Paging and Swaping

Linux implementation of virtual memory is the concept of a page.
A page is a 4Kb area of memory and is the basic unit of memory with which both the kernel and the CPU deal.

When a process attempts to access a virtual address that is not currently in memory, the processor cannot find a page table entry for the virtual page being referenced. At this point the processor notifies the operating system that a page fault has occurred.

If the faulting virtual address is invalid this means that the process has attempted to access a virtual address that it should not have. Maybe the application has gone wrong in some way,

If the faulting virtual address was valid but the page that it refers to is not currently in memory, the operating system must bring the appropriate page into memory. Linux uses demand paging to load executable images into a process's virtual memory.
   
If a process needs to bring a virtual page into physical memory and there are no free physical pages available, the operating system must make room for this page by discarding another page from physical memory. This is called Swaping.  

Linux uses a Least Recently Used (LRU) page aging technique to fairly choose pages which might be removed from the system. This scheme involves every page in the system having an age which changes as the page is accessed. The more that a page is accessed, the younger it is; the less that it is accessed, the older and more stale it becomes. Old pages are good candidates for swapping.