Re: [RFC v2 PATCH 13/13] KVM: Enable memfd based page invalidation/fallocate

[Chao Peng]

On Mon, Nov 22, 2021 at 05:16:47PM +0300, Kirill A. Shutemov wrote:
> On Fri, Nov 19, 2021 at 09:47:39PM +0800, Chao Peng wrote:
> > Since the memory backing store does not get notified when VM is
> > destroyed so need check if VM is still live in these callbacks.
> > 
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  virt/kvm/memfd.c | 22 ++++++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> > 
> > diff --git a/virt/kvm/memfd.c b/virt/kvm/memfd.c
> > index bd930dcb455f..bcfdc685ce22 100644
> > --- a/virt/kvm/memfd.c
> > +++ b/virt/kvm/memfd.c
> > @@ -12,16 +12,38 @@
> >  #include <linux/memfd.h>
> >  const static struct guest_mem_ops *memfd_ops;
> >  
> > +static bool vm_is_dead(struct kvm *vm)
> > +{
> > +	struct kvm *kvm;
> > +
> > +	list_for_each_entry(kvm, &vm_list, vm_list) {
> > +		if (kvm == vm)
> > +			return false;
> > +	}
> 
> I don't think this is enough. The struct kvm can be freed and re-allocated
> from the slab and this function will give false-negetive.

Right.

> 
> Maybe the kvm has to be tagged with a sequential id that incremented every
> allocation. This id can be checked here.

Sounds like a sequential id will be needed, no existing fields in struct
kvm can work for this.

> 
> > +
> > +	return true;
> > +}
> 
> -- 
>  Kirill A. Shutemov