> Processes could create a page of memory with known contents then, after
> waiting for the checker process (or kernel thread) to run, see if memory
> usage has increased.
I don't believe that's a real security issue at all. Absolutely anything could run in the meantime, with memory being both freed and allocated by completely unrelated causes. At best, such processes would be able to say it was possible that the page was shared, which really says nothing.