lookup_fast(): take mount traversal into callers

Current calling conventions: -E... on error, 0 on cache miss,
result of handle_mounts(nd, dentry, path, inode, seqp) on
success.  Turn that into returning ERR_PTR(-E...), NULL and dentry
resp.; deal with handle_mounts() in the callers.  The thing
is, they already do that in cache miss handling case, so we
just need to supply dentry to them and unify the mount traversal
in those cases.  Fewer arguments that way, and we get closer
to merging handle_mounts() and step_into().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1628,9 +1628,9 @@ static struct dentry *__lookup_hash(const struct qstr *name,
 	return dentry;
 }
 
-static int lookup_fast(struct nameidata *nd,
-		       struct path *path, struct inode **inode,
-		       unsigned *seqp)
+static struct dentry *lookup_fast(struct nameidata *nd,
+				  struct inode **inode,
+			          unsigned *seqp)
 {
 	struct dentry *dentry, *parent = nd->path.dentry;
 	int status = 1;
@@ -1645,8 +1645,8 @@ static int lookup_fast(struct nameidata *nd,
 		dentry = __d_lookup_rcu(parent, &nd->last, &seq);
 		if (unlikely(!dentry)) {
 			if (unlazy_walk(nd))
-				return -ECHILD;
-			return 0;
+				return ERR_PTR(-ECHILD);
+			return NULL;
 		}
 
 		/*
@@ -1655,7 +1655,7 @@ static int lookup_fast(struct nameidata *nd,
 		 */
 		*inode = d_backing_inode(dentry);
 		if (unlikely(read_seqcount_retry(&dentry->d_seq, seq)))
-			return -ECHILD;
+			return ERR_PTR(-ECHILD);
 
 		/*
 		 * This sequence count validates that the parent had no
@@ -1665,30 +1665,30 @@ static int lookup_fast(struct nameidata *nd,
 		 *  enough, we can use __read_seqcount_retry here.
 		 */
 		if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq)))
-			return -ECHILD;
+			return ERR_PTR(-ECHILD);
 
 		*seqp = seq;
 		status = d_revalidate(dentry, nd->flags);
 		if (likely(status > 0))
-			return handle_mounts(nd, dentry, path, inode, seqp);
+			return dentry;
 		if (unlazy_child(nd, dentry, seq))
-			return -ECHILD;
+			return ERR_PTR(-ECHILD);
 		if (unlikely(status == -ECHILD))
 			/* we'd been told to redo it in non-rcu mode */
 			status = d_revalidate(dentry, nd->flags);
 	} else {
 		dentry = __d_lookup(parent, &nd->last);
 		if (unlikely(!dentry))
-			return 0;
+			return NULL;
 		status = d_revalidate(dentry, nd->flags);
 	}
 	if (unlikely(status <= 0)) {
 		if (!status)
 			d_invalidate(dentry);
 		dput(dentry);
-		return status;
+		return ERR_PTR(status);
 	}
-	return handle_mounts(nd, dentry, path, inode, seqp);
+	return dentry;
 }
 
 /* Fast lookup failed, do it the slow way */
@@ -1874,19 +1874,18 @@ static int walk_component(struct nameidata *nd, int flags)
 			put_link(nd);
 		return err;
 	}
-	err = lookup_fast(nd, &path, &inode, &seq);
-	if (unlikely(err <= 0)) {
-		if (err < 0)
-			return err;
+	dentry = lookup_fast(nd, &inode, &seq);
+	if (IS_ERR(dentry))
+		return PTR_ERR(dentry);
+	if (unlikely(!dentry)) {
 		dentry = lookup_slow(&nd->last, nd->path.dentry, nd->flags);
 		if (IS_ERR(dentry))
 			return PTR_ERR(dentry);
-
-		err = handle_mounts(nd, dentry, &path, &inode, &seq);
-		if (unlikely(err < 0))
-			return err;
 	}
 
+	err = handle_mounts(nd, dentry, &path, &inode, &seq);
+	if (unlikely(err < 0))
+			return err;
 	return step_into(nd, &path, flags, inode, seq);
 }
 
@@ -3302,13 +3301,12 @@ static int do_last(struct nameidata *nd,
 		if (nd->last.name[nd->last.len])
 			nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY;
 		/* we _can_ be in RCU mode here */
-		error = lookup_fast(nd, &path, &inode, &seq);
-		if (likely(error > 0))
+		dentry = lookup_fast(nd, &inode, &seq);
+		if (IS_ERR(dentry))
+			return PTR_ERR(dentry);
+		if (likely(dentry))
 			goto finish_lookup;
 
-		if (error < 0)
-			return error;
-
 		BUG_ON(nd->inode != dir->d_inode);
 		BUG_ON(nd->flags & LOOKUP_RCU);
 	} else {
@@ -3383,10 +3381,10 @@ static int do_last(struct nameidata *nd,
 		got_write = false;
 	}
 
+finish_lookup:
 	error = handle_mounts(nd, dentry, &path, &inode, &seq);
 	if (unlikely(error < 0))
 		return error;
-finish_lookup:
 	error = step_into(nd, &path, 0, inode, seq);
 	if (unlikely(error))
 		return error;