Handle copy_from_user in the kernel

[beowulf]

Some kernel implementations require user-space accesses (e.g., any functions use copy_from_user or copy_to_user. But in popcorn, the user-page might not be at the node so that the page might be brought from remote through the memory consistency protocol. This is getting problematic especially for writev, which is used by fprintf. Thus, need to handle this in a concrete way.

[beowulf]

This issue might be happened when you try to run ps or top-like application; they access other processes' cmdline (command line) and arguments, and the page containing them might be away from the node. In this case, get_user_page() will make a nested page fault, dropping the "page server panic" dump. But the system should keep working except for the ps or top.

[acarno]

Just an additional note: ps appears to execute whenever you SSH into the system, so try to avoid doing so whenever you're running experiments!