Some blocking issues in implementing JOS

[zhongshu]

When I implement the fork in lab4. There is a step for "The
parent sets the user page fault entrypoint for the child to look like
its own.". But how to copy the _pgfault_upcall of parent environment
to the child environment ? set_pgfault_handler() can not be used as it
will not define the envid of the environment that need this page fault
handler. And call sys_env_set_pgfault_upcall() directly also don't
work.
I am also confused when I am not sure when the sys_exofork
will return the value of the child environment and when will the child
being executed by forktree. I copied the dumbfork and its duppage to
lib/fork execute the forktree. It works well.But it doesn't work if
using COW.
And the third issue is that if I enable the clock interrupt,
the fork will never return as a child because the child can not set
runnable when this interrupt begin. And at that time, it will only
execute the only runnable environment of the parent env. How to solve
this problem?

Thanks