Re: [Qemu-devel] [RFC PATCH 1/1] ceph/rbd block driver for qemu-kvm

[Anthony Liguori]

On 05/25/2010 11:16 AM, Avi Kivity wrote:
> On 05/25/2010 06:01 PM, Anthony Liguori wrote:
> > On 05/25/2010 10:00 AM, Avi Kivity wrote:
> > > The latter.  Why is it less important?  If you don't inherit the 
> > > memory, you can't access it.
> > >
> > > > You can also pass /dev/shm fd's via SCM_RIGHTs to establish shared 
> > > > memory segments dynamically.
> > >
> > > Doesn't work for anonymous memory.
> >
> > What's wrong with /dev/shm memory?
>
> The kernel treats anonymous and nonymous memory differently for 
> swapping (see /proc/sys/vm/swappiness); transparent hugepages won't 
> work for /dev/shm (though it may be argued that that's a problem with 
> thp); setup (/dev/shm defaults to half memory IIRC, we want mem+swap); 
> different cgroup handling; somewhat clunky (a minor concern to be sure).

Surely, with mmu notifiers, it wouldn't be that hard to share anonymous 
memory via an fd though, no?

Regards,

Anthony Liguori

> Nothing is a killer, but we should prefer anonymous memory.