Re: xfstests failure generic/239

[Zhao Hongjiang <zhaohongjiang@huawei.com>]

On 2013/6/9 6:30, Theodore Ts'o wrote:
> On Sat, Jun 08, 2013 at 11:13:35AM +0800, Zhao Hongjiang wrote:
>>
>> I run xfstests #239 against mainline 3.10.0-rc3, unfortunately it failure in my QEMU. I run the
>> case a hundred times, it certainly hit the failure several times. The failure msg is as follow:
>>
>> FSTYP         -- ext4
>> PLATFORM      -- Linux/x86_64  3.10.0-rc3-mainline
>>
>> generic/239 1s ... - output mismatch (see /home/zhj/xfstests/results/generic/239.out.bad)
>>     --- tests/generic/239.out   2013-06-07 22:04:09.000000000 -0400
>>     +++ /home/zff/xfstests/results/generic/239.out.bad  2013-06-07 22:04:09.000000000 -0400
>>     @@ -1,2 +1,515 @@
>>      QA output created by 239
>>     +hostname: Host name lookup failure
> 
> OK, so this hostname failure is weird; I'm not sure what's causing
> this, but this I presume unrelated to the failure at hand.
> 
>>      Silence is golden
>>     +0: 0x0
>>     +1: 0x0
>>     +2: 0x0
>>     +3: 0x0
> 
> This indicates a problem.  Test generic/239 is running
> aio-dio-hole-filling-race.c, which submits an asynchronous, direct I/O
> 4k write with a buffer containing non-zero contents to a sparse file,
> and once the I/O has completed, it uses pread to read it back, using
> the same descriptor, so it is doing the read using direct I/O.  It
> then checks to see if the read returns zero or not.  
> 
> The "XX: 0x0" lines indicates that buffer is zero, which implies that
> somehow aio_complete() is getting called before the uninitialized to
> initialized conversion is taking place.  I'm not seeing how this is
> happening, though, so I'm a bit puzzled.  If there are any unwritten
> extents, we don't call aio_complete() in ext4_end_io_dio(), but
> instead the conversion is queued via a call to ext4_add_compete_io(),
> and and aio_done() is only called on the iocb after the conversion is
> complete.
> 
> Can anyone see something that I might be missing?
> 
>     	       		      	      - Ted
> 
> P.S.  Zhao, what was the hardware that you using to find this failure?

I'am use x86 and start a qumu-kvm to run the test. 

> I'm not seeing it, but then again if the failure is only happening
> once every few hundred runs that might explain it.  I'm perhaps

And as Christoph Hellwig  said "the race is very easy to hit by using QEMU with
native AIO support on a sparse image, and the result is filesystem corruption 
in the guest", i also run the test on the host, but nerver see the failure.

> wondering if we should add a mode to aio-dio-hole-filling-race.c which
> allows it to try the race a large number of times, instead of just
> once.

This seems necessary, i'll give a patch for this.
				   - Zhao
> 
> P.P.S.  One thought.... perhaps it might be useful to have a debug
> mode where we use queue_delayed_work() to submit the conversion
> request to the workqueue.  It will of course make certain workloads
> run slow as molasses, but it might expose some races so we can see
> them more easily.
> 
> .
>