Understanding the Linux Kernel

6.1. Page Frame Management

We saw in Section 2.4 in Chapter 2 how the Intel Pentium processor can use two different page frame sizes: 4 KB and 4 MB. Linux adopts the smaller 4 KB page frame size as the standard memory allocation unit. This makes things simpler for two reasons:

• The paging circuitry automatically checks whether the page being addressed is contained in some page frame; furthermore, each page frame is hardware-protected through the flags included in the Page Table entry that points to it. By choosing a 4 KB allocation unit, the kernel can directly determine the memory allocation unit associated with the page where a page fault exception occurs.

• The 4 KB size is a multiple of most disk block sizes, so transfers of data between main memory and disks are more efficient. Yet this smaller size is much more manageable than the 4 MB size.

The kernel must keep track of the current status of each page frame. For instance, it must be able to distinguish the page frames used to contain pages belonging 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 or not. This sort of state information is kept in an array of descriptors, one for each page frame. The descriptors of type struct page have the following format:

typedef struct page { struct page *next; struct page *prev; struct inode *inode; unsigned long offset; struct page *next_hash; atomic_t count; unsigned long ...