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

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

On Thu, Sep 01, 2011 at 08:42:52AM +0100, Ian Campbell wrote:
> On Wed, 2011-08-31 at 18:07 +0100, Konrad Rzeszutek Wilk wrote:
> > On Wed, Aug 31, 2011 at 05:58:43PM +0100, David Vrabel wrote:
> > > On 26/08/11 15:44, Konrad Rzeszutek Wilk wrote:
> > > > 
> > > > So while I am still looking at the hypervisor code to figure out why
> > > > it would give me [when trying to map a grant page]:
> > > > 
> > > > (XEN) mm.c:3846:d0 Could not find L1 PTE for address fbb42000
> > > 
> > > It is failing in guest_map_l1e() because the page for the vmalloc'd
> > > virtual address PTEs is not present.
> > > 
> > > The test that fails is:
> > > 
> > > (l2e_get_flags(l2e) & (_PAGE_PRESENT | _PAGE_PSE)) != _PAGE_PRESENT
> > > 
> > > I think this is because the GNTTABOP_map_grant_ref hypercall is done
> > > when task->active_mm != &init_mm and alloc_vm_area() only adds PTEs into
> > > init_mm so when Xen looks in the page tables it doesn't find the entries
> > > because they're not there yet.
> > > 
> > > Putting a call to vmalloc_sync_all() after create_vm_area() and before
> > > the hypercall makes it work for me.  Classic Xen kernels used to have
> > > such a call.
> > 
> > That sounds quite reasonable.
> 
> I was wondering why upstream was missing the vmalloc_sync_all() in
> alloc_vm_area() since the out-of-tree kernels did have it and the
> function was added by us. I found this:
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=ef691947d8a3d479e67652312783aedcf629320a
> 
> commit ef691947d8a3d479e67652312783aedcf629320a
> Author: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
> Date:   Wed Dec 1 15:45:48 2010 -0800
> 
>     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?