Re: [PATCH V3 2/2] mm: Extend memory hotplug API to allow memory hotplug in virtual machines

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 May 2011 22:45:09 +0200
Daniel Kiper <dkiper@net-space.pl> wrote:

> On Wed, May 18, 2011 at 08:36:02PM -0700, David Rientjes wrote:
> > On Tue, 17 May 2011, Daniel Kiper wrote:
> >
> > > This patch contains online_page_callback and apropriate functions for
> > > setting/restoring online page callbacks. It allows to do some machine
> > > specific tasks during online page stage which is required to implement
> > > memory hotplug in virtual machines. Additionally, __online_page_set_limits(),
> > > __online_page_increment_counters() and __online_page_free() function
> > > was added to ease generic hotplug operation.
> >
> > There are several issues with this.
> >
> > First, this is completely racy and only allows one global callback to be
> > in use at a time without looping, which is probably why you had to pass an
> 
> One callback is allowed by design. Currently I do not see
> any real usage for more than one callback.

I'd suggest that you try using the notifier.h tools here and remove the
restriction.  Sure, we may never use the capability but I expect the
code will look nice and simple and once it's done, it's done.