Dentries: Stab at movable dentries

We should be able to move a dentry if we just fix up all reachable
references? (Good luck....). So attempt that approach for the case
that we only have a single reference (file). Track down the refs
and update the pointers under dcache lock.

Warning: This is highly experimental and avoids dealing with subsystems
having references to the dentry.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 fs/dcache.c |   60 +++++++++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 53 insertions(+), 7 deletions(-)

Index: slub/fs/dcache.c
===================================================================
--- slub.orig/fs/dcache.c	2007-05-15 21:47:28.000000000 -0700
+++ slub/fs/dcache.c	2007-05-15 21:49:40.000000000 -0700
@@ -2131,7 +2131,7 @@ static int get_dentry(struct kmem_cache 
 	 */
 	if (dentry->d_inode) {
 		dget_locked(dentry);
-		if (atomic_read(&dentry->d_count) > 2)
+		if (atomic_read(&dentry->d_count) > 3)
 			result = -EINVAL;
 	} else
 		result = 1;
@@ -2146,6 +2146,44 @@ static void put_dentry(struct kmem_cache
 	dput(dentry);
 }
 
+/* Relocate a dentry. dentry lock is held */
+static struct dentry *move_dentry(struct dentry *d)
+{
+	struct dentry *n = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
+
+	if (!n)
+		return d;
+
+	memcpy(n, d, sizeof(struct dentry));
+	list_del(&d->d_alias);
+	list_add(&n->d_alias, &n->d_inode->i_dentry);
+
+	/*
+	 * Hash list. What happens to lookups between
+	 * drop and rehash?
+	 */
+	__d_drop(d);
+	_d_rehash(n);
+
+	/* LRU */
+	if (!list_empty(&d->d_lru)) {
+                list_del_init(&d->d_lru);
+		list_add(&n->d_lru, &dentry_unused);
+	}
+
+	/* Subdirs  Skip... we do not support dirs for now */
+
+	/*
+	 * This leaves a long list of dangling pointers.
+	 * fsnotify_d_instantiate?
+	 * integrity_d_instantiate?
+	 * security_d_instantiate?
+	 */
+	/* Need to do rcu here ??? */
+	kmem_cache_free(dentry_cache, d);
+	return n;
+}
+
 /*
  * Slab has dropped all the locks. Get rid of the
  * refcount we obtained earlier and also rid of the
@@ -2154,18 +2192,26 @@ static void put_dentry(struct kmem_cache
 static int kick_dentry(struct kmem_cache *s, void *private)
 {
 	struct dentry *dentry = private;
+	int refcount;
+	int rc;
 
 	spin_lock(&dcache_lock);
 	spin_lock(&dentry->d_lock);
-	if (atomic_read(&dentry->d_count) > 1) {
-		/*
-		 * References still exist. So we cannot
-		 * do anything right now.
-		 */
+	refcount = atomic_read(&dentry->d_count);
+	if (refcount > 1) {
+		if (refcount > 2)
+			/* Too high refcount */
+			rc = -EBUSY;
+		else {
+			/* File. May be able to move dentry */
+			dentry = move_dentry(dentry);
+			rc = 0;
+		}
+
 		spin_unlock(&dentry->d_lock);
 		spin_unlock(&dcache_lock);
 		dput(dentry);
-		return -EBUSY;
+		return rc;
 	}
 
 	/* Remove from LRU */