> If we don't flush the smp tlb don't we risk that we'll insert pages in
> the unstable tree that are volatile just because the dirty bit didn't
> get set again on the spte?

Yes, this is the trade off we take, the unstable tree will be flushed 
anyway -
so this is nothing that won`t be recovered very soon after it happen...

and most of the chances the tlb will be flushed before ksm get there anyway
(specially for heavily modified page, that we don`t want in the unstable 
tree)