[Lguest] Virtio block problems on newer kernels

[Sakari Ailus]

Hi,

On Mon, Apr 07, 2014 at 11:33:23AM +0300, Sakari Ailus wrote:
> Sakari Ailus wrote:
> >On Mon, Mar 24, 2014 at 03:31:33AM +0200, Sakari Ailus wrote:
> >>I haven't had time to debug this further and bisecting will be
> >>painful as this isn't that easy to reproduce: I first noticed after
> >>running 3.13.6 for a day but first thought it was a network issue.
> >>The problem appeared between the two above versions (3.10.10 and
> >>3.13.6).
> >
> >Oops --- after having the problem awhile and then sending the e-mail to the
> >list, I now realise I'm using a launcher which is quite old. I'll replace
> >that with a newer one and see if that fixes the issues.
> 
> So I did that. No luck.
> 
> It seems that virtio block devices can just get stuck. The (guest) processes
> that try to access them will stay in uninterruptible sleep, sometimes for
> minutes, sometimes apparently forever. The appears to be per-virtio block
> device issue when it happens. The host lguest processes are simply in sleep
> state.
> 
> Host kernel version does not matter but that of the guest does. 3.14.0 seems
> to be affected, too. 3.12.0 works.
> 
> I'll try to bisect and see what I can find.

I did the bisect and found this:

commit 1cf7e9c68fe84248174e998922b39e508375e7c1
Author: Jens Axboe <axboe at kernel.dk>
Date:   Fri Nov 1 10:52:52 2013 -0600

    virtio_blk: blk-mq support
    
    Switch virtio-blk from the dual support for old-style requests and bios
    to use the block-multiqueue.
    
    Acked-by: Asias He <asias at redhat.com>
    Signed-off-by: Jens Axboe <axboe at kernel.dk>
    Signed-off-by: Christoph Hellwig <hch at lst.de>

Erratic block i/o behaviour begin when that patch is applied to the guest
kernel. I wonder if someone else has seen this. I can reproduce it
relatively easily by running bonnie++ in a guest:

	bonnie++ -n 0 -s 256M -r 10 -b

I'm using an up-to-date lguest launcher.

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ailus at iki.fi	XMPP: sailus at retiisi.org.uk