linux-ext4 - Re: generic/346 fails on ext4 DAX mount

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Date:   Mon, 5 Jun 2017 13:30:22 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Eryu Guan <eguan@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Jan Kara <jack@...e.cz>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Xiong Zhou <xzhou@...hat.com>, Theodore Ts'o <tytso@....edu>
Subject: Re: generic/346 fails on ext4 DAX mount

On Fri, Jun 02, 2017 at 01:33:19PM +0800, Eryu Guan wrote:
> Hi all,
> 
> I occasionally hit generic/344 and generic/346 failures when testing
> 4.12-rc[1-3] kernels on ext4 DAX mount.
> 
> FSTYP         -- ext4
> PLATFORM      -- Linux/x86_64 hp-xl420gen9-01 4.12.0-rc3
> MKFS_OPTIONS  -- /dev/pmem2
> MOUNT_OPTIONS -- -o dax -o context=system_u:object_r:root_t:s0 /dev/pmem2 /mnt/testarea/scratch
> 
> generic/344 1s ... 1s
> generic/346 1s ... - output mismatch (see /var/lib/xfstests/results//generic/346.out.bad)
>     --- tests/generic/346.out   2017-05-24 10:13:38.592436565 -0400
>     +++ /var/lib/xfstests/results//generic/346.out.bad  2017-06-01 12:46:50.122007818 -0400
>     @@ -10,7 +10,8 @@
>      INFO: sz = 1048576
>      INFO: thread 0 created
>      INFO: thread 1 created
>     -INFO: 0 error(s) detected
>     +ERROR: thread 0, offset 000ff400, 00000000 != 7f1068063700
>     +INFO: 1 error(s) detected
>  
>      INFO: ftruncate test...
>      INFO: sz = 1048576
> 
> And it seems generic/346 is easer to hit, usually it can be reproduced
> within 20 iterations on 4.12-rc kernels.
> 
> At first I thought it was a regression introduced in 4.12-rc1, but after
> two failed bisects (pointed first bad to unrelated networking patch), I
> enlarged the iteration count to 5000 and found that generic/346 failure
> can also be seen on 4.11 and 4.10 kernel. I haven't tried other old
> kernels yet. It's just much harder to hit on 4.10/4.11 kernels (need
> hundreds of iterations).
> 
> But the failure could only be reproduced with ext4 DAX mount, XFS DAX
> mount survived 5000 runs of generic/346 on 4.12-rc3 kernel.
> 
> I was testing with pmem device created by memmap kernel param
> "memmap=10G!5G memmap=15G!15G", but it can be reproduced with brd
> ramdisk too.
> 
> If more info is needed please let me know.
> 
> Thanks,
> Eryu

It turns out that this has already been fixed by Jan, with this commit:

https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/commit/?h=dev&id=4f8caa60a5a13a78f26198618f21774bd6aa6498

I have a test setup that was failing with this error very constantly within
100 iterations using v4.12-rc4.  I was able to run 10,000+ iterations without
issue with v4.12-rc4 + that one patch.

That patch clearly isn't upstream yet, but I think it's headed for v4.12?  
(Ted, Jan?)  It's marked for stable as well.

It's currently in the ext4/dev tree:
https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git/log/?h=dev

- Ross