Re: [PATCH 2/4] mm: introduce new VM_NOZEROPAGE flag

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Mon, 20 Oct 2014 20:14:53 +0200
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 10/18/2014 06:28 PM, Dave Hansen wrote:
> > > Currently it is an all or nothing thing, but for a future change we might want to just
> > > tag the guest memory instead of the complete user address space.
> >
> > I think it's a bad idea to reserve a flag for potential future use.  If
> > you_need_  it in the future, let's have the discussion then.  For now, I
> > think it should probably just be stored in the mm somewhere.
> 
> I agree with Dave (I thought I disagreed, but I changed my mind while 
> writing down my thoughts).  Just define mm_forbids_zeropage in 
> arch/s390/include/asm, and make it return mm->context.use_skey---with a 
> comment explaining how this is only for processes that use KVM, and then 
> only for guests that use storage keys.

The mm_forbids_zeropage() sure will work for now, but I think a vma flag
is the better solution. This is analog to VM_MERGEABLE or VM_NOHUGEPAGE,
the best solution would be to only mark those vmas that are mapped to
the guest. That we have not found a way to do that yet in a sensible way
does not change the fact that "no-zero-page" is a per-vma property, no?

But if you insist we go with the mm_forbids_zeropage() until we find a
clever way to distinguish the guest vmas from the qemu ones.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.