linux文件系统初探--Day6

今天的内容是关于简单的inode操作。

源码分析

今天的源码只涉及四个新函数，其中较关键的只有两个，分别是sfs_lookup和sfs_mknod。

#include <linux/module.h>
#include <linux/fs.h>
#include "samplefs.h"

extern struct dentry_operations sfs_dentry_ops;
extern struct dentry_operations sfs_ci_dentry_ops;
extern struct inode *samplefs_get_inode(struct super_block *sb, int mode, 
					dev_t dev);

/*
 * Lookup the data, if the dentry didn't already exist, it must be
 * negative.  Set d_op to delete negative dentries to save memory
 * (and since it does not help performance for in memory filesystem).
 */

static struct dentry *sfs_lookup(struct inode *dir, struct dentry *dentry, unsigned int len)
{
	struct samplefs_sb_info *sfs_sb = SFS_SB(dir->i_sb);
	if (dentry->d_name.len > NAME_MAX)
		return ERR_PTR(-ENAMETOOLONG);
	if (sfs_sb->flags & SFS_MNT_CASE)
		dentry->d_op = &sfs_ci_dentry_ops;
	else
		dentry->d_op = &sfs_dentry_ops;

	d_add(dentry, NULL);
	return NULL;
}

在inode_operations中的lookup函数主要作用是根据名称查找其inode实例，并将dentry->d_inode指向inode实例。在实际实验中，d_op一直为sfs_dentry_ops。

sfs_mknod

这个函数主要是新建一个inode并将其连接到dentry上，而mknod命令被用来创建Linux中的字符设备文件和块设备文件。

static int
sfs_mknod(struct inode *dir, struct dentry *dentry, umode_t mode, dev_t dev)
{
	struct inode *inode = samplefs_get_inode(dir->i_sb, mode, dev);
	struct timespec64 ts;
	int error = -ENOSPC;
	
	printk(KERN_INFO "samplefs: mknod
");
	if (inode) {
		if (dir->i_mode & S_ISGID) {
			inode->i_gid = dir->i_gid;
			if (S_ISDIR(mode))
				inode->i_mode |= S_ISGID;
		}
		d_instantiate(dentry, inode);
		dget(dentry);   /* Extra count - pin the dentry in core */
		error = 0;
		ktime_get_ts64(&ts);
		dir->i_mtime = dir->i_ctime = ts;

		/* real filesystems would normally use i_size_write function */
		dir->i_size += 0x20;  /* bogus small size for each dir entry */
	}
	return error;
}


static int sfs_mkdir(struct inode * dir, struct dentry * dentry, umode_t mode)
{
	int retval = 0;
	
	retval = sfs_mknod(dir, dentry, mode | S_IFDIR, 0);

	/* link count is two for dir, for dot and dot dot */
	if (!retval)
		dir->__i_nlink++;
	return retval;
}

static int sfs_create(struct inode *dir, struct dentry *dentry, umode_t mode,
			bool flag)
{
	return sfs_mknod(dir, dentry, mode | S_IFREG, 0);
}


struct inode_operations sfs_dir_inode_ops = {
	.create         = sfs_create,
	.lookup         = sfs_lookup,
	.unlink         = simple_unlink,
	.mkdir          = sfs_mkdir,
	.rmdir          = simple_rmdir,
	.mknod          = sfs_mknod,
	.rename         = simple_rename,
};

参考资料

Linux内核之设备驱动-sysfs
mknod命令