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

[santosh shilimkar]

On 5/18/2015 4:14 PM, Ming Lei wrote:
> 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
Our emails crossed. I did port that one patch alone and confirm
that the issue is getting fixed. That patch also should go
to v4.0 stable then along with above 2.

Regards,
Santosh
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