Re: [Regression] Guest fs corruption with 'block: loop: improve performance via blk-mq'

[Ming Lei]

On Tue, May 19, 2015 at 2:07 AM, santosh shilimkar
<santosh.shilimkar@xxxxxxxxxx> wrote:
> On 5/17/2015 6:26 PM, Ming Lei wrote:
>>
>> Hi Santosh,
>>
>> Thanks for your report!
>>
>> On Sun, May 17, 2015 at 4:13 AM, santosh shilimkar
>> <santosh.shilimkar@xxxxxxxxxx> wrote:
>>>
>>> Hi Ming Lei, Jens,
>>>
>>> While doing few tests with recent kernels with Xen Server,
>>> we saw guests(DOMU) disk image getting corrupted while booting it.
>>> Strangely the issue is seen so far only with disk image over ocfs2
>>> volume. If the same image kept on the EXT3/4 drive, no corruption
>>> is observed. The issue is easily reproducible. You see the flurry
>>> of errors while guest is mounting the file systems.
>>>
>>> After doing some debug and bisects, we zeroed down the issue with
>>> commit "b5dd2f6 block: loop: improve performance via blk-mq". With
>>> that commit reverted the corruption goes away.
>>>
>>> Some more details on the test setup:
>>> 1. OVM(XEN) Server kernel(DOM0) upgraded to more recent kernel
>>> which includes commit b5dd2f6. Boot the Server.
>>> 2. On DOM0 file system create a ocfs2 volume
>>> 3. Keep the Guest(VM) disk image on ocfs2 volume.
>>> 4. Boot guest image. (xm create vm.cfg)
>>
>>
>> I am not familiar with xen, so is the image accessed via
>> loop block inside of guest VM? Is he loop block created
>> in DOM0 or guest VM?
>>
> Guest. The Guest disk image is represented as a file by loop
> device.
>
>>> 5. Observe the VM boot console log. VM itself use the EXT3 fs.
>>> You will see errors like below and after this boot, that file
>>> system/disk-image gets corrupted and mostly won't boot next time.
>>
>>
>> OK, that means the image is corrupted by VM booting.
>>
> Right
>
> [...]
>
>>>
>>>  From the debug of the actual data on the disk vs what is read by
>>> the guest VM, we suspect the *reads* are actually not going all
>>> the way to disk and possibly returning the wrong data. Because
>>> the actual data on ocfs2 volume at those locations seems
>>> to be non-zero where as the guest seems to be read it as zero.
>>
>>
>> Two big changes in the patchset are: 1) use blk-mq request based IO;
>> 2) submit I/O concurrently(write vs. write is still serialized)
>>
>> Could you apply the patch in below link to see if it can fix the issue?
>> BTW, this patch only removes concurrent submission.
>>
>> http://marc.info/?t=143093223200004&r=1&w=2
>>
> What kernel is this patch generated against ? It doesn't apply against
> v4.0. Does this need the AIO/DIO conversion patches as well. Do you
> have the dependent patch-set I can't apply it against v4.0.

My fault, the patch is against -next tree, but you just need another two
patches for applying this one:

http://git.kernel.org/cgit/linux/kernel/git/axboe/linux-block.git/commit/?h=for-next&id=f4aa4c7bbac6c4afdd4adccf90898c1a3685396d

http://git.kernel.org/cgit/linux/kernel/git/axboe/linux-block.git/commit/?h=for-next&id=4d4e41aef9429872ea3b105e83426941f7185ab6


Thanks,
Ming
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