Most tracing GC solutions would run into the same problem. A common implementation strategy of the classic three-color collector puts the grey bit in the objec

Most tracing GC solutions would run into the same problem. A common implementation strategy of the classic three-color collector puts the grey bit in the object header (and often also the white/black bit). A two finger (Cheney-style) collector avoids an explicit grey bit, but moves objects around, which also triggers CoW of shared pages. If you want CoW-friendly GC, you need to move your color bits / reference counts to be all together in the headers of your GC arenas instead of being in the object headers. That way, your high mutation bits are all together in a small number of pages. Those pages with the color bits/counters end up not being shared across processes, but at least they're all packed together to minimize the number of affected pages.