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Linux Tutorial - The Operating System - The Kernel - Processes - Creating a Process
  Scheduling in Multiprocessor Systems ---- Executing Programs  

Creating a Process

When the system starts up it is running in kernel mode and there is, in a sense, only one process, the initial process. Like all processes, the initial process has a machine state represented by stacks, registers and so on. These will be saved in the initial process's task_struct data structure when other processes in the system are created and run. At the end of system initialization, the initial process starts up a kernel thread (called init) and then sits in an idle loop doing nothing. Whenever there is nothing else to do the scheduler will run this, idle, process. The idle process's task_struct is the only one that is not dynamically allocated, it is statically defined at kernel build time and is, rather confusingly, called init_task.

The init kernel thread or process has a process identifier of 1 as it is the system's first real process. It does some initial setting up of the system (such as opening the system console and mounting the root file system) and then executes the system initialization program. This is one of /etc/init, /bin/init or /sbin/init depending on your system. The init program uses /etc/inittab as a script file to create new processes within the system. These new processes may themselves go on to create new processes. For example the getty process may create a login process when a user attempts to login. All of the processes in the system are descended from the init kernel thread.

New processes are created by cloning old processes, or rather by cloning the current process. A new task is created by a system call (fork or clone)

and the cloning happens within the kernel in kernel mode. At the end of the system call there is a new process waiting to run once the scheduler chooses it. A new task_struct data structure is allocated from the system's physical memory with one or more physical pages for the cloned process's stacks (user and kernel). A new process identifier may be created, one that is unique within the set of process identifiers in the system. However, it is perfectly reasonable for the cloned process to keep its parents process identifier. The new task_struct is entered into the task vector and the contents of the old (current) process's task_struct are copied into the cloned task_struct.

When cloning processes Linux allows the two processes to share resources rather than have two separate copies. This applies to the process's files, signal handlers and virtual memory. When the resources are to be shared their respective count fields are incremented so that Linux will not deallocate these resources until both processes have finished using them. So, for example, if the cloned process is to share virtual memory, its task_struct will contain a pointer to the mm_struct of the original process and that mm_struct has its count field incremented to show the number of current processes sharing it.

Cloning a process's virtual memory is rather tricky. A new set of vm_area_struct data structures must be generated together with their owning mm_struct data structure and the cloned process's page tables. None of the process's virtual memory is copied at this point. That would be a rather difficult and lengthy task for some of that virtual memory would be in physical memory, some in the executable image that the process is currently executing and possibly some would be in the swap file. Instead Linux uses a technique called ``copy on write'' which means that virtual memory will only be copied when one of the two processes tries to write to it. Any virtual memory that is not written to, even if it can be, will be shared between the two processes without any harm occuring. The read only memory, for example the executable code, will always be shared. For ``copy on write'' to work, the writeable areas have their page table entries marked as read only and the vm_area_struct data structures describing them are marked as ``copy on write''. When one of the processes attempts to write to this virtual memory a page fault will occur. It is at this point that Linux will make a copy of the memory and fix up the two processes' page tables and virtual memory data structures.

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