vfs: add d_prune dentry operation

This adds a d_prune dentry operation that is called by the VFS prior to
pruning (i.e. unhashing and killing) a hashed dentry from the dcache.
Wrap dentry_lru_del() and use the new _prune() helper in the cases where we
are about to unhash and kill the dentry.

This will be used by Ceph to maintain a flag indicating whether the
complete contents of a directory are contained in the dcache, allowing it
to satisfy lookups and readdir without addition server communication.

Renumber a few DCACHE_* #defines to group DCACHE_OP_PRUNE with the other
DCACHE_OP_ bits.

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Documentation/filesystems/Locking

@@ -29,6 +29,7 @@ d_hash		no		no		no		maybe
 d_compare:	yes		no		no		maybe
 d_delete:	no		yes		no		no
 d_release:	no		no		yes		no
+d_prune:        no              yes             no              no
 d_iput:		no		no		yes		no
 d_dname:	no		no		no		no
 d_automount:	no		no		yes		no

fs/dcache.c

@@ -225,7 +225,7 @@ static void dentry_unlink_inode(struct dentry * dentry)
 }
 
 /*
- * dentry_lru_(add|del|move_tail) must be called with d_lock held.
+ * dentry_lru_(add|del|prune|move_tail) must be called with d_lock held.
  */
 static void dentry_lru_add(struct dentry *dentry)
 {
@@ -245,6 +245,9 @@ static void __dentry_lru_del(struct dentry *dentry)
 	dentry_stat.nr_unused--;
 }
 
+/*
+ * Remove a dentry with references from the LRU.
+ */
 static void dentry_lru_del(struct dentry *dentry)
 {
 	if (!list_empty(&dentry->d_lru)) {
@@ -254,6 +257,23 @@ static void dentry_lru_del(struct dentry *dentry)
 	}
 }
 
+/*
+ * Remove a dentry that is unreferenced and about to be pruned
+ * (unhashed and destroyed) from the LRU, and inform the file system.
+ * This wrapper should be called _prior_ to unhashing a victim dentry.
+ */
+static void dentry_lru_prune(struct dentry *dentry)
+{
+	if (!list_empty(&dentry->d_lru)) {
+		if (dentry->d_flags & DCACHE_OP_PRUNE)
+			dentry->d_op->d_prune(dentry);
+
+		spin_lock(&dcache_lru_lock);
+		__dentry_lru_del(dentry);
+		spin_unlock(&dcache_lru_lock);
+	}
+}
+
 static void dentry_lru_move_tail(struct dentry *dentry)
 {
 	spin_lock(&dcache_lru_lock);
@@ -403,8 +423,12 @@ static inline struct dentry *dentry_kill(struct dentry *dentry, int ref)
 
 	if (ref)
 		dentry->d_count--;
-	/* if dentry was on the d_lru list delete it from there */
-	dentry_lru_del(dentry);
+	/*
+	 * if dentry was on the d_lru list delete it from there.
+	 * inform the fs via d_prune that this dentry is about to be
+	 * unhashed and destroyed.
+	 */
+	dentry_lru_prune(dentry);
 	/* if it was on the hash then remove it */
 	__d_drop(dentry);
 	return d_kill(dentry, parent);
@@ -854,8 +878,12 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 		do {
 			struct inode *inode;
 
-			/* detach from the system */
-			dentry_lru_del(dentry);
+			/*
+			 * remove the dentry from the lru, and inform
+			 * the fs that this dentry is about to be
+			 * unhashed and destroyed.
+			 */
+			dentry_lru_prune(dentry);
 			__d_shrink(dentry);
 
 			if (dentry->d_count != 0) {
@@ -1283,6 +1311,8 @@ void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 		dentry->d_flags |= DCACHE_OP_REVALIDATE;
 	if (op->d_delete)
 		dentry->d_flags |= DCACHE_OP_DELETE;
+	if (op->d_prune)
+		dentry->d_flags |= DCACHE_OP_PRUNE;
 
 }
 EXPORT_SYMBOL(d_set_d_op);

include/linux/dcache.h

@@ -165,6 +165,7 @@ struct dentry_operations {
 			unsigned int, const char *, const struct qstr *);
 	int (*d_delete)(const 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 *);
@@ -184,8 +185,9 @@ struct dentry_operations {
 #define DCACHE_OP_COMPARE	0x0002
 #define DCACHE_OP_REVALIDATE	0x0004
 #define DCACHE_OP_DELETE	0x0008
+#define DCACHE_OP_PRUNE         0x0010
 
-#define	DCACHE_DISCONNECTED	0x0010
+#define	DCACHE_DISCONNECTED	0x0020
      /* 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
@@ -196,8 +198,8 @@ struct dentry_operations {
       * dentry into place and return that dentry rather than the passed one,
       * typically using d_splice_alias. */
 
-#define DCACHE_REFERENCED	0x0020  /* Recently used, don't discard. */
-#define DCACHE_RCUACCESS	0x0040	/* Entry has ever been RCU-visible */
+#define DCACHE_REFERENCED	0x0040  /* Recently used, don't discard. */
+#define DCACHE_RCUACCESS	0x0080	/* Entry has ever been RCU-visible */
 
 #define DCACHE_CANT_MOUNT	0x0100
 #define DCACHE_GENOCIDE		0x0200