I really shouldn't break COW if you don't request LOCAL_WRITE.
I think it's a kernel bug, and apparently has been there in the code since the
first version: get_user_pages parameters swapped.
I'll send a patch. If it's applied, you should also
change your code from
+ IBV_ACCESS_LOCAL_WRITE |
+ IBV_ACCESS_REMOTE_WRITE |
+ IBV_ACCESS_REMOTE_READ);
to
+ IBV_ACCESS_REMOTE_READ);
on send side.
Then, each time we detect a page has changed we must make sure to
unregister and re-register it. Or if you want to be very
smart, check that the PFN didn't change and reregister
if it did.
This will make overcommit work.