Understanding the Linux Kernel, 3rd Edition

Creating and Deleting a Process Address Space

Of the six typical cases mentioned earlier in the section "The Process’s Address Space,” in which a process gets new memory regions, the first one—issuing a fork( ) system call—requires the creation of a whole new address space for the child process. Conversely, when a process terminates, the kernel destroys its address space. In this section, we discuss how these two activities are performed by Linux.

Creating a Process Address Space

In the section "The clone( ), fork( ), and vfork( ) System Calls" in Chapter 3, we mentioned that the kernel invokes the copy_mm( ) function while creating a new process. This function creates the process address space by setting up all Page Tables and memory descriptors of the new process.

Each process usually has its own address space, but lightweight processes can be created by calling clone( ) with the CLONE_VM flag set. These processes share the same address space; that is, they are allowed to address the same set of pages.

Following the COW approach described earlier, traditional processes inherit the address space of their parent: pages stay shared as long as they are only read. When one of the processes attempts to write one of them, however, the page is duplicated; after some time, a forked process usually gets its own address space that is different from that of the parent process. Lightweight processes, on the other hand, use the address space of their parent process. Linux implements them simply ...