進程僅僅能訪問合法的地址空間，假設一個進程訪問了不合法的地址空間。內核就會終止該進程。並返回“段錯誤”。

虛擬內存的合法地址空間在哪而呢？我們先來看看進程虛擬地址空間的劃分：

struct mm_struct {
    struct vm_area_struct * mmap;        /* list of VMAs */
    struct rb_root mm_rb;
    struct vm_area_struct * mmap_cache;    /* last find_vma result */
    unsigned long (*get_unmapped_area) (struct file *filp,
                unsigned long addr, unsigned long len,
                unsigned long pgoff, unsigned long flags);
    void (*unmap_area) (struct mm_struct *mm, unsigned long addr);
    unsigned long mmap_base;        /* base of mmap area */
    unsigned long task_size;        /* size of task vm space */
    unsigned long cached_hole_size;     /* if non-zero, the largest hole below free_area_cache */
    unsigned long free_area_cache;        /* first hole of size cached_hole_size or larger */
    pgd_t * pgd;
    atomic_t mm_users;            /* How many users with user space? */
    atomic_t mm_count;            /* How many references to "struct mm_struct" (users count as 1) */
    int map_count;                /* number of VMAs */
    struct rw_semaphore mmap_sem;
    spinlock_t page_table_lock;        /* Protects page tables and some counters */

    struct list_head mmlist;        /* List of maybe swapped mm‘s.    These are globally strung
                         * together off init_mm.mmlist, and are protected
                         * by mmlist_lock
                         */

    /* Special counters, in some configurations protected by the
     * page_table_lock, in other configurations by being atomic.
     */
    mm_counter_t _file_rss;
    mm_counter_t _anon_rss;

    unsigned long hiwater_rss;    /* High-watermark of RSS usage */
    unsigned long hiwater_vm;    /* High-water virtual memory usage */

    unsigned long total_vm, locked_vm, shared_vm, exec_vm;
    unsigned long stack_vm, reserved_vm, def_flags, nr_ptes;
    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 saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */

    struct linux_binfmt *binfmt;

    cpumask_t cpu_vm_mask;

    /* Architecture-specific MM context */
    mm_context_t context;

    /* Swap token stuff */
    /*
     * Last value of global fault stamp as seen by this process.
     * In other words, this value gives an indication of how long
     * it has been since this task got the token.
     * Look at mm/thrash.c
     */
    unsigned int faultstamp;
    unsigned int token_priority;
    unsigned int last_interval;

    unsigned long flags; /* Must use atomic bitops to access the bits */

    struct core_state *core_state; /* coredumping support */
#ifdef CONFIG_AIO
    spinlock_t        ioctx_lock;
    struct hlist_head    ioctx_list;
#endif
#ifdef CONFIG_MM_OWNER
    /*
     * "owner" points to a task that is regarded as the canonical
     * user/owner of this mm. All of the following must be true in
     * order for it to be changed:
     *
     * current == mm->owner
     * current->mm != mm
     * new_owner->mm == mm
     * new_owner->alloc_lock is held
     */
    struct task_struct *owner;
#endif

#ifdef CONFIG_PROC_FS
    /* store ref to file /proc/<pid>/exe symlink points to */
    struct file *exe_file;
    unsigned long num_exe_file_vmas;
#endif
#ifdef CONFIG_MMU_NOTIFIER
    struct mmu_notifier_mm *mmu_notifier_mm;
#endif
};

/*
 * This struct defines a memory VMM memory area. There is one of these
 * per VM-area/task.  A VM area is any part of the process virtual memory
 * space that has a special rule for the page-fault handlers (ie a shared
 * library, the executable area etc).
 */
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;

    pgprot_t vm_page_prot;        /* Access permissions of this VMA. */
    unsigned long vm_flags;        /* Flags, see mm.h. */

    struct rb_node vm_rb;

    /*
     * For areas with an address space and backing store,
     * linkage into the address_space->i_mmap prio tree, or
     * linkage to the list of like vmas hanging off its node, or
     * linkage of vma in the address_space->i_mmap_nonlinear list.
     */
    union {
        struct {
            struct list_head list;
            void *parent;    /* aligns with prio_tree_node parent */
            struct vm_area_struct *head;
        } vm_set;

        struct raw_prio_tree_node prio_tree_node;
    } shared;

    /*
     * A file‘s MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
     * list, after a COW of one of the file pages.    A MAP_SHARED vma
     * can only be in the i_mmap tree.  An anonymous MAP_PRIVATE, stack
     * or brk vma (with NULL file) can only be in an anon_vma list.
     */
    struct list_head anon_vma_node;    /* Serialized by anon_vma->lock */
    struct anon_vma *anon_vma;    /* Serialized by page_table_lock */

    /* Function pointers to deal with this struct. */
    const struct vm_operations_struct *vm_ops;

    /* Information about our backing store: */
    unsigned long vm_pgoff;        /* Offset (within vm_file) in PAGE_SIZE
                       units, *not* PAGE_CACHE_SIZE */
    struct file * vm_file;        /* File we map to (can be NULL). */
    void * vm_private_data;        /* was vm_pte (shared mem) */
    unsigned long vm_truncate_count;/* truncate_count or restart_addr */

#ifndef CONFIG_MMU
    struct vm_region *vm_region;    /* NOMMU mapping region */
#endif
#ifdef CONFIG_NUMA
    struct mempolicy *vm_policy;    /* NUMA policy for the VMA */
#endif
};

# cat /proc/17192/maps     //顯示該進程地址空間中所有內存區域
001e3000-00201000 r-xp 00000000 fd:00 789547     /lib/ld-2.12.so
00201000-00202000 r--p 0001d000 fd:00 789547     /lib/ld-2.12.so
00202000-00203000 rw-p 0001e000 fd:00 789547     /lib/ld-2.12.so
00209000-00399000 r-xp 00000000 fd:00 789548     /lib/libc-2.12.so
00399000-0039a000 ---p 00190000 fd:00 789548     /lib/libc-2.12.so
0039a000-0039c000 r--p 00190000 fd:00 789548     /lib/libc-2.12.so
0039c000-0039d000 rw-p 00192000 fd:00 789548     /lib/libc-2.12.so
0039d000-003a0000 rw-p 00000000 00:00 0
08048000-08049000 r-xp 00000000 fd:00 1191771    /home/allen/Myprojects/blog/conn_user_kernel/test/a.out
08049000-0804a000 rw-p 00000000 fd:00 1191771    /home/allen/Myprojects/blog/conn_user_kernel/test/a.out
b7755000-b7756000 rw-p 00000000 00:00 0
b776d000-b776e000 rw-p 00000000 00:00 0
b776e000-b776f000 r-xp 00000000 00:00 0          [vdso]
bfc9f000-bfcb4000 rw-p 00000000 00:00 0          [stack]
# 

# pmap 17192
17192:   ./a.out
001e3000    120K r-x--  /lib/ld-2.12.so    //本行和以下兩行為動態鏈接程序ld.so的代碼段、數據段、bss段
00201000      4K r----  /lib/ld-2.12.so
00202000      4K rw---  /lib/ld-2.12.so
00209000   1600K r-x--  /lib/libc-2.12.so    //本行和以下為C庫中libc.so的代碼段、數據段和bss段
00399000      4K -----  /lib/libc-2.12.so
0039a000      8K r----  /lib/libc-2.12.so
0039c000      4K rw---  /lib/libc-2.12.so
0039d000     12K rw---    [ anon ]
08048000      4K r-x--  /home/allen/Myprojects/blog/conn_user_kernel/test/a.out    //可運行對象的代碼段
08049000      4K rw---  /home/allen/Myprojects/blog/conn_user_kernel/test/a.out    //可運行對象的數據段
b7755000      4K rw---    [ anon ]
b776d000      4K rw---    [ anon ]
b776e000      4K r-x--    [ anon ]
bfc9f000     84K rw---    [ stack ]    //堆棧段
 total     1860K

/*
 * These are the virtual MM functions - opening of an area, closing and
 * unmapping it (needed to keep files on disk up-to-date etc), pointer
 * to the functions called when a no-page or a wp-page exception occurs.
 */
struct vm_operations_struct {
    void (*open)(struct vm_area_struct * area);    //指定內存區域被載入到一個地址空間時函數被調用
    void (*close)(struct vm_area_struct * area);    //指定內存區域從地址空間刪除時函數被調用
    int (*fault)(struct vm_area_struct *vma, struct vm_fault *vmf); //沒有出如今物理內存中的頁面被訪問時，頁面故障處理調用該函數

    /* notification that a previously read-only page is about to become
     * writable, if an error is returned it will cause a SIGBUS */
    int (*page_mkwrite)(struct vm_area_struct *vma, struct vm_fault *vmf);

    /* called by access_process_vm when get_user_pages() fails, typically
     * for use by special VMAs that can switch between memory and hardware
     */
    int (*access)(struct vm_area_struct *vma, unsigned long addr,
              void *buf, int len, int write);
#ifdef CONFIG_NUMA
    ......
#endif
};

/* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
{
    struct vm_area_struct *vma = NULL;

    if (mm) {
        /* 首先檢查近期使用的內存區域，看緩存的VMA是否包括所需地址 */
        /* (命中錄接近35%.) */
        vma = mm->mmap_cache;
        //假設緩存中不包括未包括希望的VMA，該函數搜索紅-黑樹。
        if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
             struct rb_node * rb_node;

            rb_node = mm->mm_rb.rb_node;
            vma = NULL;

            while (rb_node) {
                struct vm_area_struct * vma_tmp;

                vma_tmp = rb_entry(rb_node,
                        struct vm_area_struct, vm_rb);

                if (vma_tmp->vm_end > addr) {
                    vma = vma_tmp;
                    if (vma_tmp->vm_start <= addr)
                        break;
                    rb_node = rb_node->rb_left;
                } else
                    rb_node = rb_node->rb_right;
            }
            if (vma)
                mm->mmap_cache = vma;
        }
    }
    return vma;
}

static inline unsigned long do_mmap(struct file *file, unsigned long addr,
    unsigned long len, unsigned long prot,
    unsigned long flag, unsigned long offset)
{
    unsigned long ret = -EINVAL;
    if ((offset + PAGE_ALIGN(len)) < offset)
        goto out;
    if (!(offset & ~PAGE_MASK))
        ret = do_mmap_pgoff(file, addr, len, prot, flag, offset >> PAGE_SHIFT);
out:
    return ret;
}