I'd just like to mention that the is_vmalloc_addr() test depends only on the virtual address, so can be done when the memory is queued, before the grace period. That is, you could have kfree_rcu(), vfree_rcu(), and a kvfree_rcu() function which decides between them. One advantage of that would be that you could put the vfree() sanity checks into vfree_rcu(), so errors would have a useful backtrace, and just do the __vunmap when the grace period expires. Or you could build a mixed queue and do the is_vmalloc_addr() test at the end of the grace period. That was my original idea, because it avoids the need for a separate queue. (In the _rcu_bulk_data structure, you can do a quicksort-style partitioning on is_vmalloc_addr(), so the kfree_bulk() pointers are all contiguous.) Just an implementation option.
