The virtual memory subsystem of Linux is, without any doubt, the most complex and performance-critical component of the whole kernel.
In previous chapters, we explained how the kernel handles dynamic memory by keeping track of free and busy page frames. We have also discussed how every process in User Mode has its own linear address space so that page frames can be assigned to the process at the very last possible moment. Finally, we have also described how dynamic memory is used to cache the data of the slow block devices.
In this chapter, we complete our description of the virtual memory subsystem by discussing page frame reclaiming. As we saw in Chapter 14, the cache systems grab more and more page frames but never release any of them. This is reasonable because cache systems don’t know if and when processes will reuse some of the cached data and are therefore unable to identify the portions of cache that should be released. Moreover, thanks to the demand paging mechanism described in Chapter 8, User Mode processes get page frames as long as they proceed with their execution; however, demand paging has no way to force processes to release the page frames whenever they are no longer used. Page frame reclaiming is a remedy for this problem.
The kernel developers’ worst nightmare is to encounter a situation in which no free page frame exists. When this happens, the kernel might be easily trapped in a deadly chain of memory requests—to free a page frame, ...