dcache.h

linux的文件系统的部分代码的详细注释

#ifndef __LINUX_DCACHE_H
#define __LINUX_DCACHE_H

#ifdef __KERNEL__

#include <asm/atomic.h>
#include <linux/mount.h>

/*
 * linux/include/linux/dcache.h
 *
 * Dirent cache data structures
 *
 * (C) Copyright 1997 Thomas Schoebel-Theuer,
 * with heavy changes by Linus Torvalds
 */

#define IS_ROOT(x) ((x) == (x)->d_parent)

/*
 * "quick string" -- eases parameter passing, but more importantly
 * saves "metadata" about the string (ie length and the hash).
 */
struct qstr {
	const unsigned char * name;/*名字*/
	unsigned int len;/*长度*/
	unsigned int hash;/*哈希号*/
};

struct dentry_stat_t {/*目录状态结构*/
	int nr_dentry;/*目录个数*/
	int nr_unused;/*未使用的目录项个数*/
	int age_limit;          /* age in seconds ??时间限制??*/
	int want_pages;         /* pages requested by system 系统需要页*/
	int dummy[2];  /*dummy 虚拟*/
};
extern struct dentry_stat_t dentry_stat;

/* Name hashing routines. Initial hash value */
/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
#define init_name_hash()		0

/* partial hash update function. Assume roughly 4 bits per character */
static __inline__ unsigned long partial_name_hash(unsigned long c, unsigned long prevhash)
{
	return (prevhash + (c << 4) + (c >> 4)) * 11;
}

/* Finally: cut down the number of bits to a int value (and try to avoid losing bits) */
static __inline__ unsigned long end_name_hash(unsigned long hash)
{
	return (unsigned int) hash;
}

/* Compute the hash for a name string. */
static __inline__ unsigned int full_name_hash(const unsigned char * name, unsigned int len)
{
	unsigned long hash = init_name_hash();
	while (len--)
		hash = partial_name_hash(*name++, hash);
	return end_name_hash(hash);
}

#define DNAME_INLINE_LEN 16

struct dentry {
	atomic_t d_count; /*目录项引用的计数*/
	unsigned int d_flags;/*目录项标志*/
	struct inode  * d_inode;	/* Where the name belongs to - NULL is negative 与文件名关联的索引节点*/
	struct dentry * d_parent;	/* parent directory 父目录的目录项对象*/
	struct list_head d_hash;	/* lookup hash list 目录项形成的哈希表*/
	struct list_head d_lru;		/* d_count = 0 LRU list  未使用的LRU链表*/
	struct list_head d_child;	/* child of parent list 父目录项的子目录项所形成的链表*/
	struct list_head d_subdirs;	/* our children 该目录项的子目录项形成的链表*/
	struct list_head d_alias;	/* inode alias list 索引节点别名的链表*/
	int d_mounted;/*目录项的安装点*/
	struct qstr d_name;/*目录项名，可快速查找*/
	unsigned long d_time;		/* used by d_revalidate 由d_reavlidate函数使用*/
	struct dentry_operations  *d_op;/*目录项的函数集*/
	struct super_block * d_sb;	/* The root of the dentry tree 目录项树的根，即文件的超级块*/
	unsigned long d_vfs_flags;
	void * d_fsdata;		/* fs-specific data 具体文件系统的数据*/
	unsigned char d_iname[DNAME_INLINE_LEN]; /* small names 段文件名*/
};

struct dentry_operations {
	int (*d_revalidate)(struct dentry *, int);/*判定目录项是否有效，默认情况下，vfs函数什么也不做，而网络文件系统可以指定自己的函数*/
	int (*d_hash) (struct dentry *, struct qstr *);/*生成哈希值，对目录项哈希表而言，这是一个具体文件系统的哈希函数*/
	int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);/*比较两个文件名*/
	int (*d_delete)(struct dentry *);/*如果对目录项的最后一个引用被删除(d_count=0)，就调用该方法*/
	void (*d_release)(struct dentry *);/*当要释放一个目录项时(放入slab分配器)，就调用该方法*/
	void (*d_iput)(struct dentry *, struct inode *);/*当要丢弃目录项对应的索引节点时，就调用该方法。默认情况下，vfs的这个函数调用iput()释放索引节点*/
};

/* the dentry parameter passed to d_hash and d_compare is the parent
 * directory of the entries to be compared. It is used in case these
 * functions need any directory specific information for determining
 * equivalency classes.  Using the dentry itself might not work, as it
 * might be a negative dentry which has no information associated with
 * it */

/*
locking rules:
		big lock	dcache_lock	may block
d_revalidate:	no		no		yes
d_hash		no		no		yes
d_compare:	no		yes		no
d_delete:	       no		yes		no
d_release:	no		no		yes
d_iput:		no		no		yes
 */

/* d_flags entries */
#define DCACHE_AUTOFS_PENDING 0x0001    /* autofs: "under construction" */
#define DCACHE_NFSFS_RENAMED  0x0002    /* this dentry has been "silly
					 * renamed" and has to be
					 * deleted on the last dput()
					 */
#define	DCACHE_NFSD_DISCONNECTED 0x0004	/* This dentry is not currently connected to the
					 * dcache tree. Its parent will either be itself,
					 * or will have this flag as well.
					 * If this dentry points to a directory, then
					 * s_nfsd_free_path semaphore will be down
					 */
#define DCACHE_REFERENCED	0x0008  /* Recently used, don't discard. */

extern spinlock_t dcache_lock;

/**
 * d_drop - drop a dentry
 * @dentry: dentry to drop
 *
 * d_drop() unhashes the entry from the parent
 * dentry hashes, so that it won't be found through
 * a VFS lookup any more. Note that this is different
 * from deleting the dentry - d_delete will try to
 * mark the dentry negative if possible, giving a
 * successful _negative_ lookup, while d_drop will
 * just make the cache lookup fail.
 *
 * d_drop() is used mainly for stuff that wants
 * to invalidate a dentry for some reason (NFS
 * timeouts or autofs deletes).
 */

static __inline__ void d_drop(struct dentry * dentry)/*摘下目录项*/
{
	spin_lock(&dcache_lock);/*加锁*/
	list_del(&dentry->d_hash);/*删除该目录项的哈希表*/
	INIT_LIST_HEAD(&dentry->d_hash);/*初始化哈希表*/
	spin_unlock(&dcache_lock);/*解锁*/
}

static __inline__ int dname_external(struct dentry *d)
{
	return d->d_name.name != d->d_iname; 
}

/*
 * These are the low-level FS interfaces to the dcache..
 */
extern void d_instantiate(struct dentry *, struct inode *);
extern void d_delete(struct dentry *);

/* allocate/de-allocate */
extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
extern void shrink_dcache_sb(struct super_block *);
extern void shrink_dcache_parent(struct dentry *);
extern int d_invalidate(struct dentry *);

#define shrink_dcache() prune_dcache(0)
struct zone_struct;
/* dcache memory management */
extern int shrink_dcache_memory(int, unsigned int);
extern void prune_dcache(int);

/* icache memory management (defined in linux/fs/inode.c) */
extern int shrink_icache_memory(int, int);
extern void prune_icache(int);

/* quota cache memory management (defined in linux/fs/dquot.c) */
extern int shrink_dqcache_memory(int, unsigned int);

/* only used at mount-time */
extern struct dentry * d_alloc_root(struct inode *);

/* <clickety>-<click> the ramfs-type tree */
extern void d_genocide(struct dentry *);

extern struct dentry *d_find_alias(struct inode *);
extern void d_prune_aliases(struct inode *);

/* test whether we have any submounts in a subdir tree */
extern int have_submounts(struct dentry *);

/*
 * This adds the entry to the hash queues.
 */
extern void d_rehash(struct dentry *);

/**
 * d_add - add dentry to hash queues
 * @entry: dentry to add
 * @inode: The inode to attach to this dentry
 *
 * This adds the entry to the hash queues and initializes @inode.
 * The entry was actually filled in earlier during d_alloc().
 */
 
static __inline__ void d_add(struct dentry * entry, struct inode * inode)
{
	d_instantiate(entry, inode);
	d_rehash(entry);
}

/* used for rename() and baskets */
extern void d_move(struct dentry *, struct dentry *);

/* appendix may either be NULL or be used for transname suffixes */
extern struct dentry * d_lookup(struct dentry *, struct qstr *);

/* validate "insecure" dentry pointer */
extern int d_validate(struct dentry *, struct dentry *);

extern char * __d_path(struct dentry *, struct vfsmount *, struct dentry *,
	struct vfsmount *, char *, int);
  
/* Allocation counts.. */

/**
 *	dget, dget_locked	-	get a reference to a dentry
 *	@dentry: dentry to get a reference to
 *
 *	Given a dentry or %NULL pointer increment the reference count
 *	if appropriate and return the dentry. A dentry will not be 
 *	destroyed when it has references. dget() should never be
 *	called for dentries with zero reference counter. For these cases
 *	(preferably none, functions in dcache.c are sufficient for normal
 *	needs and they take necessary precautions) you should hold dcache_lock
 *	and call dget_locked() instead of dget().
 */
 
static __inline__ struct dentry * dget(struct dentry *dentry)
{
	if (dentry) {
		if (!atomic_read(&dentry->d_count))
			BUG();
		atomic_inc(&dentry->d_count);
	}
	return dentry;
}

extern struct dentry * dget_locked(struct dentry *);

/**
 *	d_unhashed -	is dentry hashed
 *	@dentry: entry to check
 *
 *	Returns true if the dentry passed is not currently hashed.
 */
 
static __inline__ int d_unhashed(struct dentry *dentry)/*把哈希表置为空链表*/
{
	return list_empty(&dentry->d_hash);/*调用list_empty()函数把哈希表置为空链表*/
}

extern void dput(struct dentry *);

static __inline__ int d_mountpoint(struct dentry *dentry)/*得到安装点*/
{
	return dentry->d_mounted;/*返回目录项的安装点*/
}

extern struct vfsmount *lookup_mnt(struct vfsmount *, struct dentry *);
#endif /* __KERNEL__ */

#endif	/* __LINUX_DCACHE_H */