UserfaultFD: Extension for non cooperative uffd usage

Hi,

On the recent LSF Andrea presented his userfault-fd patches and I had shown some issues that appear in usage scenarios when the monitor task and mm task do not cooperate to each other on VM changes (and fork()-s).

Here's the implementation of the extended uffd API that would help us to address those issues.

As proof of concept I've implemented only fix for fork() case, but I also plan to add the mremap() and exit() notifications, both are also required for such non-cooperative usage.

More details about the extension itself is in patch #2 and the fork() notification description is in patch #3.

Comments and suggestion are warmly welcome :)

Andrea, what's the best way to go on with the patches -- would you prefer to include them in your git tree or should I instead continue with them on my own, re-sending them when required? Either way would be OK for me.

Thanks,
Pavel