How to know whether a copy-on-write page is an actual copy?

Question

When I create a copy-on-write mapping (a MAP_PRIVATE) using mmap, then some pages of this mapping will be copied as soon as I write to specific addresses. At a certain point

Answer 1

I usually use mprotect to set my tracked copy-on-write pages to read-only, then handle the resulting SIGSEGVs by marking the given page dirty and enabling writing.

It isn't ideal, but the overhead is quite manageable and it can be used in combination with mincore, etc. to do more complicated optimizations, like manage your working set size or to approximate pointer information for pages you expect to have swap out, which lets the runtime system cooperate with the kernel rather than fight it.