linux内核源码分析之虚拟文件系统VFS

Posted 2022-04-05 为了维护世界和平_

系统调用

read()->ksys_read()

ssize_t ksys_read(unsigned int fd, char __user *buf, size_t count)

	struct fd f = fdget_pos(fd);//根据fd号从task的文件描述符中获取file指针
	ssize_t ret = -EBADF;

	if (f.file) 
		loff_t pos, *ppos = file_ppos(f.file);//找到文件当前读写位置
		if (ppos) 
			pos = *ppos;
			ppos = &pos;
		
		ret = vfs_read(f.file, buf, count, ppos);//执行file->f_op->read/->read_iter
		if (ret >= 0 && ppos)
			f.file->f_pos = pos;//更新读取位置
		fdput_pos(f);//
	
	return ret;


SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)

	return ksys_read(fd, buf, count);

vfs_read()->__vfs_read()

ssize_t __vfs_read(struct file *file, char __user *buf, size_t count,
		   loff_t *pos)

	if (file->f_op->read)
		return file->f_op->read(file, buf, count, pos);
	else if (file->f_op->read_iter)
		return new_sync_read(file, buf, count, pos);
	else
		return -EINVAL;

重要！！！结构体之间的关联

• vfs_read会调用file->f_op->read_iter函数。
• file->f_op是在文件打开时，在do_dentry_open中赋值为inode->i__fop
• inode->i_fop是在初始化inode时，在ext4_iget中赋值为&ext4_file_operations。

const struct file_operations ext2_file_operations = 
	.llseek		= generic_file_llseek,
	.read_iter	= ext2_file_read_iter,
	.write_iter	= ext2_file_write_iter,
	.unlocked_ioctl = ext2_ioctl,
#ifdef CONFIG_COMPAT
	.compat_ioctl	= ext2_compat_ioctl,
#endif
	.mmap		= ext2_file_mmap,
	.open		= dquot_file_open,
	.release	= ext2_release_file,
	.fsync		= ext2_fsync,
	.get_unmapped_area = thp_get_unmapped_area,
	.splice_read	= generic_file_splice_read,
	.splice_write	= iter_file_splice_write,
;

void ext2_set_file_ops(struct inode *inode)

	inode->i_op = &ext2_file_inode_operations;
	inode->i_fop = &ext2_file_operations;
	if (IS_DAX(inode))
		inode->i_mapping->a_ops = &ext2_dax_aops;
	else if (test_opt(inode->i_sb, NOBH))
		inode->i_mapping->a_ops = &ext2_nobh_aops;
	else
		inode->i_mapping->a_ops = &ext2_aops;

 参考链接

VFS(一) 虚拟文件系统概述 - 知乎 (zhihu.com)

VFS(二) 读文件的过程中发生了什么 - 知乎 (zhihu.com)

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