Understanding page frames and pages

Memory in Linux is organized in the form of pages (typically 4 KB in size). Contiguous linear addresses within a page are mapped onto contiguous physical addresses on the RAM chip. However contiguous pages can be present anywhere on the physical RAM. Access rights and physical address mapping in the kernel is done at a page level rather than for every linear address. A page refers both to the set of linear addresses that it contains as well as to the data contained in this group of addresses.


The paging unit thinks of all physical RAM as partitioned into fixed-length page frames. Each page frame contains a page. A page frame is a constituent of main memory, and hence it is a storage area. It is important to distinguish a page from a page frame; the former is just a block of data, which may be stored in any page frame or on disk. The paging unit translates linear addresses into physical ones. One key task in the unit is to check the requested access type against the access rights of the linear address. If the memory access is not valid, it generates a Page Fault exception (see Chapter 4 and Chapter 8). The data structures that map linear to physical addresses are called page tables ; they are stored in main memory and must be properly initialized by the kernel before enabling the paging unit.


Pages can optionally be 4 MB in size. However this is not advised except for applications where the expected data unit is large.


The kernel considers the following page frames as reserved:


Those falling in the unavailable physical address ranges
Those containing the kernel's code and initialized data structures
A page contained in a reserved page frame can never be dynamically assigned or swapped to disk. As a general rule, the Linux kernel is installed in RAM starting from the physical address 0x00100000 i.e., from the second megabyte. The total number of page frames required depends on how the kernel is configured. A typical configuration yields a kernel that can be loaded in less than 3 MB of RAM


The remaining portion of the RAM barring the reserved page frames is called dynamic memory. It is a valuable resource, needed not only by the processes but also by the kernel itself. In fact, the performance of the entire system depends on how efficiently dynamic memory is managed. Therefore, all current multitasking operating systems try to optimize the use of dynamic memory, assigning it only when it is needed and freeing it as soon as possible.


The kernel must keep track of the current status of each page frame. For instance, it must be able to distinguish the page frames that are used to contain pages that belong to processes from those that contain kernel code or kernel data structures. Similarly, it must be able to determine whether a page frame in dynamic memory is free. A page frame in dynamic memory is free if it does not contain any useful data. It is not free when the page frame contains data of a User Mode process, data of a software cache, dynamically allocated kernel data structures, buffered data of a device driver, code of a kernel module, and so on