Understanding the Linux Kernel

7.3. Memory Regions

Linux implements memory regions by means of descriptors of type vm_area_struct:

struct vm_area_struct { 
    struct mm_struct * vm_mm; 
    unsigned long vm_start; 
    unsigned long vm_end; 
    struct vm_area_struct *vm_next; 
    pgprot_t vm_page_prot; 
    unsigned short vm_flags; 
    short vm_avl_height; 
    struct vm_area_struct *vm_avl_left, *vm_avl_right; 
    struct vm_area_struct *vm_next_share, **vm_pprev_share; 
    struct vm_operations_struct * vm_ops; 
    unsigned long vm_offset; 
    struct file * vm_file; 
    unsigned long vm_pte; 
};

Each memory region descriptor identifies a linear address interval. The vm_start field contains the first linear address of the interval, while the vm_end field contains the first linear address outside of the interval; vm_end - vm_start thus denotes the length of the memory region. The vm_mm field points to the mm_struct memory descriptor of the process that owns the region. We shall describe the other fields of vm_area_struct later.

Memory regions owned by a process never overlap, and the kernel tries to merge regions when a new one is allocated right next to an existing one. Two adjacent regions can be merged if their access rights match.

As shown in Figure 7-1, when a new range of linear addresses is added to the process address space, the kernel checks whether an already existing memory region can be enlarged (case a). If not, a new memory region is created (case b). Similarly, if a range of linear addresses is removed from the process address space, the kernel resizes ...