mm_fork_context() currently maps shared chdev mappings 1:1 when forking
a range. The issue with this approach is that chdev mappings are not
guaranteed to be backed by physical memory, and thus cannot safely be
refcounted. This can lead to issues on devices with more complex buffer
lifetimes.
To allow for internal book-keeping, do not map shared chdev memory on
fork and instead allow it to go through the entire fault-in process
again.
Matheus