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Linux Tutorial - The Operating System - The Kernel - Memory Management - Memory Mapping
  Page Allocation and Deallocation ---- The Linux Page Cache  

Memory Mapping

When an image is executed, the contents of the executable image must be brought into the process' virtual address space. The same is also true of any shared libraries that the executable image has been linked to use. The executable file is not actually brought into physical memory, instead it is merely linked into the process' virtual memory. Then, as the parts of the program are referenced by the running application, the image is brought into memory from the executable image. This linking of an image into a process' virtual address space is known as memory mapping.

Figure: Areas of Virtual Memory

Every process' virtual memory is represented by an mm_struct data structure. This contains information about the image that it is currently executing (for example bash) and also has pointers to a number of vm_area_struct data structures. Each vm_area_struct data structure describes the start and end of the area of virtual memory, the process' access rights to that memory and a set of operations for that memory. These operations are a set of routines that Linux must use when manipulating this area of virtual memory. For example, one of the virtual memory operations performs the correct actions when the process has attempted to access this virtual memory but finds (via a page fault) that the memory is not actually in physical memory. This operation is the nopage operation. The nopage operation is used when Linux demand pages the pages of an executable image into memory.

When an executable image is mapped into a process' virtual address a set of vm_area_struct data structures is generated. Each vm_area_struct data structure represents a part of the executable image; the executable code, initialized data (variables), unintialized data and so on. Linux supports a number of standard virtual memory operations and, as the vm_area_struct data structures are created, the correct set of virtual memory operations are associated with them.

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