configfs: improve item creation performance

As the size of a directory increases item creation slows down.
Optimizing access to s_children removes this bottleneck.

dirents are already pinned into the cache, there is no need to scan the
s_children list looking for duplicate Items. The configfs_dirent_exists
check is moved to a location where it is called only during subsystem
initialization.

d_lookup will only need to call configfs_lookup in the case where the
item in question is not pinned to dcache. The only items not pinned to
dcache are attributes. These are placed at the front of the s_children
list, whilst pinned items are inserted at the back. configfs_lookup
stops scanning when it encounters the first pinned entry in s_children.

The assumption of the above optimizations is that there will be few
attributes, but potentially many Items in a given directory.

Signed-off-by: Seamus Connor <sconnor@purestorage.com>
Reviewed-by: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Author: Seamus Connor

fs/configfs/configfs_internal.h

@@ -55,6 +55,8 @@ struct configfs_dirent {
 #define CONFIGFS_USET_IN_MKDIR	0x0200
 #define CONFIGFS_USET_CREATING	0x0400
 #define CONFIGFS_NOT_PINNED	(CONFIGFS_ITEM_ATTR | CONFIGFS_ITEM_BIN_ATTR)
+#define CONFIGFS_PINNED \
+	(CONFIGFS_ROOT | CONFIGFS_DIR | CONFIGFS_ITEM_LINK)
 
 extern struct mutex configfs_symlink_mutex;
 extern spinlock_t configfs_dirent_lock;

fs/configfs/dir.c

@@ -207,7 +207,17 @@ static struct configfs_dirent *configfs_new_dirent(struct configfs_dirent *paren
 		return ERR_PTR(-ENOENT);
 	}
 	sd->s_frag = get_fragment(frag);
-	list_add(&sd->s_sibling, &parent_sd->s_children);
+
+	/*
+	 * configfs_lookup scans only for unpinned items. s_children is
+	 * partitioned so that configfs_lookup can bail out early.
+	 * CONFIGFS_PINNED and CONFIGFS_NOT_PINNED are not symmetrical.  readdir
+	 * cursors still need to be inserted at the front of the list.
+	 */
+	if (sd->s_type & CONFIGFS_PINNED)
+		list_add_tail(&sd->s_sibling, &parent_sd->s_children);
+	else
+		list_add(&sd->s_sibling, &parent_sd->s_children);
 	spin_unlock(&configfs_dirent_lock);
 
 	return sd;
@@ -220,10 +230,11 @@ static struct configfs_dirent *configfs_new_dirent(struct configfs_dirent *paren
  *
  * called with parent inode's i_mutex held
  */
-static int configfs_dirent_exists(struct configfs_dirent *parent_sd,
-				  const unsigned char *new)
+static int configfs_dirent_exists(struct dentry *dentry)
 {
-	struct configfs_dirent * sd;
+	struct configfs_dirent *parent_sd = dentry->d_parent->d_fsdata;
+	const unsigned char *new = dentry->d_name.name;
+	struct configfs_dirent *sd;
 
 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
 		if (sd->s_element) {
@@ -289,10 +300,6 @@ static int configfs_create_dir(struct config_item *item, struct dentry *dentry,
 
 	BUG_ON(!item);
 
-	error = configfs_dirent_exists(p->d_fsdata, dentry->d_name.name);
-	if (unlikely(error))
-		return error;
-
 	error = configfs_make_dirent(p->d_fsdata, dentry, item, mode,
 				     CONFIGFS_DIR | CONFIGFS_USET_CREATING,
 				     frag);
@@ -451,6 +458,18 @@ static struct dentry * configfs_lookup(struct inode *dir,
 
 	spin_lock(&configfs_dirent_lock);
 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
+
+		/*
+		 * s_children is partitioned, see configfs_new_dirent. The first
+		 * pinned item indicates we can stop scanning.
+		 */
+		if (sd->s_type & CONFIGFS_PINNED)
+			break;
+
+		/*
+		 * Note: CONFIGFS_PINNED and CONFIGFS_NOT_PINNED are asymmetric.
+		 * there may be a readdir cursor in this list
+		 */
 		if ((sd->s_type & CONFIGFS_NOT_PINNED) &&
 		    !strcmp(configfs_get_name(sd), dentry->d_name.name)) {
 			struct configfs_attribute *attr = sd->s_element;
@@ -1885,8 +1904,11 @@ int configfs_register_subsystem(struct configfs_subsystem *subsys)
 	if (dentry) {
 		d_add(dentry, NULL);
 
-		err = configfs_attach_group(sd->s_element, &group->cg_item,
-					    dentry, frag);
+		err = configfs_dirent_exists(dentry);
+		if (!err)
+			err = configfs_attach_group(sd->s_element,
+						    &group->cg_item,
+						    dentry, frag);
 		if (err) {
 			BUG_ON(d_inode(dentry));
 			d_drop(dentry);