上次讲了Linux进程描述符，这次我们在讲讲内存描述符，Linux的内存描述符也用一个结构体来表示，具体是mm_struct结构体，在linux/mm_types.h里定义。Linux操作系统就是用这个结构体实现内存的管理，我们先看一下具体的字段情况。
struct mm_struct {
//拥有该结构体的进程的虚拟地址空间的大小
unsigned long task_size;
/* if non-zero, the largest hole below free_area_cache */
unsigned long cached_hole_size;
/* first hole of size cached_hole_size or larger */
unsigned long free_area_cache;
//在用户空间下有多少用户使用该内存
atomic_t mm_users; /* How many users with user space? */
//该结构体的引用计数值
atomic_t mm_count;
/*又出现了list_head,详情请参考，简单来讲，list_head和list_for_each、list_entry共同实现了其“包含体”的链表。*/
struct list_head mmlist;
/*以下是关于代码段、数据段，栈、环境变量、参数的起始地址或结束地址等*/
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;
/*
* 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;
};
该结构体可以通过进程描述符来得到，比如current是当前进程的描述符，则current->mm是指向内存的指针。mm是指向struct mm_struct的指针。若还想对Linux的进程描述符有一定了解，可以参考task_struct。