The Linux kernel operates in a virtual memorymode: for every virtual page there is a corresponding physical page of memory in the system.
Linux 内核工作于虚拟内存模式：每一个虚拟页对应一个相应的系统内存的物理页。
2
The whole user modepage doesn't need to be locked in memory as with Direct I/O, which is the plus side of this.
整个用户态的页不需要再像直接访问输入输出那样被锁定，这是它的好的方面。
3
A set of kernel mode procedures that executes as the result of a page fault. The pager and the image activator provide the operating system's memory management functions.