Re: Kernel bug from 3.0 (was phy disks and vifs timing out in DomU)

[Ian Campbell <Ian.Campbell@eu.citrix.com>]

On Thu, 2011-09-01 at 16:37 +0100, Konrad Rzeszutek Wilk wrote:
> > >>     vmalloc: remove vmalloc_sync_all() from alloc_vm_area()
> > >>     
> > >>     There's no need for it: it will get faulted into the current pagetable
> > >>     as needed.
> > >>     
> > >>     Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
> > >>
> > >> The flaw in the reasoning here is that you cannot take a kernel fault
> > >> while processing a hypercall, so hypercall arguments must have been
> > >> faulted in beforehand and that is what the sync_all was for.
> > >>
> > >> It's probably fair to say that the Xen specific caller should take care
> > >> of that Xen-specific requirement rather than pushing it into common
> > >> code. On the other hand Xen is the only user and creating a Xen specific
> > >> helper/wrapper seems a bit pointless.
> > > 
> > > Perhaps then doing the vmalloc_sync_all() (or are more precise one:
> > > vmalloc_sync_one) should be employed in the netback code then?
> > > 
> > > And obviously guarded by the CONFIG_HIGHMEM case?
> > 
> > Perhaps. But I think the correct thing to do initially is revert the
> > change and then look at possible improvements.  Particularly as the fix
> > needs to be a backported to stable.
> 
> I disagree. Ian pointed out properly that this a Xen requirment - and there
> is no reason for us to slow down non-Xen runs with vmalloc_sync_all plucked in
> a generic path.

There is literally no other caller of alloc_vm_area than xen so you
won't be slowing anyone else down.

Maybe we should add alloc_vm_area_sync and use that everywhere?

Ian.