segment & vma

references:

1. Process Segments and VMA
2. Anatomy of a Program in Memory

Each virtual memory area (VMA) is a contiguous range of virtual addresses; these areas never overlap. An instance of vm_area_struct fully describes a memory area, including its start and end addresses, flags to determine access rights and behaviors, and the vm_file field to specify which file is being mapped by the area, if any. A VMA that does not map a file is anonymous. Each memory segment above (e.g., heap, stack) corresponds to a single VMA, with the exception of the memory mapping segment. This is not a requirement, though it is usual in x86 machines. VMAs do not care which segment they are in.

segments

The VMAs are stored in struct vm_area_struct defined in linux/mm.h:

struct vm_area_struct {
        struct mm_struct * vm_mm;       /* The address space we belong to. */
        unsigned long vm_start;         /* Our start address within vm_mm. */
        unsigned long vm_end;           /* The first byte after our end address
                                           within vm_mm. */
        ....
        ....
        ....
        /* linked list of VM areas per task, sorted by address */
        struct vm_area_struct *vm_next;
        struct file *vm_file;
        ....
        ....
} 

The kernel keeps track of the segments which have been allocated to a particular process using the above structures. For each segment, the kernel allocates a VMA. It keeps track of these segments in the mm_struct structures. The kernel tracks the data segment using two variables: start_data and end_data. The code segment boundaries are in the start_code and end_code variables. The stack segment is covered by the single variable start_stack. There is no special variable to keep track of the BSS segment — the VMA corresponding to the BSS accounts for it.