On Wed, 15 Aug 2012 14:28:51 +0300, "Michael S. Tsirkin" [off-list ref] wrote:

On Wed, Aug 15, 2012 at 12:16:51PM +0100, Mel Gorman wrote:

quoted

I was thinking of exactly that page->mapping == balloon_mapping check. As I
do not know how many active balloon drivers there might be I cannot guess
in advance how much of a scalability problem it will be.

Not at all sure multiple drivers are worth supporting, but multiple
*devices* is I think worth supporting, if for no other reason than that
they can work today. For that, we need a device pointer which Rafael
wants to put into the mapping, this means multiple balloon mappings.

Rafael, please make sure that the balloon driver fails on the second and
subsequent balloon devices.

Michael, we only allow multiple balloon devices because it fell out of
the implementation.  If it causes us even the slightest issue, we should
not support it.  It's not a sensible setup.

Cheers,
Rusty.