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

[Jens Axboe]

On 05/18/2015 05:38 PM, santosh shilimkar wrote:
> On 5/18/2015 4:25 PM, Ming Lei wrote:
> > On Tue, May 19, 2015 at 7:13 AM, santosh shilimkar
> > <santosh.shilimkar@xxxxxxxxxx> wrote:
> > > On 5/18/2015 11:07 AM, santosh shilimkar 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.
> > > Anyways, I created patch(end of the email) against v4.0, based on
> > > your patch
> > > and tested it. The corruption is no more seen so it does fix
> > > the issue after backing out concurrent submission changes from
> > > commit b5dd2f6. Let me know whats you plan with it since linus
> > > tip as well as v4.0 needs this fix.
> >
> > If your issue is caused by concurrent IO submittion, it might be one
> > issue of ocfs2. As you see, there isn't such problem for ext3/ext4.
> As we speak, I got to know about another regression with XFS as well
> and am quite confident based on symptom that its similar issue.
> I will get a confirmation on the same by tomorrow whether the patch
> fixes it or not.
>
> > And the single thread patch is introduced for aio/dio support, which
> > shouldn't have been a fix patch.
>
> Well before the loop blk-mq conversion commit b5dd2f6, the loop driver
> was single threaded and as you see that issue seen with that
> commit. Now with this experiment, it also proves that those work-queue
> split changes are problematic. So am not sure why do you say that it
> shouldn't be a fix patch.

There should be no issue with having concurrent submissions. If 
something relies on serialization of some sort, then that is broken and 
should be fixed up. That's not a problem with the loop driver. That's 
why it's not a fix.

--
Jens Axboe

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