Re: [Xen-devel] Tagging Xen 4.0.0 first release candidate / pygrub dom0 caching bug

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Jan 21, 2010 at 07:37:27PM +0000, Ian Campbell wrote:
> On Thu, 2010-01-21 at 19:16 +0000, Daniel Stodden wrote: 
> > On Thu, 2010-01-21 at 13:44 -0500, Daniel Stodden wrote:
> > > On Thu, 2010-01-21 at 07:28 -0500, Pasi Kärkkäinen wrote:
> > > > On Wed, Jan 06, 2010 at 03:50:05PM +0200, Pasi Kärkkäinen wrote:
> > > > > On Tue, Jan 05, 2010 at 06:42:05AM +0000, Keir Fraser wrote:
> > > > > > I plan to tag -rc1 later this week. If you have any outstanding 
> > > > > > patches,
> > > > > > please send them to the list now.
> > > > > > 
> > > > > 
> > > > > Hmm.. I just remembered this pygrub bug:
> > > > > https://bugzilla.redhat.com/show_bug.cgi?id=466681
> > > > > 
> > > > > pygrub doesn't use O_DIRECT so sometimes it gets old information
> > > > > from dom0 kernel cache - and fails to use the updated domU grub.conf.
> > > > > 
> > > > > Redhat seems to have patches available for testing.. not for
> > > > > xen-unstable though.
> > > > > 
> > > > > I've personally hit this bug many times.
> > > > > 
> > > > 
> > > > It seems Redhat guys have a fix available.. they fixed the problem by 
> > > > patching dom0 kernel blkback.
> > > > 
> > > > More details about the fix here:
> > > > https://bugzilla.redhat.com/show_bug.cgi?id=466681
> > > > 
> > > > Should this be applied to 2.6.18-xen and pv_ops dom0 kernels aswell? 
> > > 
> > > Only to 2.6.18. 
> > > 
> > > It's obsolete after 2.6.27.
> > > O_DIRECT gained page cache invalidation in the meantime.
> > 
> > Aiiee, sorry. I guess this one only applies to tapdisks. The page cache
> > invalidation only covers the filemap. That obviously won't fix blkback
> > bios on raw devices.
> > 
> > Ian Campbell recently noted he came across a different fix, which adds
> > direct-io to e2fsprogs.
> 
> I noted the thread because the root problem seemed interesting and
> worthy of investigation, but I should have made it clear that I didn't
> think messing with direct-io in e2fsprogs was the correct solution. I
> think the majority of the participants in the thread thought that too.
> The biggest problem is that it only solves the issue in the one specific
> case of things which use e2fsprogs and not in general, we can't go round
> adding O_DIRECT to everything which might be used to access these disks.
> 

Yeah, it should be fixed in blkback.. who knows, some users might be using 
other tools in dom0 aswell, not just pygrub.

-- Pasi


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