Understanding the Linux Kernel, Second Edition

Noncontiguous Memory Area Management

We already know that it is preferable to map memory areas into sets of contiguous page frames, thus making better use of the cache and achieving lower average memory access times. Nevertheless, if the requests for memory areas are infrequent, it makes sense to consider an allocation schema based on noncontiguous page frames accessed through contiguous linear addresses. The main advantage of this schema is to avoid external fragmentation, while the disadvantage is that it is necessary to fiddle with the kernel Page Tables. Clearly, the size of a noncontiguous memory area must be a multiple of 4,096. Linux uses noncontiguous memory areas in several ways — for instance, to allocate data structures for active swap areas (see Section 16.2.3), to allocate space for a module (see Appendix B), or to allocate buffers to some I/O drivers.

Linear Addresses of Noncontiguous Memory Areas

To find a free range of linear addresses, we can look in the area starting from PAGE_OFFSET (usually 0xc0000000, the beginning of the fourth gigabyte). Figure 7-7 shows how the fourth gigabyte linear addresses are used:

• The beginning of the area includes the linear addresses that map the first 896 MB of RAM (see Section 2.5.4); the linear address that corresponds to the end of the directly mapped physical memory is stored in the high_memory variable.

• The end of the area contains the fix-mapped linear addresses (see Section 2.5.6).

• Starting from PKMAP_BASE (0xfe000000