8/0x220
> > >>>>[c000000079a4b850] [c0000000004d5b3c]
> > >>>>.btrfs_commit_transaction+0x37c/0xca0 [c000000079a4b960]
> > >>>>[c00000000049824c] .btrfs_sync_fs+0x6c/0x1a0
> > >>>>[c000000079a4ba00] [c000000000255270] .sync_filesystem+0xd0/0x100
> > >>>>[c000000079a4ba80] [c000000000218070]
> > >>>>.generic_shutdown_super+0x40/0x170
> > >>>>[c000000079a4bb10] [c000000000218598] .kill_anon_super+0x18/0x30
> > >>>>[c000000079a4bb90] [c000000000498418] .btrfs_kill_super+0x18/0xc0
> > >>>>[c000000079a4bc10] [c000000000218ac8]
> > >>>>.deactivate_locked_super+0x98/0xe0
> > >>>>[c000000079a4bc90] [c00000000023e744] .cleanup_mnt+0x54/0xa0
> > >>>>[c000000079a4bd10] [c0000000000b7d14] .task_work_run+0x114/0x150
> > >>>>[c000000079a4bdb0] [c000000000015f84] .do_notify_resume+0x74/0x80
> > >>>>[c000000079a4be30] [c000000000009838] .ret_from_except_lite+0x64/0x68
> > >>>>Instruction dump:
> > >>>>ebc1fff0 ebe1fff8 4bfffb28 60000000 3ce2ffcd 38e7e818 4bffffbc
> > >>>>3c62ffd2
> > >>>>7fa4eb78 3863b808 48589e1d 60000000 <0fe00000> 4bfffedc 60000000
> > >>>>60000000
> > >>>>BTRFS: error (device dm-0) in __btrfs_run_delayed_items:1188: errno=-5
> > >>>>IO
> > >>>>failure
> > >>>>
> > >>>>
> > >>>>The call trace is seen when executing _run_test() for the 8th time.
> > >>>>The above trace is actually a false-positive failure as indicated
> > >>>>below,
> > >>>>
> > >>>> fsync-tester
> > >>>>
> > >>>> fsync(fd)
> > >>>> Write delayed inode item to fs tree
> > >>>>
> > >>>> (assume transid to be 160)
> > >>>> (assume tree block to start at logical address 29720576)
> > >>>>
> > >>>> md5sum $testfile
> > >>>>
> > >>>> This causes a delayed inode to be added
> > >>>>
> > >>>> Load flakey table
> > >>>>
> > >>>> i.e. drop writes that are initiated from now onwards
> > >>>>
> > >>>> Unmount filesystem
> > >>>>
> > >>>> btrfs_sync_fs is invoked
> > >>>>
> > >>>> Write 29720576 metadata block to disk
> > >>>> free_extent_buffer(29720576)
> > >>>>
> > >>>> release_extent_buffer(29720576)
> > >>>>
> > >>>> Start writing delayed inode
> > >>>>
> > >>>> Traverse the fs tree
> > >>>>
> > >>>> (assume the parent tree block of 29720576 is still in memory)
> > >>>> When reading 29720576 from disk, parent's blkptr will have
> > >>>> generation
> > >>>> set to 160. But the on-disk tree block will have an older
> > >>>> generation (say, 158). Transid verification fails and hence
> > >>>> the
> > >>>> transaction gets aborted
> > >>>>
> > >>>>The test only cares about the FS instance before the unmount
> > >>>>operation (i.e. the synced FS). Hence to get the test to pass, ignore
> > >>>>the
> > >>>>false-positive trace that could be generated.
> > >>>>
> > >>>>Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
> > >>>>---
> > >>>>
> > >>>> tests/generic/311 | 2 ++
> > >>>> 1 file changed, 2 insertions(+)
> > >>>>
> > >>>>diff --git a/tests/generic/311 b/tests/generic/311
> > >>>>index d21b6eb..cd6391d 100755
> > >>>>--- a/tests/generic/311
> > >>>>+++ b/tests/generic/311
> > >>>>@@ -64,6 +64,8 @@ _require_xfs_io_command "falloc"
> > >>>>
> > >>>> [ -x $here/src/fsync-tester ] || _notrun "fsync-tester not built"
> > >>>>
> > >>>>+_disable_dmesg_check
> > >>>>+
> > >>>
> > >>>Hmm, I don't think this is something we'd want to do unconditionally.
> > >>>E.g., if something hits the logs for xfs or ext4, we probably want to
> > >>>hear about it.
> > >>
> > >>Sorry, I forgot that I was dealing with generic tests. I agree with what
> > >>you say.
> > >>
> > >>>Perhaps check that the fs is btrfs and possibly the fs params are such
> > >>>that the known warning occurs..? I'd defer to the btrfs folks on how
> > >>>best to check that, so long as it doesn't affect other fs'.
> > >>
> > >>The above trace is independent of any Btrfs parameters. Its just that it
> > >>was easily reproducible on ppc64 with the subpagesize-blocksize
> > >>patchset applied. So IMHO disabling call trace check when testing Btrfs
> > >>filesystem is the way to go. But again, as you said, we can wait for
> > >>comments from other Btrfs devs.
> > >
> > >As a general principle, we don't disable warnings/errors in tests
> > >just to make tests pass. The bug in the code being tested should be
> > >fixed so the warning/error is not generated. If you do not want
> > >to see the error, then expung the test from your
> > >subpagesize-blocksize test runs.
> >
> > The same problem would happen if you did echo 3>
> > /proc/sys/vm/drop_caches right after dropping writes before doing
> > the unmount.
>
> Nice, you have multiple ways of reproducing the problem....

Dave, Apologies if I am sounding repetitive. The call trace that is showing up
when executing the test isn't indicating a failure in Btrfs' code. Its a
result of,

1. Perform fsync(fd).
2. Load dm-flakey.
i.e. drop writes from now onwards.
3. Write a dirty metadata btree block to disk.
The data however doesn't reach the disk. The corresponding block on disk
still has old data.
4. Evict the btree block from memory.
5. Read the btree block from disk.
Verification of the btree block read from the disk fails since the
transaction id expected from the btree block is different from the one
stored in the parent btree block. This causes the transaction to be aborted
and hence a call trace gets printed.

>From the above we see that there is no issue in Btrfs. The code in fact is
doing the right thing by aborting the transaction. Unfortunately the test
notices the call trace in dmesg and gets marked as a failure.

>
> > The sub page blocksize stuff is just exposing the race
> > and thus causing this warning to happen. The warning has nothing to
> > do with the test, it happens after drop writes so I think it's fine
> > to ignore it. Maybe a different mechanism could be put into place
> > to say "hey ignore dmesg for this part" or something like that.
> >
> > That all being said, it is a little weird that the sub page
> > blocksize stuff hits this so consistently, I wonder why things are
> > getting evicted that quickly. Thanks,
>
> Josef, you've just explained exactly why silencing/ignoring dmesg is
> the wrong thing to do. The test is exposing some problem (even a
> known problem) via a behaviour that is not understood. And when I
> hear "there's a race condition, and this test hits it easily" I see
> a test that can be used to verify that the problem has been fixed.
>
> i.e. we don't silence warnings just because a test triggers it
> easily; having a test fail is a clear reminder that either:
>
> a) there is a problem that needs fixing; or
> b) there is a warning being emitted under normal operating
> situations that users are going to notice and report. And
> if users can see it, then see a).

IMHO the problem lies in the test i.e. the test assumes that a btree block
that was written to the disk after dm-flakey was loaded (with write operations
being dropped) would never be evicted from memory and re-read again.

>
> Cheers,
>
> Dave.

--
chandan