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

[santosh shilimkar]

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.

Regards,
Santosh

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