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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

Ian.

> 
> http://www.spinics.net/lists/linux-ext4/msg16992.html
> 
> Any opinions on the tradeoff?
> 
> Daniel
> 
> 
> 
> 
> 
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