Understanding the Linux Kernel

7.2. The Memory Descriptor

All information related to the process address space is included in a table referenced by the mm field of the process descriptor. This table is a structure of type mm_struct as follows:

struct mm_struct { 
    struct vm_area_struct *mmap, *mmap_avl, *mmap_cache; 
    pgd_t * pgd; 
    atomic_t count; 
    int map_count; 
    struct semaphore mmap_sem; 
    unsigned long context; 
    unsigned long start_code, end_code, start_data, end_data; 
    unsigned long start_brk, brk, start_stack; 
    unsigned long arg_start, arg_end, env_start, env_end; 
    unsigned long rss, total_vm, locked_vm; 
    unsigned long def_flags; 
    unsigned long cpu_vm_mask; 
    unsigned long swap_cnt; 
    unsigned long swap_address; 
    void * segments; 
};

For the present discussion, the most important fields are:

pgd and segments

Point, respectively, to the Page Global Directory and Local Descriptor Table of the process.

rss

Specifies the number of page frames allocated to the process.

total_vm

Denotes the size of the process address space expressed as a number of pages.

locked_vm

Counts the number of "locked" pages, that is, pages that cannot be swapped out (see Chapter 16).

count

Denotes the number of processes that share the same struct mm_struct descriptor. If count is greater than 1, the processes are lightweight processes sharing the same address space, that is, using the same memory descriptor.

The mm_alloc( ) function is invoked to get a new memory descriptor. Since these descriptors are stored in a slab allocator cache, mm_alloc( ) ...