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SubjectRe: Direct I/O bug in kernel
----- Original Message -----
> From: Hillf Danton <dhillf@gmail.com>
> To: Victor Meyerson <calculuspenguin@yahoo.com>
> Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>; Ralf Baechle <ralf@linux-mips.org>; LKML <linux-kernel@vger.kernel.org>
> Sent: Tuesday, July 24, 2012 6:04 AM
> Subject: Re: Direct I/O bug in kernel
>
> On Sun, Jul 22, 2012 at 10:05 AM, Victor Meyerson
> <calculuspenguin@yahoo.com> wrote:
>> Hi,
>>
>> I recently found a bug related to direct io in post 3.3 linux kernels. 
> Fortunately, my hardware (a Cobalt Qube2) is supported by the vanilla kernel so
> I did not need additional patch sets to get the machine to boot.  I ran git
> bisect on the main tree[1] and tested the various bisect results until git
> reported the first bad commit.  After several bisects and many reboots, git
> reported that [2] was the first bad commit.
>>
>> In testing this I came up with a repeatable process.  Unfortunately, I do
> not have any other MIPS hardware to test this on and I believe that based on the
> commit in question that it is MIPS related.  My procedure is as follows:
>>
>> 1) Create a random file to be used on the two kernels (one before the
> commit, and one that includes the commit)
>> $ dd if=/dev/urandom of=random-file bs=512 count=30720
>> 30720+0 records in
>> 30720+0 records out
>> 15728640 bytes (16 MB) copied, 60.7035 s, 259 kB/s
>> $ chmod -w random-file
>>
>> 2) Reboot to the kernel before the commit and run dd with direct io. 
> Repeat.
>> $ uname -a
>> Linux horadric 3.2.0-dirty #2 Fri Jul 13 06:20:22 PDT 2012 mips64 Nevada
> V10.0 FPU V10.0 Cobalt Qube2 GNU/Linux
>> $ dd if=random-file of=portion-of-random-3.2.0 bs=512 count=20480
> iflag=direct
>> 20480+0 records in
>> 20480+0 records out
>> 10485760 bytes (10 MB) copied, 42.3636 s, 248 kB/s
>> $ reboot
>> $ dd if=random-file of=portion-of-random-3.2.0-2 bs=512 count=20480
> iflag=direct
>> 20480+0 records in
>> 20480+0 records out
>> 10485760 bytes (10 MB) copied, 42.5252 s, 247 kB/s
>>
>> 3) Reboot to the kernel with the commit and run dd with direct io.  Repeat.
>> $ uname -a
>> Linux horadric 3.2.0-rc4-00003-gb1c10be-dirty #15 Fri Jul 20 15:05:13 PDT
> 2012 mips64 Nevada V10.0 FPU V10.0 Cobalt Qube2 GNU/Linux
>> $ dd if=random-file of=portion-of-random-3.2.0-rc4 bs=512 count=20480
> iflag=direct
>> 20480+0 records in
>> 20480+0 records out
>> 10485760 bytes (10 MB) copied, 40.6226 s, 258 kB/s
>> $ reboot
>> $ dd if=random-file of=portion-of-random-3.2.0-rc4-2 bs=512 count=20480
> iflag=direct
>> 20480+0 records in
>> 20480+0 records out
>> 10485760 bytes (10 MB) copied, 40.8856 s, 256 kB/s
>>
> Hi Victor,
>
> Create files with
>
>     dd if=random-file of=portion-of-random-3.2.0-rc4    bs=8k
> count=1280 iflag=direct
>     dd if=random-file of=portion-of-random-3.2.0-rc4-2 bs=8k
> count=1280 iflag=direct
>
> without reboot(why reboot needed?), then see the changes in checksums.
>
> Thanks
> Hillf
>

Hi Hillf,

I rebooted in an attempt to make sure nothing was cached between runs.  In any case, here are the results without a reboot:

$ dd if=random-file of=portion-of-random-3.2.0-rc4 bs=8k count=1280 iflag=direct
1280+0 records in
1280+0 records out
10485760 bytes (10 MB) copied, 6.00599 s, 1.7 MB/s
$ dd if=random-file of=portion-of-random-3.2.0-rc4-2 bs=8k count=1280 iflag=direct
1280+0 records in
1280+0 records out
10485760 bytes (10 MB) copied, 5.25964 s, 2.0 MB/s
$ sha256sum portion-of-random-3.2.0-rc4*
4c56820030ce22e6cc96127a53f6025d11a78f2fd3b0c1dec44f6d6746f70bbd  portion-of-random-3.2.0-rc4
05c41d626a67b9bcddb0e7b905533c63a0866092b819bf01cdb2a80f29c2b162  portion-of-random-3.2.0-rc4-2

Still different checksums and I used the same random-file from my first test.

Victor

>> 4) Compare checksums of the resulting files.
>> $ sha256sum portion-of-random-3.2.0*
>> c98a6e949b36448842a21f68e7c6a5daff1f161e1eb3e3529176cf56bf5af89e 
> portion-of-random-3.2.0
>> c98a6e949b36448842a21f68e7c6a5daff1f161e1eb3e3529176cf56bf5af89e 
> portion-of-random-3.2.0-2
>> dca27da87a78580b8a34bbff2790ae80d3aa880d5d00fc2126f109d6fff9e056 
> portion-of-random-3.2.0-rc4
>> 703cf02d4fa90679d4a75900e7e5a3b8c3000a65bfc475610b10f17bb88bedbc 
> portion-of-random-3.2.0-rc4-2
>>
>> Notice how the last two files have different checksums between themselves
> and even different from the first two files.  This lead me to believe that there
> is a problem with direct io.  All the files are the same size and should include
> the same portion of the random file created in step 1).
>>
>> My configuration is the Cobalt Qube2 with a 64-bit kernel and an n32
> userspace.  Hopefully someone with a much more deeper understanding of the
> kernel can confirm and provide a fix for this (assuming one has not been created
> yet).
>>
>> Thanks.  Let me know if there is any additional information that may help
> with the investigation.
>>
>> Victor
>>
>>
>> [1] http://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
>> [2]
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=b1c10bea620f79109b5cc9935267bea4f6f29ac6
>
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