内核实现mmap的关键点

mmap系统调用在内核的入口是sys_mmap_pgoff，主要实现在do_mmap_pgoff函数里。 sys_mmap_pgoff的调用关系如下：
sys_mmap_pgoff->do_mmap_pgoff ->get_unmapped_area
                                                    ->mmap_region

SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
        unsigned long, prot, unsigned long, flags,
        unsigned long, fd, unsigned long, pgoff)
{


    down_write(¤t->mm->mmap_sem);
    retval = do_mmap_pgoff(file, addr, len, prot, flags, pgoff);
    up_write(¤t->mm->mmap_sem);

}

do_mmap_pgoff函数有两个关键点：
一个是get_unmapped_area函数
一个是mmap_region函数
unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
            unsigned long len, unsigned long prot,
            unsigned long flags, unsigned long pgoff)
{
....
    /* Obtain the address to map to. we verify (or select) it and ensure
     * that it represents a valid section of the address space.
     */
    addr = get_unmapped_area(file, addr, len, pgoff, flags | ((prot & PROT_EXEC) ? MAP_EXECUTABLE : 0));
    if (addr & ~PAGE_MASK)
        return addr;

.....

    return mmap_region(file, addr, len, flags, vm_flags, pgoff);

}