Since programs are only permitted read and or modify addresses within allocations handed out by malloc, they are oblivious to the merge process as pieces of ano

Since programs are only permitted read and or modify addresses within allocations handed out by malloc, they are oblivious to the merge process as pieces of another page are copied to virtual addresses near an existing allocation in a page they are accessing So long as mprotect() is used to ensure the source page is stable for the duration of the copy and until the mremap() is complete, it should not be possible for the program to observe any difference in behaviour -- except perhaps if it tries to modify an allocation on a page that is currently being merged by malloc() running in another thread (presumably they have some kind of SIGSEGV handler to catch this)