Re: blkfront problem in pvops kernel when barriers enabled

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Sep 06, 2011 at 07:16:34PM +0200, Marek Marczykowski wrote:
> On 06.09.2011 18:32, Konrad Rzeszutek Wilk wrote:
> > On Sun, Sep 04, 2011 at 12:49:42PM +0200, Marek Marczykowski wrote:
> >> Hello,
> >>
> >> Pvops block frontend (tested vanilla 3.0.3, 3.1rc2, Konrad's testing
> >> branch) produces a lot of I/O errors when barriers are enabled but
> >> cannot be used.
> >>
> >> On xenlinux I've got message:
> >> [   15.036921] blkfront: xvdb: empty write barrier op failed
> >> [   15.036936] blkfront: xvdb: barriers disabled
> >>
> >> and after that, everything works fine. On pvops - I/O errors.
> >> As backend I've used 2.6.38.3 xenlinux (based on SUSE package) and
> >> 3.1rc2 with same result.
> > 
> > Hm, and the 'feature-barrier' was enabled on in those backends?
> > That is really bizzare considering that those backends don't actually
> > support WRITE_BARRIER anymore.
> 
> At least in 2.6.38.3 xenlinux  (SUSE). Now I'm not sure if 3.1rc2 also
> needed this modification (can't find it now).
> 
> >> When I disable barriers (patching blkbackend to set feature-barrier=0)
> >> everything works fine with all above versions.
> > 
> > Ok, and the patch you sent "[PATCH] Initialize vars in blkfront_connect"
> > as well?
> 
> Yes.
> I've noticed now that this patch was needed only on your testing branch
> (not vanilla kernel).

Oooo. Let me check what went wrong. Perhaps the fix is already applied in
my local tree.
> 
> >> My setup is xen-4.1.1 (if it matters), backends: phy from device-mapper
> >> device and phy from loop device; frontends covered by device-mapper
> >> snapshot, which is set up in domU initramfs.
> >>
> >> It looks like some race condition, because when I setup device-mapper in
> >> domU and mount it manually (which cause some delays between steps), it
> >> works fine...
> >>
> >> Have you idea why it happens? What additional data can I provide debug it?
> >>
> >> In addition it should be possible to disable barrier without patching
> >> module... Perhaps some pciback module parameter? Or leave feature-*
> > 
> > Not sure why you would touch pciback.. 
> 
> I mean blkback of course.
> 
> > But the barrier should _not_
> > be enabled in those backends. The 'feature-flush-cache' should be.
> 
> (on 3.1rc2) Looking to xenstore now there is 'feature-flush-cache=1' and
> no 'feature-barrier'. So it is ok.

<scratches head>

I can only think of 2.6.38-3 XenOLinux doing it - and it is a bug
to do it. It really ought to _not_ advertise 'feature-barrier' and
instead advertise 'feature-flush-cache'.