Merge patch series "fs: tidy up step_into() & friends before inlining"

Cleanup step_into() and walk_component() and inline them both.

* patches from https://patch.msgid.link/20251120003803.2979978-1-mjguzik@gmail.com:
  fs: inline step_into() and walk_component()
  fs: tidy up step_into() & friends before inlining

Link: https://patch.msgid.link/20251120003803.2979978-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

fs/namei.c

@@ -1672,13 +1672,15 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
 	path->dentry = dentry;
 	if (nd->flags & LOOKUP_RCU) {
 		unsigned int seq = nd->next_seq;
+		if (likely(!d_managed(dentry)))
+			return 0;
 		if (likely(__follow_mount_rcu(nd, path)))
 			return 0;
 		// *path and nd->next_seq might've been clobbered
 		path->mnt = nd->path.mnt;
 		path->dentry = dentry;
 		nd->next_seq = seq;
-		if (!try_to_unlazy_next(nd, dentry))
+		if (unlikely(!try_to_unlazy_next(nd, dentry)))
 			return -ECHILD;
 	}
 	ret = traverse_mounts(path, &jumped, &nd->total_link_count, nd->flags);
@@ -1941,13 +1943,23 @@ static int reserve_stack(struct nameidata *nd, struct path *link)
 
 enum {WALK_TRAILING = 1, WALK_MORE = 2, WALK_NOFOLLOW = 4};
 
-static const char *pick_link(struct nameidata *nd, struct path *link,
+static noinline const char *pick_link(struct nameidata *nd, struct path *link,
 		     struct inode *inode, int flags)
 {
 	struct saved *last;
 	const char *res;
-	int error = reserve_stack(nd, link);
+	int error;
 
+	if (nd->flags & LOOKUP_RCU) {
+		/* make sure that d_is_symlink from step_into_slowpath() matches the inode */
+		if (read_seqcount_retry(&link->dentry->d_seq, nd->next_seq))
+			return ERR_PTR(-ECHILD);
+	} else {
+		if (link->mnt == nd->path.mnt)
+			mntget(link->mnt);
+	}
+
+	error = reserve_stack(nd, link);
 	if (unlikely(error)) {
 		if (!(nd->flags & LOOKUP_RCU))
 			path_put(link);
@@ -2021,14 +2033,15 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
  *
  * NOTE: dentry must be what nd->next_seq had been sampled from.
  */
-static const char *step_into(struct nameidata *nd, int flags,
+static noinline const char *step_into_slowpath(struct nameidata *nd, int flags,
 		     struct dentry *dentry)
 {
 	struct path path;
 	struct inode *inode;
-	int err = handle_mounts(nd, dentry, &path);
+	int err;
 
-	if (err < 0)
+	err = handle_mounts(nd, dentry, &path);
+	if (unlikely(err < 0))
 		return ERR_PTR(err);
 	inode = path.dentry->d_inode;
 	if (likely(!d_is_symlink(path.dentry)) ||
@@ -2050,15 +2063,32 @@ static const char *step_into(struct nameidata *nd, int flags,
 		nd->seq = nd->next_seq;
 		return NULL;
 	}
-	if (nd->flags & LOOKUP_RCU) {
-		/* make sure that d_is_symlink above matches inode */
-		if (read_seqcount_retry(&path.dentry->d_seq, nd->next_seq))
+	return pick_link(nd, &path, inode, flags);
+}
+
+static __always_inline const char *step_into(struct nameidata *nd, int flags,
+                    struct dentry *dentry)
+{
+	/*
+	 * In the common case we are in rcu-walk and traversing over a non-mounted on
+	 * directory (as opposed to e.g., a symlink).
+	 *
+	 * We can handle that and negative entries with the checks below.
+	 */
+	if (likely((nd->flags & LOOKUP_RCU) &&
+	    !d_managed(dentry) && !d_is_symlink(dentry))) {
+		struct inode *inode = dentry->d_inode;
+		if (read_seqcount_retry(&dentry->d_seq, nd->next_seq))
 			return ERR_PTR(-ECHILD);
-	} else {
-		if (path.mnt == nd->path.mnt)
-			mntget(path.mnt);
+		if (unlikely(!inode))
+			return ERR_PTR(-ENOENT);
+		nd->path.dentry = dentry;
+		/* nd->path.mnt is retained on purpose */
+		nd->inode = inode;
+		nd->seq = nd->next_seq;
+		return NULL;
 	}
-	return pick_link(nd, &path, inode, flags);
+	return step_into_slowpath(nd, flags, dentry);
 }
 
 static struct dentry *follow_dotdot_rcu(struct nameidata *nd)
@@ -2171,7 +2201,7 @@ static const char *handle_dots(struct nameidata *nd, int type)
 	return NULL;
 }
 
-static const char *walk_component(struct nameidata *nd, int flags)
+static __always_inline const char *walk_component(struct nameidata *nd, int flags)
 {
 	struct dentry *dentry;
 	/*