linux内核中的文件描述符(六)--fd的分配--expand_files

Kernel version：2.6.14

CPU architecture：ARM920T

Author：ce123(http://blog.csdn.net/ce123)

我们先贴出expand_files函数的源码：

int expand_files(struct files_struct *files, int nr)
{
	int err, expand = 0;
	struct fdtable *fdt;

	fdt = files_fdtable(files);
	if (nr >= fdt->max_fdset || nr >= fdt->max_fds) {   //我们在前面的文章中已经分析过，初始时max_fdset = 1024，max_fds = 32
		if (fdt->max_fdset >= NR_OPEN ||   //#define NR_OPEN (1024*1024)	/* Absolute upper limit on fd num */
			fdt->max_fds >= NR_OPEN || nr >= NR_OPEN) {
			err = -EMFILE;  //max_fdset和max_fds都不能大于 NR_OPEN，否则返回 -EMFILE，即打开太多的文件

			goto out;
		}
		expand = 1;
		if ((err = expand_fdtable(files, nr)))//真正进行扩展
			goto out;
	}
	err = expand;
out:
	return err;
}

expand_files函数进行一些检查后调用expand_fdtable进行文件描述符表的扩展，下面分析expand_fdtable函数。

static int expand_fdtable(struct files_struct *files, int nr)
	__releases(files->file_lock)
	__acquires(files->file_lock)
{
	int error = 0;
	struct fdtable *fdt;
	struct fdtable *nfdt = NULL;

	spin_unlock(&files->file_lock);
	nfdt = alloc_fdtable(nr);//根据nr重新创建一个新的fdtable
	if (!nfdt) {
		error = -ENOMEM;
		spin_lock(&files->file_lock);
		goto out;
	}

	spin_lock(&files->file_lock);
	fdt = files_fdtable(files);
	/*
	 * Check again since another task may have expanded the
	 * fd table while we dropped the lock
	 */
	if (nr >= fdt->max_fds || nr >= fdt->max_fdset) { //nr值必须大于max_fds和max_fdset值，这里再次进行检查是防止另一个进程进行了expand
		copy_fdtable(nfdt, fdt); //将旧的fdtable中的内容拷贝至新的fdtable
	} else {
		/* Somebody expanded while we dropped file_lock */
		spin_unlock(&files->file_lock);
		__free_fdtable(nfdt);
		spin_lock(&files->file_lock);
		goto out;
	}
	rcu_assign_pointer(files->fdt, nfdt);//用新的fdtable替换旧的fdtable  
	free_fdtable(fdt);//释放旧的fdtable 
out:
	return error;
}

static struct fdtable *alloc_fdtable(int nr)
{
	struct fdtable *fdt = NULL;
	int nfds = 0;
  	fd_set *new_openset = NULL, *new_execset = NULL;
	struct file **new_fds;

	fdt = kmalloc(sizeof(*fdt), GFP_KERNEL);
	if (!fdt)
  		goto out;
	memset(fdt, 0, sizeof(*fdt));

	nfds = __FD_SETSIZE;  //#define __FD_SETSIZE	1024
　　　　		      //  #define PAGE_SHIFT		12
　　　　		      //  #define PAGE_SIZE		(1UL <<