include/linux/dcache.h - kernel/msm-4.19 - Gitiles

 #ifndef __LINUX_DCACHE_H
 #define __LINUX_DCACHE_H

 #include <linux/atomic.h>
 #include <linux/list.h>
 #include <linux/rculist.h>
 #include <linux/rculist_bl.h>
 #include <linux/spinlock.h>
 #include <linux/seqlock.h>
 #include <linux/cache.h>
 #include <linux/rcupdate.h>
 #include <linux/lockref.h>
 #include <linux/stringhash.h>
 #include <linux/wait.h>

 struct path;
 struct vfsmount;

 /*
  * 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)

 /* The hash is always the low bits of hash_len */
 #ifdef __LITTLE_ENDIAN
  #define HASH_LEN_DECLARE u32 hash; u32 len
  #define bytemask_from_count(cnt)	(~(~0ul << (cnt)*8))
 #else
  #define HASH_LEN_DECLARE u32 len; u32 hash
  #define bytemask_from_count(cnt)	(~(~0ul >> (cnt)*8))
 #endif

 /*
  * "quick string" -- eases parameter passing, but more importantly
  * saves "metadata" about the string (ie length and the hash).
  *
  * hash comes first so it snuggles against d_parent in the
  * dentry.
  */
 struct qstr {
 	union {
 		struct {
 			HASH_LEN_DECLARE;
 		};
 		u64 hash_len;
 	};
 	const unsigned char *name;
 };

 #define QSTR_INIT(n,l) { { { .len = l } }, .name = n }

 extern const char empty_string[];
 extern const struct qstr empty_name;
 extern const char slash_string[];
 extern const struct qstr slash_name;

 struct dentry_stat_t {
 	long nr_dentry;
 	long nr_unused;
 	long age_limit;          /* age in seconds */
 	long want_pages;         /* pages requested by system */
 	long dummy[2];
 };
 extern struct dentry_stat_t dentry_stat;

 /*
  * Try to keep struct dentry aligned on 64 byte cachelines (this will
  * give reasonable cacheline footprint with larger lines without the
  * large memory footprint increase).
  */
 #ifdef CONFIG_64BIT
 # define DNAME_INLINE_LEN 32 /* 192 bytes */
 #else
 # ifdef CONFIG_SMP
 #  define DNAME_INLINE_LEN 36 /* 128 bytes */
 # else
 #  define DNAME_INLINE_LEN 40 /* 128 bytes */
 # endif
 #endif

 #define d_lock	d_lockref.lock

 struct dentry {
 	/* RCU lookup touched fields */
 	unsigned int d_flags;		/* protected by d_lock */
 	seqcount_t d_seq;		/* per dentry seqlock */
 	struct hlist_bl_node d_hash;	/* lookup hash list */
 	struct dentry *d_parent;	/* parent directory */
 	struct qstr d_name;
 	struct inode *d_inode;		/* Where the name belongs to - NULL is
 					 * negative */
 	unsigned char d_iname[DNAME_INLINE_LEN];	/* small names */

 	/* Ref lookup also touches following */
 	struct lockref d_lockref;	/* per-dentry lock and refcount */
 	const struct dentry_operations *d_op;
 	struct super_block *d_sb;	/* The root of the dentry tree */
 	unsigned long d_time;		/* used by d_revalidate */
 	void *d_fsdata;			/* fs-specific data */

 	union {
 		struct list_head d_lru;		/* LRU list */
 		wait_queue_head_t *d_wait;	/* in-lookup ones only */
 	};
 	struct list_head d_child;	/* child of parent list */
 	struct list_head d_subdirs;	/* our children */
 	/*
 	 * d_alias and d_rcu can share memory
 	 */
 	union {
 		struct hlist_node d_alias;	/* inode alias list */
 		struct hlist_bl_node d_in_lookup_hash;	/* only for in-lookup ones */
 	 	struct rcu_head d_rcu;
 	} d_u;
 } __randomize_layout;

 /*
  * dentry->d_lock spinlock nesting subclasses:
  *
  * 0: normal
  * 1: nested
  */
 enum dentry_d_lock_class
 {
 	DENTRY_D_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
 	DENTRY_D_LOCK_NESTED
 };

 struct dentry_operations {
 	int (*d_revalidate)(struct dentry *, unsigned int);
 	int (*d_weak_revalidate)(struct dentry *, unsigned int);
 	int (*d_hash)(const struct dentry *, struct qstr *);
 	int (*d_compare)(const struct dentry *,
 			unsigned int, const char *, const struct qstr *);
 	int (*d_delete)(const struct dentry *);
 	int (*d_init)(struct dentry *);
 	void (*d_release)(struct dentry *);
 	void (*d_prune)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
 	char *(*d_dname)(struct dentry *, char *, int);
 	struct vfsmount *(*d_automount)(struct path *);
 	int (*d_manage)(const struct path *, bool);
 	struct dentry *(*d_real)(struct dentry *, const struct inode *,
 				 unsigned int);
 } ____cacheline_aligned;

 /*
  * Locking rules for dentry_operations callbacks are to be found in
  * Documentation/filesystems/Locking. Keep it updated!
  *
  * FUrther descriptions are found in Documentation/filesystems/vfs.txt.
  * Keep it updated too!
  */

 /* d_flags entries */
 #define DCACHE_OP_HASH			0x00000001
 #define DCACHE_OP_COMPARE		0x00000002
 #define DCACHE_OP_REVALIDATE		0x00000004
 #define DCACHE_OP_DELETE		0x00000008
 #define DCACHE_OP_PRUNE			0x00000010

 #define	DCACHE_DISCONNECTED		0x00000020
      /* This dentry is possibly not currently connected to the dcache tree, in
       * which case its parent will either be itself, or will have this flag as
       * well.  nfsd will not use a dentry with this bit set, but will first
       * endeavour to clear the bit either by discovering that it is connected,
       * or by performing lookup operations.   Any filesystem which supports
       * nfsd_operations MUST have a lookup function which, if it finds a
       * directory inode with a DCACHE_DISCONNECTED dentry, will d_move that
       * dentry into place and return that dentry rather than the passed one,
       * typically using d_splice_alias. */

 #define DCACHE_REFERENCED		0x00000040 /* Recently used, don't discard. */
 #define DCACHE_RCUACCESS		0x00000080 /* Entry has ever been RCU-visible */

 #define DCACHE_CANT_MOUNT		0x00000100
 #define DCACHE_GENOCIDE			0x00000200
 #define DCACHE_SHRINK_LIST		0x00000400

 #define DCACHE_OP_WEAK_REVALIDATE	0x00000800

 #define DCACHE_NFSFS_RENAMED		0x00001000
      /* this dentry has been "silly renamed" and has to be deleted on the last
       * dput() */
 #define DCACHE_COOKIE			0x00002000 /* For use by dcookie subsystem */
 #define DCACHE_FSNOTIFY_PARENT_WATCHED	0x00004000
      /* Parent inode is watched by some fsnotify listener */

 #define DCACHE_DENTRY_KILLED		0x00008000

 #define DCACHE_MOUNTED			0x00010000 /* is a mountpoint */
 #define DCACHE_NEED_AUTOMOUNT		0x00020000 /* handle automount on this dir */
 #define DCACHE_MANAGE_TRANSIT		0x00040000 /* manage transit from this dirent */
 #define DCACHE_MANAGED_DENTRY \
 	(DCACHE_MOUNTED|DCACHE_NEED_AUTOMOUNT|DCACHE_MANAGE_TRANSIT)

 #define DCACHE_LRU_LIST			0x00080000

 #define DCACHE_ENTRY_TYPE		0x00700000
 #define DCACHE_MISS_TYPE		0x00000000 /* Negative dentry (maybe fallthru to nowhere) */
 #define DCACHE_WHITEOUT_TYPE		0x00100000 /* Whiteout dentry (stop pathwalk) */
 #define DCACHE_DIRECTORY_TYPE		0x00200000 /* Normal directory */
 #define DCACHE_AUTODIR_TYPE		0x00300000 /* Lookupless directory (presumed automount) */
 #define DCACHE_REGULAR_TYPE		0x00400000 /* Regular file type (or fallthru to such) */
 #define DCACHE_SPECIAL_TYPE		0x00500000 /* Other file type (or fallthru to such) */
 #define DCACHE_SYMLINK_TYPE		0x00600000 /* Symlink (or fallthru to such) */

 #define DCACHE_MAY_FREE			0x00800000
 #define DCACHE_FALLTHRU			0x01000000 /* Fall through to lower layer */
 #define DCACHE_ENCRYPTED_WITH_KEY	0x02000000 /* dir is encrypted with a valid key */
 #define DCACHE_OP_REAL			0x04000000

 #define DCACHE_PAR_LOOKUP		0x10000000 /* being looked up (with parent locked shared) */
 #define DCACHE_DENTRY_CURSOR		0x20000000

 extern seqlock_t rename_lock;

 /*
  * These are the low-level FS interfaces to the dcache..
  */
 extern void d_instantiate(struct dentry *, struct inode *);
 extern struct dentry * d_instantiate_unique(struct dentry *, struct inode *);
 extern int d_instantiate_no_diralias(struct dentry *, struct inode *);
 extern void __d_drop(struct dentry *dentry);
 extern void d_drop(struct dentry *dentry);
 extern void d_delete(struct dentry *);
 extern void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op);

 /* allocate/de-allocate */
 extern struct dentry * d_alloc(struct dentry *, const struct qstr *);
 extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *);
 extern struct dentry * d_alloc_parallel(struct dentry *, const struct qstr *,
 					wait_queue_head_t *);
 extern struct dentry * d_splice_alias(struct inode *, struct dentry *);
 extern struct dentry * d_add_ci(struct dentry *, struct inode *, struct qstr *);
 extern struct dentry * d_exact_alias(struct dentry *, struct inode *);
 extern struct dentry *d_find_any_alias(struct inode *inode);
 extern struct dentry * d_obtain_alias(struct inode *);
 extern struct dentry * d_obtain_root(struct inode *);
 extern void shrink_dcache_sb(struct super_block *);
 extern void shrink_dcache_parent(struct dentry *);
 extern void shrink_dcache_for_umount(struct super_block *);
 extern void d_invalidate(struct dentry *);

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

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

 extern void d_tmpfile(struct dentry *, struct inode *);

 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 path_has_submounts(const struct path *);

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

 extern void d_add(struct dentry *, struct inode *);

 extern void dentry_update_name_case(struct dentry *, const struct qstr *);

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

 /* appendix may either be NULL or be used for transname suffixes */
 extern struct dentry *d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *d_hash_and_lookup(struct dentry *, struct qstr *);
 extern struct dentry *__d_lookup(const struct dentry *, const struct qstr *);
 extern struct dentry *__d_lookup_rcu(const struct dentry *parent,
 				const struct qstr *name, unsigned *seq);

 static inline unsigned d_count(const struct dentry *dentry)
 {
 	return dentry->d_lockref.count;
 }

 /*
  * helper function for dentry_operations.d_dname() members
  */
 extern __printf(4, 5)
 char *dynamic_dname(struct dentry *, char *, int, const char *, ...);
 extern char *simple_dname(struct dentry *, char *, int);

 extern char *__d_path(const struct path *, const struct path *, char *, int);
 extern char *d_absolute_path(const struct path *, char *, int);
 extern char *d_path(const struct path *, char *, int);
 extern char *dentry_path_raw(struct dentry *, char *, int);
 extern char *dentry_path(struct dentry *, char *, int);

 /* Allocation counts.. */

 /**
  *	dget, dget_dlock -	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.
  */
 static inline struct dentry *dget_dlock(struct dentry *dentry)
 {
 	if (dentry)
 		dentry->d_lockref.count++;
 	return dentry;
 }

 static inline struct dentry *dget(struct dentry *dentry)
 {
 	if (dentry)
 		lockref_get(&dentry->d_lockref);
 	return dentry;
 }

 extern struct dentry *dget_parent(struct dentry *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(const struct dentry *dentry)
 {
 	return hlist_bl_unhashed(&dentry->d_hash);
 }

 static inline int d_unlinked(const struct dentry *dentry)
 {
 	return d_unhashed(dentry) && !IS_ROOT(dentry);
 }

 static inline int cant_mount(const struct dentry *dentry)
 {
 	return (dentry->d_flags & DCACHE_CANT_MOUNT);
 }

 static inline void dont_mount(struct dentry *dentry)
 {
 	spin_lock(&dentry->d_lock);
 	dentry->d_flags |= DCACHE_CANT_MOUNT;
 	spin_unlock(&dentry->d_lock);
 }

 extern void __d_lookup_done(struct dentry *);

 static inline int d_in_lookup(struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_PAR_LOOKUP;
 }

 static inline void d_lookup_done(struct dentry *dentry)
 {
 	if (unlikely(d_in_lookup(dentry))) {
 		spin_lock(&dentry->d_lock);
 		__d_lookup_done(dentry);
 		spin_unlock(&dentry->d_lock);
 	}
 }

 extern void dput(struct dentry *);

 static inline bool d_managed(const struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_MANAGED_DENTRY;
 }

 static inline bool d_mountpoint(const struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_MOUNTED;
 }

 /*
  * Directory cache entry type accessor functions.
  */
 static inline unsigned __d_entry_type(const struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_ENTRY_TYPE;
 }

 static inline bool d_is_miss(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_MISS_TYPE;
 }

 static inline bool d_is_whiteout(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_WHITEOUT_TYPE;
 }

 static inline bool d_can_lookup(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_DIRECTORY_TYPE;
 }

 static inline bool d_is_autodir(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_AUTODIR_TYPE;
 }

 static inline bool d_is_dir(const struct dentry *dentry)
 {
 	return d_can_lookup(dentry) || d_is_autodir(dentry);
 }

 static inline bool d_is_symlink(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_SYMLINK_TYPE;
 }

 static inline bool d_is_reg(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_REGULAR_TYPE;
 }

 static inline bool d_is_special(const struct dentry *dentry)
 {
 	return __d_entry_type(dentry) == DCACHE_SPECIAL_TYPE;
 }

 static inline bool d_is_file(const struct dentry *dentry)
 {
 	return d_is_reg(dentry) || d_is_special(dentry);
 }

 static inline bool d_is_negative(const struct dentry *dentry)
 {
 	// TODO: check d_is_whiteout(dentry) also.
 	return d_is_miss(dentry);
 }

 static inline bool d_is_positive(const struct dentry *dentry)
 {
 	return !d_is_negative(dentry);
 }

 /**
  * d_really_is_negative - Determine if a dentry is really negative (ignoring fallthroughs)
  * @dentry: The dentry in question
  *
  * Returns true if the dentry represents either an absent name or a name that
  * doesn't map to an inode (ie. ->d_inode is NULL).  The dentry could represent
  * a true miss, a whiteout that isn't represented by a 0,0 chardev or a
  * fallthrough marker in an opaque directory.
  *
  * Note!  (1) This should be used *only* by a filesystem to examine its own
  * dentries.  It should not be used to look at some other filesystem's
  * dentries.  (2) It should also be used in combination with d_inode() to get
  * the inode.  (3) The dentry may have something attached to ->d_lower and the
  * type field of the flags may be set to something other than miss or whiteout.
  */
 static inline bool d_really_is_negative(const struct dentry *dentry)
 {
 	return dentry->d_inode == NULL;
 }

 /**
  * d_really_is_positive - Determine if a dentry is really positive (ignoring fallthroughs)
  * @dentry: The dentry in question
  *
  * Returns true if the dentry represents a name that maps to an inode
  * (ie. ->d_inode is not NULL).  The dentry might still represent a whiteout if
  * that is represented on medium as a 0,0 chardev.
  *
  * Note!  (1) This should be used *only* by a filesystem to examine its own
  * dentries.  It should not be used to look at some other filesystem's
  * dentries.  (2) It should also be used in combination with d_inode() to get
  * the inode.
  */
 static inline bool d_really_is_positive(const struct dentry *dentry)
 {
 	return dentry->d_inode != NULL;
 }

 static inline int simple_positive(struct dentry *dentry)
 {
 	return d_really_is_positive(dentry) && !d_unhashed(dentry);
 }

 extern void d_set_fallthru(struct dentry *dentry);

 static inline bool d_is_fallthru(const struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_FALLTHRU;
 }


 extern int sysctl_vfs_cache_pressure;

 static inline unsigned long vfs_pressure_ratio(unsigned long val)
 {
 	return mult_frac(val, sysctl_vfs_cache_pressure, 100);
 }

 /**
  * d_inode - Get the actual inode of this dentry
  * @dentry: The dentry to query
  *
  * This is the helper normal filesystems should use to get at their own inodes
  * in their own dentries and ignore the layering superimposed upon them.
  */
 static inline struct inode *d_inode(const struct dentry *dentry)
 {
 	return dentry->d_inode;
 }

 /**
  * d_inode_rcu - Get the actual inode of this dentry with ACCESS_ONCE()
  * @dentry: The dentry to query
  *
  * This is the helper normal filesystems should use to get at their own inodes
  * in their own dentries and ignore the layering superimposed upon them.
  */
 static inline struct inode *d_inode_rcu(const struct dentry *dentry)
 {
 	return ACCESS_ONCE(dentry->d_inode);
 }

 /**
  * d_backing_inode - Get upper or lower inode we should be using
  * @upper: The upper layer
  *
  * This is the helper that should be used to get at the inode that will be used
  * if this dentry were to be opened as a file.  The inode may be on the upper
  * dentry or it may be on a lower dentry pinned by the upper.
  *
  * Normal filesystems should not use this to access their own inodes.
  */
 static inline struct inode *d_backing_inode(const struct dentry *upper)
 {
 	struct inode *inode = upper->d_inode;

 	return inode;
 }

 /**
  * d_backing_dentry - Get upper or lower dentry we should be using
  * @upper: The upper layer
  *
  * This is the helper that should be used to get the dentry of the inode that
  * will be used if this dentry were opened as a file.  It may be the upper
  * dentry or it may be a lower dentry pinned by the upper.
  *
  * Normal filesystems should not use this to access their own dentries.
  */
 static inline struct dentry *d_backing_dentry(struct dentry *upper)
 {
 	return upper;
 }

 /**
  * d_real - Return the real dentry
  * @dentry: the dentry to query
  * @inode: inode to select the dentry from multiple layers (can be NULL)
  * @flags: open flags to control copy-up behavior
  *
  * If dentry is on a union/overlay, then return the underlying, real dentry.
  * Otherwise return the dentry itself.
  *
  * See also: Documentation/filesystems/vfs.txt
  */
 static inline struct dentry *d_real(struct dentry *dentry,
 				    const struct inode *inode,
 				    unsigned int flags)
 {
 	if (unlikely(dentry->d_flags & DCACHE_OP_REAL))
 		return dentry->d_op->d_real(dentry, inode, flags);
 	else
 		return dentry;
 }

 /**
  * d_real_inode - Return the real inode
  * @dentry: The dentry to query
  *
  * If dentry is on a union/overlay, then return the underlying, real inode.
  * Otherwise return d_inode().
  */
 static inline struct inode *d_real_inode(const struct dentry *dentry)
 {
 	/* This usage of d_real() results in const dentry */
 	return d_backing_inode(d_real((struct dentry *) dentry, NULL, 0));
 }

 struct name_snapshot {
 	const unsigned char *name;
 	unsigned char inline_name[DNAME_INLINE_LEN];
 };
 void take_dentry_name_snapshot(struct name_snapshot *, struct dentry *);
 void release_dentry_name_snapshot(struct name_snapshot *);

 #endif	/* __LINUX_DCACHE_H */
	#ifndef __LINUX_DCACHE_H
	#define __LINUX_DCACHE_H

	#include <linux/atomic.h>
	#include <linux/list.h>
	#include <linux/rculist.h>
	#include <linux/rculist_bl.h>
	#include <linux/spinlock.h>
	#include <linux/seqlock.h>
	#include <linux/cache.h>
	#include <linux/rcupdate.h>
	#include <linux/lockref.h>
	#include <linux/stringhash.h>
	#include <linux/wait.h>

	struct path;
	struct vfsmount;

	/*
	* 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)

	/* The hash is always the low bits of hash_len */
	#ifdef __LITTLE_ENDIAN
	#define HASH_LEN_DECLARE u32 hash; u32 len
	#define bytemask_from_count(cnt) (~(~0ul << (cnt)*8))
	#else
	#define HASH_LEN_DECLARE u32 len; u32 hash
	#define bytemask_from_count(cnt) (~(~0ul >> (cnt)*8))
	#endif

	/*
	* "quick string" -- eases parameter passing, but more importantly
	* saves "metadata" about the string (ie length and the hash).
	*
	* hash comes first so it snuggles against d_parent in the
	* dentry.
	*/
	struct qstr {
	union {
	struct {
	HASH_LEN_DECLARE;
	};
	u64 hash_len;
	};
	const unsigned char *name;
	};

	#define QSTR_INIT(n,l) { { { .len = l } }, .name = n }

	extern const char empty_string[];
	extern const struct qstr empty_name;
	extern const char slash_string[];
	extern const struct qstr slash_name;

	struct dentry_stat_t {
	long nr_dentry;
	long nr_unused;
	long age_limit; /* age in seconds */
	long want_pages; /* pages requested by system */
	long dummy[2];
	};
	extern struct dentry_stat_t dentry_stat;

	/*
	* Try to keep struct dentry aligned on 64 byte cachelines (this will
	* give reasonable cacheline footprint with larger lines without the
	* large memory footprint increase).
	*/
	#ifdef CONFIG_64BIT
	# define DNAME_INLINE_LEN 32 /* 192 bytes */
	#else
	# ifdef CONFIG_SMP
	# define DNAME_INLINE_LEN 36 /* 128 bytes */
	# else
	# define DNAME_INLINE_LEN 40 /* 128 bytes */
	# endif
	#endif

	#define d_lock d_lockref.lock

	struct dentry {
	/* RCU lookup touched fields */
	unsigned int d_flags; /* protected by d_lock */
	seqcount_t d_seq; /* per dentry seqlock */
	struct hlist_bl_node d_hash; /* lookup hash list */
	struct dentry d_parent; / parent directory */
	struct qstr d_name;
	struct inode d_inode; / Where the name belongs to - NULL is
	* negative */
	unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */

	/* Ref lookup also touches following */
	struct lockref d_lockref; /* per-dentry lock and refcount */
	const struct dentry_operations *d_op;
	struct super_block d_sb; / The root of the dentry tree */
	unsigned long d_time; /* used by d_revalidate */
	void d_fsdata; / fs-specific data */

	union {
	struct list_head d_lru; /* LRU list */
	wait_queue_head_t d_wait; / in-lookup ones only */
	};
	struct list_head d_child; /* child of parent list */
	struct list_head d_subdirs; /* our children */
	/*
	* d_alias and d_rcu can share memory
	*/
	union {
	struct hlist_node d_alias; /* inode alias list */
	struct hlist_bl_node d_in_lookup_hash; /* only for in-lookup ones */
	struct rcu_head d_rcu;
	} d_u;
	} __randomize_layout;

	/*
	* dentry->d_lock spinlock nesting subclasses:
	*
	* 0: normal
	* 1: nested
	*/
	enum dentry_d_lock_class
	{
	DENTRY_D_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
	DENTRY_D_LOCK_NESTED
	};

	struct dentry_operations {
	int (d_revalidate)(struct dentry , unsigned int);
	int (d_weak_revalidate)(struct dentry , unsigned int);
	int (d_hash)(const struct dentry , struct qstr *);
	int (d_compare)(const struct dentry ,
	unsigned int, const char , const struct qstr );
	int (d_delete)(const struct dentry );
	int (d_init)(struct dentry );
	void (d_release)(struct dentry );
	void (d_prune)(struct dentry );
	void (d_iput)(struct dentry , struct inode *);
	char (d_dname)(struct dentry , char , int);
	struct vfsmount (d_automount)(struct path *);
	int (d_manage)(const struct path , bool);
	struct dentry (d_real)(struct dentry , const struct inode ,
	unsigned int);
	} ____cacheline_aligned;

	/*
	* Locking rules for dentry_operations callbacks are to be found in
	* Documentation/filesystems/Locking. Keep it updated!
	*
	* FUrther descriptions are found in Documentation/filesystems/vfs.txt.
	* Keep it updated too!
	*/

	/* d_flags entries */
	#define DCACHE_OP_HASH 0x00000001
	#define DCACHE_OP_COMPARE 0x00000002
	#define DCACHE_OP_REVALIDATE 0x00000004
	#define DCACHE_OP_DELETE 0x00000008
	#define DCACHE_OP_PRUNE 0x00000010

	#define DCACHE_DISCONNECTED 0x00000020
	/* This dentry is possibly not currently connected to the dcache tree, in
	* which case its parent will either be itself, or will have this flag as
	* well. nfsd will not use a dentry with this bit set, but will first
	* endeavour to clear the bit either by discovering that it is connected,
	* or by performing lookup operations. Any filesystem which supports
	* nfsd_operations MUST have a lookup function which, if it finds a
	* directory inode with a DCACHE_DISCONNECTED dentry, will d_move that
	* dentry into place and return that dentry rather than the passed one,
	* typically using d_splice_alias. */

	#define DCACHE_REFERENCED 0x00000040 /* Recently used, don't discard. */
	#define DCACHE_RCUACCESS 0x00000080 /* Entry has ever been RCU-visible */

	#define DCACHE_CANT_MOUNT 0x00000100
	#define DCACHE_GENOCIDE 0x00000200
	#define DCACHE_SHRINK_LIST 0x00000400

	#define DCACHE_OP_WEAK_REVALIDATE 0x00000800

	#define DCACHE_NFSFS_RENAMED 0x00001000
	/* this dentry has been "silly renamed" and has to be deleted on the last
	* dput() */
	#define DCACHE_COOKIE 0x00002000 /* For use by dcookie subsystem */
	#define DCACHE_FSNOTIFY_PARENT_WATCHED 0x00004000
	/* Parent inode is watched by some fsnotify listener */

	#define DCACHE_DENTRY_KILLED 0x00008000

	#define DCACHE_MOUNTED 0x00010000 /* is a mountpoint */
	#define DCACHE_NEED_AUTOMOUNT 0x00020000 /* handle automount on this dir */
	#define DCACHE_MANAGE_TRANSIT 0x00040000 /* manage transit from this dirent */
	#define DCACHE_MANAGED_DENTRY \
	(DCACHE_MOUNTED\|DCACHE_NEED_AUTOMOUNT\|DCACHE_MANAGE_TRANSIT)

	#define DCACHE_LRU_LIST 0x00080000

	#define DCACHE_ENTRY_TYPE 0x00700000
	#define DCACHE_MISS_TYPE 0x00000000 /* Negative dentry (maybe fallthru to nowhere) */
	#define DCACHE_WHITEOUT_TYPE 0x00100000 /* Whiteout dentry (stop pathwalk) */
	#define DCACHE_DIRECTORY_TYPE 0x00200000 /* Normal directory */
	#define DCACHE_AUTODIR_TYPE 0x00300000 /* Lookupless directory (presumed automount) */
	#define DCACHE_REGULAR_TYPE 0x00400000 /* Regular file type (or fallthru to such) */
	#define DCACHE_SPECIAL_TYPE 0x00500000 /* Other file type (or fallthru to such) */
	#define DCACHE_SYMLINK_TYPE 0x00600000 /* Symlink (or fallthru to such) */

	#define DCACHE_MAY_FREE 0x00800000
	#define DCACHE_FALLTHRU 0x01000000 /* Fall through to lower layer */
	#define DCACHE_ENCRYPTED_WITH_KEY 0x02000000 /* dir is encrypted with a valid key */
	#define DCACHE_OP_REAL 0x04000000

	#define DCACHE_PAR_LOOKUP 0x10000000 /* being looked up (with parent locked shared) */
	#define DCACHE_DENTRY_CURSOR 0x20000000

	extern seqlock_t rename_lock;

	/*
	* These are the low-level FS interfaces to the dcache..
	*/
	extern void d_instantiate(struct dentry , struct inode );
	extern struct dentry * d_instantiate_unique(struct dentry , struct inode );
	extern int d_instantiate_no_diralias(struct dentry , struct inode );
	extern void __d_drop(struct dentry *dentry);
	extern void d_drop(struct dentry *dentry);
	extern void d_delete(struct dentry *);
	extern void d_set_d_op(struct dentry dentry, const struct dentry_operations op);

	/* allocate/de-allocate */
	extern struct dentry * d_alloc(struct dentry , const struct qstr );
	extern struct dentry * d_alloc_pseudo(struct super_block , const struct qstr );
	extern struct dentry * d_alloc_parallel(struct dentry , const struct qstr ,
	wait_queue_head_t *);
	extern struct dentry * d_splice_alias(struct inode , struct dentry );
	extern struct dentry * d_add_ci(struct dentry , struct inode , struct qstr *);
	extern struct dentry * d_exact_alias(struct dentry , struct inode );
	extern struct dentry d_find_any_alias(struct inode inode);
	extern struct dentry * d_obtain_alias(struct inode *);
	extern struct dentry * d_obtain_root(struct inode *);
	extern void shrink_dcache_sb(struct super_block *);
	extern void shrink_dcache_parent(struct dentry *);
	extern void shrink_dcache_for_umount(struct super_block *);
	extern void d_invalidate(struct dentry *);

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

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

	extern void d_tmpfile(struct dentry , struct inode );

	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 path_has_submounts(const struct path *);

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

	extern void d_add(struct dentry , struct inode );

	extern void dentry_update_name_case(struct dentry , const struct qstr );

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

	/* appendix may either be NULL or be used for transname suffixes */
	extern struct dentry d_lookup(const struct dentry , const struct qstr *);
	extern struct dentry d_hash_and_lookup(struct dentry , struct qstr *);
	extern struct dentry __d_lookup(const struct dentry , const struct qstr *);
	extern struct dentry __d_lookup_rcu(const struct dentry parent,
	const struct qstr name, unsigned seq);

	static inline unsigned d_count(const struct dentry *dentry)
	{
	return dentry->d_lockref.count;
	}

	/*
	* helper function for dentry_operations.d_dname() members
	*/
	extern __printf(4, 5)
	char dynamic_dname(struct dentry , char , int, const char , ...);
	extern char simple_dname(struct dentry , char *, int);

	extern char __d_path(const struct path , const struct path , char , int);
	extern char d_absolute_path(const struct path , char *, int);
	extern char d_path(const struct path , char *, int);
	extern char dentry_path_raw(struct dentry , char *, int);
	extern char dentry_path(struct dentry , char *, int);

	/* Allocation counts.. */

	/**
	* dget, dget_dlock - 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.
	*/
	static inline struct dentry dget_dlock(struct dentry dentry)
	{
	if (dentry)
	dentry->d_lockref.count++;
	return dentry;
	}

	static inline struct dentry dget(struct dentry dentry)
	{
	if (dentry)
	lockref_get(&dentry->d_lockref);
	return dentry;
	}

	extern struct dentry dget_parent(struct dentry 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(const struct dentry *dentry)
	{
	return hlist_bl_unhashed(&dentry->d_hash);
	}

	static inline int d_unlinked(const struct dentry *dentry)
	{
	return d_unhashed(dentry) && !IS_ROOT(dentry);
	}

	static inline int cant_mount(const struct dentry *dentry)
	{
	return (dentry->d_flags & DCACHE_CANT_MOUNT);
	}

	static inline void dont_mount(struct dentry *dentry)
	{
	spin_lock(&dentry->d_lock);
	dentry->d_flags \|= DCACHE_CANT_MOUNT;
	spin_unlock(&dentry->d_lock);
	}

	extern void __d_lookup_done(struct dentry *);

	static inline int d_in_lookup(struct dentry *dentry)
	{
	return dentry->d_flags & DCACHE_PAR_LOOKUP;
	}

	static inline void d_lookup_done(struct dentry *dentry)
	{
	if (unlikely(d_in_lookup(dentry))) {
	spin_lock(&dentry->d_lock);
	__d_lookup_done(dentry);
	spin_unlock(&dentry->d_lock);
	}
	}

	extern void dput(struct dentry *);

	static inline bool d_managed(const struct dentry *dentry)
	{
	return dentry->d_flags & DCACHE_MANAGED_DENTRY;
	}

	static inline bool d_mountpoint(const struct dentry *dentry)
	{
	return dentry->d_flags & DCACHE_MOUNTED;
	}

	/*
	* Directory cache entry type accessor functions.
	*/
	static inline unsigned __d_entry_type(const struct dentry *dentry)
	{
	return dentry->d_flags & DCACHE_ENTRY_TYPE;
	}

	static inline bool d_is_miss(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_MISS_TYPE;
	}

	static inline bool d_is_whiteout(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_WHITEOUT_TYPE;
	}

	static inline bool d_can_lookup(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_DIRECTORY_TYPE;
	}

	static inline bool d_is_autodir(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_AUTODIR_TYPE;
	}

	static inline bool d_is_dir(const struct dentry *dentry)
	{
	return d_can_lookup(dentry) \|\| d_is_autodir(dentry);
	}

	static inline bool d_is_symlink(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_SYMLINK_TYPE;
	}

	static inline bool d_is_reg(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_REGULAR_TYPE;
	}

	static inline bool d_is_special(const struct dentry *dentry)
	{
	return __d_entry_type(dentry) == DCACHE_SPECIAL_TYPE;
	}

	static inline bool d_is_file(const struct dentry *dentry)
	{
	return d_is_reg(dentry) \|\| d_is_special(dentry);
	}

	static inline bool d_is_negative(const struct dentry *dentry)
	{
	// TODO: check d_is_whiteout(dentry) also.
	return d_is_miss(dentry);
	}

	static inline bool d_is_positive(const struct dentry *dentry)
	{
	return !d_is_negative(dentry);
	}

	/**
	* d_really_is_negative - Determine if a dentry is really negative (ignoring fallthroughs)
	* @dentry: The dentry in question
	*
	* Returns true if the dentry represents either an absent name or a name that
	* doesn't map to an inode (ie. ->d_inode is NULL). The dentry could represent
	* a true miss, a whiteout that isn't represented by a 0,0 chardev or a
	* fallthrough marker in an opaque directory.
	*
	* Note! (1) This should be used only by a filesystem to examine its own
	* dentries. It should not be used to look at some other filesystem's
	* dentries. (2) It should also be used in combination with d_inode() to get
	* the inode. (3) The dentry may have something attached to ->d_lower and the
	* type field of the flags may be set to something other than miss or whiteout.
	*/
	static inline bool d_really_is_negative(const struct dentry *dentry)
	{
	return dentry->d_inode == NULL;
	}

	/**
	* d_really_is_positive - Determine if a dentry is really positive (ignoring fallthroughs)
	* @dentry: The dentry in question
	*
	* Returns true if the dentry represents a name that maps to an inode
	* (ie. ->d_inode is not NULL). The dentry might still represent a whiteout if
	* that is represented on medium as a 0,0 chardev.
	*
	* Note! (1) This should be used only by a filesystem to examine its own
	* dentries. It should not be used to look at some other filesystem's
	* dentries. (2) It should also be used in combination with d_inode() to get
	* the inode.
	*/
	static inline bool d_really_is_positive(const struct dentry *dentry)
	{
	return dentry->d_inode != NULL;
	}

	static inline int simple_positive(struct dentry *dentry)
	{
	return d_really_is_positive(dentry) && !d_unhashed(dentry);
	}

	extern void d_set_fallthru(struct dentry *dentry);

	static inline bool d_is_fallthru(const struct dentry *dentry)
	{
	return dentry->d_flags & DCACHE_FALLTHRU;
	}


	extern int sysctl_vfs_cache_pressure;

	static inline unsigned long vfs_pressure_ratio(unsigned long val)
	{
	return mult_frac(val, sysctl_vfs_cache_pressure, 100);
	}

	/**
	* d_inode - Get the actual inode of this dentry
	* @dentry: The dentry to query
	*
	* This is the helper normal filesystems should use to get at their own inodes
	* in their own dentries and ignore the layering superimposed upon them.
	*/
	static inline struct inode d_inode(const struct dentry dentry)
	{
	return dentry->d_inode;
	}

	/**
	* d_inode_rcu - Get the actual inode of this dentry with ACCESS_ONCE()
	* @dentry: The dentry to query
	*
	* This is the helper normal filesystems should use to get at their own inodes
	* in their own dentries and ignore the layering superimposed upon them.
	*/
	static inline struct inode d_inode_rcu(const struct dentry dentry)
	{
	return ACCESS_ONCE(dentry->d_inode);
	}

	/**
	* d_backing_inode - Get upper or lower inode we should be using
	* @upper: The upper layer
	*
	* This is the helper that should be used to get at the inode that will be used
	* if this dentry were to be opened as a file. The inode may be on the upper
	* dentry or it may be on a lower dentry pinned by the upper.
	*
	* Normal filesystems should not use this to access their own inodes.
	*/
	static inline struct inode d_backing_inode(const struct dentry upper)
	{
	struct inode *inode = upper->d_inode;

	return inode;
	}

	/**
	* d_backing_dentry - Get upper or lower dentry we should be using
	* @upper: The upper layer
	*
	* This is the helper that should be used to get the dentry of the inode that
	* will be used if this dentry were opened as a file. It may be the upper
	* dentry or it may be a lower dentry pinned by the upper.
	*
	* Normal filesystems should not use this to access their own dentries.
	*/
	static inline struct dentry d_backing_dentry(struct dentry upper)
	{
	return upper;
	}

	/**
	* d_real - Return the real dentry
	* @dentry: the dentry to query
	* @inode: inode to select the dentry from multiple layers (can be NULL)
	* @flags: open flags to control copy-up behavior
	*
	* If dentry is on a union/overlay, then return the underlying, real dentry.
	* Otherwise return the dentry itself.
	*
	* See also: Documentation/filesystems/vfs.txt
	*/
	static inline struct dentry d_real(struct dentry dentry,
	const struct inode *inode,
	unsigned int flags)
	{
	if (unlikely(dentry->d_flags & DCACHE_OP_REAL))
	return dentry->d_op->d_real(dentry, inode, flags);
	else
	return dentry;
	}

	/**
	* d_real_inode - Return the real inode
	* @dentry: The dentry to query
	*
	* If dentry is on a union/overlay, then return the underlying, real inode.
	* Otherwise return d_inode().
	*/
	static inline struct inode d_real_inode(const struct dentry dentry)
	{
	/* This usage of d_real() results in const dentry */
	return d_backing_inode(d_real((struct dentry *) dentry, NULL, 0));
	}

	struct name_snapshot {
	const unsigned char *name;
	unsigned char inline_name[DNAME_INLINE_LEN];
	};
	void take_dentry_name_snapshot(struct name_snapshot , struct dentry );
	void release_dentry_name_snapshot(struct name_snapshot *);

	#endif /* __LINUX_DCACHE_H */