[PATCH] dentry qstr consolidation

When dentries are given an external name we currently allocate an entire qstr
for the external name.

This isn't needed.  We can use the internal qstr and kmalloc only the string
itself.  This saves 12 bytes from externally-allocated names and 4 bytes from
the dentry itself.

The saving of 4 bytes from the dentry doesn't actually decrease the dentry's
storage requirements, but it makes four more bytes available for internal
names, taking the internal/external ratio from 89% up to 93% on my 1.5M files.


Fix:

The qstr consolidation wasn't quite right, because it can cause qstr->len to
be unstable during lookup lockless traverasl.

Fix that up by taking d_lock earlier in lookup.  This serialises against
d_move.

Take the lock after comparing the parent and hash to preserve the
mostly-lockless behaviour.

This obsoletes d_movecount, which is removed.

fs/dcache.c

@@ -73,9 +73,8 @@ static void d_callback(void *arg)
 {
 	struct dentry * dentry = (struct dentry *)arg;
 
-	if (dname_external(dentry)) {
-		kfree(dentry->d_qstr);
-	}
+	if (dname_external(dentry))
+		kfree(dentry->d_name.name);
 	kmem_cache_free(dentry_cache, dentry); 
 }
 
@@ -682,34 +681,30 @@ static int shrink_dcache_memory(int nr, unsigned int gfp_mask)
  * copied and the copy passed in may be reused after this call.
  */
  
-struct dentry * d_alloc(struct dentry * parent, const struct qstr *name)
+struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 {
-	char * str;
 	struct dentry *dentry;
-	struct qstr * qstr;
+	char *dname;
 
 	dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL); 
 	if (!dentry)
 		return NULL;
 
 	if (name->len > DNAME_INLINE_LEN-1) {
-		qstr = kmalloc(sizeof(*qstr) + name->len + 1, GFP_KERNEL);
-		if (!qstr) {
+		dname = kmalloc(name->len + 1, GFP_KERNEL);
+		if (!dname) {
 			kmem_cache_free(dentry_cache, dentry); 
 			return NULL;
 		}
-		qstr->name = qstr->name_str;
-		qstr->len = name->len;
-		qstr->hash = name->hash;
-		dentry->d_qstr = qstr;
-		str = qstr->name_str;
 	} else  {
-		dentry->d_qstr = &dentry->d_name;
-		str = dentry->d_iname;
+		dname = dentry->d_iname;
 	}	
+	dentry->d_name.name = dname;
 
-	memcpy(str, name->name, name->len);
-	str[name->len] = 0;
+	dentry->d_name.len = name->len;
+	dentry->d_name.hash = name->hash;
+	memcpy(dname, name->name, name->len);
+	dname[name->len] = 0;
 
 	atomic_set(&dentry->d_count, 1);
 	dentry->d_vfs_flags = DCACHE_UNHASHED;
@@ -717,11 +712,7 @@ struct dentry * d_alloc(struct dentry * parent, const struct qstr *name)
 	dentry->d_flags = 0;
 	dentry->d_inode = NULL;
 	dentry->d_parent = NULL;
-	dentry->d_move_count = 0;
 	dentry->d_sb = NULL;
-	dentry->d_name.name = str;
-	dentry->d_name.len = name->len;
-	dentry->d_name.hash = name->hash;
 	dentry->d_op = NULL;
 	dentry->d_fsdata = NULL;
 	dentry->d_mounted = 0;
@@ -788,7 +779,8 @@ struct dentry * d_alloc_root(struct inode * root_inode)
 	struct dentry *res = NULL;
 
 	if (root_inode) {
-		static const struct qstr name = { .name = "/", .len = 1, .hash = 0 };
+		static const struct qstr name = { .name = "/", .len = 1 };
+
 		res = d_alloc(NULL, &name);
 		if (res) {
 			res->d_sb = root_inode->i_sb;
@@ -829,7 +821,7 @@ static inline struct hlist_head *d_hash(struct dentry *parent,
 
 struct dentry * d_alloc_anon(struct inode *inode)
 {
-	static const struct qstr anonstring = { "", 0, 0};
+	static const struct qstr anonstring = { .name = "" };
 	struct dentry *tmp;
 	struct dentry *res;
 
@@ -935,8 +927,7 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
  *
  * __d_lookup is dcache_lock free. The hash list is protected using RCU.
  * Memory barriers are used while updating and doing lockless traversal. 
- * To avoid races with d_move while rename is happening, d_move_count is 
- * used. 
+ * To avoid races with d_move while rename is happening, d_lock is used.
  *
  * Overflows in memcmp(), while d_move, are avoided by keeping the length
  * and name pointer in one structure pointed by d_qstr.
@@ -979,8 +970,7 @@ struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
 	
 	hlist_for_each (node, head) { 
 		struct dentry *dentry; 
-		unsigned long move_count;
-		struct qstr * qstr;
+		struct qstr *qstr;
 
 		smp_read_barrier_depends();
 		dentry = hlist_entry(node, struct dentry, d_hash);
@@ -991,11 +981,6 @@ struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
 		if (unlikely(dentry->d_bucket != head))
 			break;
 
-		/*
-		 * We must take a snapshot of d_move_count followed by
-		 * read memory barrier before any search key comparison 
-		 */
-		move_count = dentry->d_move_count;
 		smp_rmb();
 
 		if (dentry->d_name.hash != hash)
@@ -1003,29 +988,36 @@ struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
 		if (dentry->d_parent != parent)
 			continue;
 
-		qstr = dentry->d_qstr;
+		spin_lock(&dentry->d_lock);
+
+		/*
+		 * Recheck the dentry after taking the lock - d_move may have
+		 * changed things.  Don't bother checking the hash because we're
+		 * about to compare the whole name anyway.
+		 */
+		if (dentry->d_parent != parent)
+			goto next;
+
+		qstr = &dentry->d_name;
 		smp_read_barrier_depends();
 		if (parent->d_op && parent->d_op->d_compare) {
 			if (parent->d_op->d_compare(parent, qstr, name))
-				continue;
+				goto next;
 		} else {
 			if (qstr->len != len)
-				continue;
+				goto next;
 			if (memcmp(qstr->name, str, len))
-				continue;
+				goto next;
 		}
-		spin_lock(&dentry->d_lock);
-		/*
-		 * If dentry is moved, fail the lookup
-		 */ 
-		if (likely(move_count == dentry->d_move_count)) {
+
 		if (!d_unhashed(dentry)) {
 			atomic_inc(&dentry->d_count);
 			found = dentry;
 		}
-		}
 		spin_unlock(&dentry->d_lock);
 		break;
+next:
+		spin_unlock(&dentry->d_lock);
  	}
  	rcu_read_unlock();
 
@@ -1147,28 +1139,40 @@ void d_rehash(struct dentry * entry)
  * then no longer matches the actual (corrupted) string of the target.
  * The hash value has to match the hash queue that the dentry is on..
  */
-static inline void switch_names(struct dentry * dentry, struct dentry * target)
+static void switch_names(struct dentry *dentry, struct dentry *target)
 {
-	const unsigned char *old_name, *new_name;
-	struct qstr *old_qstr, *new_qstr;
-
-	memcpy(dentry->d_iname, target->d_iname, DNAME_INLINE_LEN); 
-	old_qstr = target->d_qstr;
-	old_name = target->d_name.name;
-	new_qstr = dentry->d_qstr;
-	new_name = dentry->d_name.name;
-	if (old_name == target->d_iname) {
-		old_name = dentry->d_iname;
-		old_qstr = &dentry->d_name;
-	}
-	if (new_name == dentry->d_iname) {
-		new_name = target->d_iname;
-		new_qstr = &target->d_name;
-	}
-	target->d_name.name = new_name;
-	dentry->d_name.name = old_name;
-	target->d_qstr = new_qstr;
-	dentry->d_qstr = old_qstr;
+	if (dname_external(target)) {
+		if (dname_external(dentry)) {
+			/*
+			 * Both external: swap the pointers
+			 */
+			do_switch(target->d_name.name, dentry->d_name.name);
+		} else {
+			/*
+			 * dentry:internal, target:external.  Steal target's
+			 * storage and make target internal.
+			 */
+			dentry->d_name.name = target->d_name.name;
+			target->d_name.name = target->d_iname;
+		}
+	} else {
+		if (dname_external(dentry)) {
+			/*
+			 * dentry:external, target:internal.  Give dentry's
+			 * storage to target and make dentry internal
+			 */
+			memcpy(dentry->d_iname, target->d_name.name,
+					target->d_name.len + 1);
+			target->d_name.name = dentry->d_name.name;
+			dentry->d_name.name = dentry->d_iname;
+		} else {
+			/*
+			 * Both are internal.  Just copy target to dentry
+			 */
+			memcpy(dentry->d_iname, target->d_name.name,
+					target->d_name.len + 1);
+		}
+	}
 }
 
 /*
@@ -1246,7 +1250,6 @@ void d_move(struct dentry * dentry, struct dentry * target)
 	}
 
 	list_add(&dentry->d_child, &dentry->d_parent->d_subdirs);
-	dentry->d_move_count++;
 	spin_unlock(&target->d_lock);
 	spin_unlock(&dentry->d_lock);
 	write_sequnlock(&rename_lock);
@@ -1344,6 +1347,7 @@ char * d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
 	char *res;
 	struct vfsmount *rootmnt;
 	struct dentry *root;
+
 	read_lock(&current->fs->lock);
 	rootmnt = mntget(current->fs->rootmnt);
 	root = dget(current->fs->root);

include/linux/dcache.h

@@ -29,10 +29,9 @@ struct vfsmount;
  * saves "metadata" about the string (ie length and the hash).
  */
 struct qstr {
-	const unsigned char * name;
+	const unsigned char *name;
 	unsigned int len;
 	unsigned int hash;
-	char name_str[0];
 };
 
 struct dentry_stat_t {
@@ -93,8 +92,6 @@ struct dentry {
 	void * d_fsdata;		/* fs-specific data */
  	struct rcu_head d_rcu;
 	struct dcookie_struct * d_cookie; /* cookie, if any */
-	unsigned long d_move_count;	/* to indicated moved dentry while lockless lookup */
-	struct qstr * d_qstr;		/* quick str ptr used in lockless lookup and concurrent d_move */
 	struct dentry * d_parent;	/* parent directory */
 	struct qstr d_name;
 	struct hlist_node d_hash;	/* lookup hash list */	
@@ -120,13 +117,13 @@ struct dentry_operations {
 
 /*
 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
+		big lock	dcache_lock	d_lock   may block
+d_revalidate:	no		no		no       yes
+d_hash		no		no		no       yes
+d_compare:	no		yes		yes      no
+d_delete:	no		yes		no       no
+d_release:	no		no		no       yes
+d_iput:		no		no		no       yes
  */
 
 /* d_flags entries */
@@ -184,9 +181,9 @@ static inline void d_drop(struct dentry *dentry)
 	spin_unlock(&dcache_lock);
 }
 
-static inline int dname_external(struct dentry *d)
+static inline int dname_external(struct dentry *dentry)
 {
-	return d->d_name.name != d->d_iname; 
+	return dentry->d_name.name != dentry->d_iname;
 }
 
 /*