fork.c

一份精简的linux内核源代码

/*
 *  linux/kernel/fork.c
 *
 *  (C) 1991  Linus Torvalds
 */

/*
 *  'fork.c' contains the help-routines for the 'fork' system call
 * (see also system_call.s), and some misc functions ('verify_area').
 * Fork is rather simple, once you get the hang of it, but the memory
 * management can be a bitch. See 'mm/mm.c': 'copy_page_tables()'
 */
#include <errno.h>

#include <linux/sched.h>
#include <linux/kernel.h>
#include <asm/segment.h>
#include <asm/system.h>

extern void write_verify(unsigned long address);

long last_pid=0;
/*特权级0的代码在执行时不会理会用户空间中的页面是否是页保护的*/
void verify_area(void * addr,int size)/*对add开始，长度为size的页面进行写验证*/
{
	unsigned long start;

	start = (unsigned long) addr;
	size += start & 0xfff;
	start &= 0xfffff000;
	start += get_base(current->ldt[2]);
	while (size>0) {
		size -= 4096;
		write_verify(start);/*页面写验证*/
		start += 4096;
	}
}

int copy_mem(int nr,struct task_struct * p)/*设置新进程的代码和数据段基址、限长、并复制当前进程内存分页管理的页表*/
{
	unsigned long old_data_base,new_data_base,data_limit;
	unsigned long old_code_base,new_code_base,code_limit;

	code_limit=get_limit(0x0f);
	data_limit=get_limit(0x17);
	old_code_base = get_base(current->ldt[1]);
	old_data_base = get_base(current->ldt[2]);
	if (old_data_base != old_code_base)
		panic("We don't support separate I&D");
	if (data_limit < code_limit)
		panic("Bad data_limit");
	new_data_base = new_code_base = nr * 0x4000000;
	p->start_code = new_code_base;
	set_base(p->ldt[1],new_code_base);
	set_base(p->ldt[2],new_data_base);
	if (copy_page_tables(old_data_base,new_data_base,data_limit)) {
		free_page_tables(new_data_base,data_limit);
		return -ENOMEM;
	}
	return 0;
}

/*
 *  Ok, this is the main fork-routine. It copies the system process
 * information (task[nr]) and sets up the necessary registers. It
 * also copies the data segment in it's entirety.
 */
int copy_process(int nr,long ebp,long edi,long esi,long gs,long none,
		long ebx,long ecx,long edx,
		long fs,long es,long ds,
		long eip,long cs,long eflags,long esp,long ss)
{
	struct task_struct *p;
	int i;
	struct file *f;

	p = (struct task_struct *) get_free_page();/*为新进程申请一页内存来存放其PCB*/
	if (!p)
		return -EAGAIN;
	task[nr] = p;
	*p = *current;	/*将当前进程的PCB复制给新建的进程*//* NOTE! this doesn't copy the supervisor stack */
	p->state = TASK_UNINTERRUPTIBLE;/*开始修改新进程的PCB，首先将新进程置为不可中断*/
	p->pid = last_pid;/*当前进程的下一个进程设为新进程*/
	p->father = current->pid;/*把当前进程设置为新进程的父进程*/
	p->counter = p->priority;/*设置初始的进程运行时间片*/
	p->signal = 0;
	p->alarm = 0;
	p->leader = 0;		/* process leadership doesn't inherit */
	p->utime = p->stime = 0;
	p->cutime = p->cstime = 0;
	p->start_time = jiffies;/*进程开始时间（从开机到当前为止的时间长度）*/
	p->tss.back_link = 0;
	p->tss.esp0 = PAGE_SIZE + (long) p;/*新进程的内核堆栈栈底指针，指向保存PCB(task_struct数据结构)页面的末端*/
	p->tss.ss0 = 0x10;                 /*新进程的内核堆栈段地址，被设为内核数据段选择符（0x10）*/
	p->tss.eip = eip;
	p->tss.eflags = eflags;
	p->tss.eax = 0;/*新进程的返回值*/
	p->tss.ecx = ecx;
	p->tss.edx = edx;
	p->tss.ebx = ebx;
	p->tss.esp = esp;/*新进程内核态堆栈被设为新进程PCB所在内存页面的顶端*/
	p->tss.ebp = ebp;
	p->tss.esi = esi;
	p->tss.edi = edi;
	p->tss.es = es & 0xffff;
	p->tss.cs = cs & 0xffff;
	p->tss.ss = ss & 0xffff;/*堆栈段被设为内核数据段选择符*/
	p->tss.ds = ds & 0xffff;
	p->tss.fs = fs & 0xffff;
	p->tss.gs = gs & 0xffff;
	p->tss.ldt = _LDT(nr);/*ldt被设置为局部表描述符在gdt中的索引值*/
	p->tss.trace_bitmap = 0x80000000;
	if (last_task_used_math == current)/*如果当前进程使用了协处理器，则还需要把协处理器的完整状态保存到新进程的tss.i386结构中*/
		__asm__("clts ; fnsave %0"::"m" (p->tss.i387));
	if (copy_mem(nr,p)) {
		task[nr] = NULL;
		free_page((long) p);
		return -EAGAIN;
	}
	for (i=0; i<NR_OPEN;i++)  /*子进程将打开父进程已打开的文件，故这里将文件计数加1*/
		if (f=p->filp[i])
			f->f_count++;
	if (current->pwd)/*更新当前进程工作目录所含的节点数*/
		current->pwd->i_count++;
	if (current->root)/*更新当前进程根目录所含的节点数*/
		current->root->i_count++;
	if (current->executable)/*更新当前进程打开文件的被复用次数*/
		current->executable->i_count++;
	set_tss_desc(gdt+(nr<<1)+FIRST_TSS_ENTRY,&(p->tss));/*在gdt中设置新任务的tss和ldt*/
	set_ldt_desc(gdt+(nr<<1)+FIRST_LDT_ENTRY,&(p->ldt));
	p->state = TASK_RUNNING;/*状态置为可运行*/	/* do this last, just in case */
	return last_pid;/*返回新进程的进程号*/
}

int find_empty_process(void)/*找新进程号*/
{
	int i;

	repeat:
		if ((++last_pid)<0) last_pid=1;
		for(i=0 ; i<NR_TASKS ; i++)
			if (task[i] && task[i]->pid == last_pid) goto repeat;
	for(i=1 ; i<NR_TASKS ; i++)
		if (!task[i])
			return i;
	return -EAGAIN;
}