fs: add predicts based on nd->depth

Stats from nd->depth usage during the venerable kernel build collected like so:
bpftrace -e 'kprobe:terminate_walk,kprobe:walk_component,kprobe:legitimize_links
{ @[probe] = lhist(((struct nameidata *)arg0)->depth, 0, 8, 1); }'

@[kprobe:legitimize_links]:
[0, 1)           6554906 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2)              3534 |                                                    |

@[kprobe:terminate_walk]:
[0, 1)          12153664 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

@[kprobe:walk_component]:
[0, 1)          53075749 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2)            971421 |                                                    |
[2, 3)             84946 |                                                    |

Additionally a custom probe was added for depth within link_path_walk():
bpftrace -e 'kprobe:link_path_walk_probe { @[probe] = lhist(arg0, 0, 8, 1); }'
@[kprobe:link_path_walk_probe]:
[0, 1)           7528231 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2)            407905 |@@                                                  |

Given these results:
1. terminate_walk() is called towards the end of the lookup and in this
   test it never had any links to clean up.
2. legitimize_links() is also called towards the end of lookup and most
   of the time there s 0 depth. Patch consumers to avoid calling into it
   in that case.
3. walk_component() is typically called with WALK_MORE and zero depth,
   checked in that order. Check depth first and predict it is 0.
4. link_path_walk() also does not deal with a symlink most of the time
   when !*name

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251119142954.2909394-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: mjguzik

fs/namei.c

@@ -785,7 +785,8 @@ static void leave_rcu(struct nameidata *nd)
 
 static void terminate_walk(struct nameidata *nd)
 {
-	drop_links(nd);
+	if (unlikely(nd->depth))
+		drop_links(nd);
 	if (!(nd->flags & LOOKUP_RCU)) {
 		int i;
 		path_put(&nd->path);
@@ -882,7 +883,7 @@ static bool try_to_unlazy(struct nameidata *nd)
 
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
-	if (unlikely(!legitimize_links(nd)))
+	if (unlikely(nd->depth && !legitimize_links(nd)))
 		goto out1;
 	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
 		goto out;
@@ -917,7 +918,7 @@ static bool try_to_unlazy_next(struct nameidata *nd, struct dentry *dentry)
 	int res;
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
-	if (unlikely(!legitimize_links(nd)))
+	if (unlikely(nd->depth && !legitimize_links(nd)))
 		goto out2;
 	res = __legitimize_mnt(nd->path.mnt, nd->m_seq);
 	if (unlikely(res)) {
@@ -2179,7 +2180,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
 	 * parent relationships.
 	 */
 	if (unlikely(nd->last_type != LAST_NORM)) {
-		if (!(flags & WALK_MORE) && nd->depth)
+		if (unlikely(nd->depth) && !(flags & WALK_MORE))
 			put_link(nd);
 		return handle_dots(nd, nd->last_type);
 	}
@@ -2191,7 +2192,7 @@ static const char *walk_component(struct nameidata *nd, int flags)
 		if (IS_ERR(dentry))
 			return ERR_CAST(dentry);
 	}
-	if (!(flags & WALK_MORE) && nd->depth)
+	if (unlikely(nd->depth) && !(flags & WALK_MORE))
 		put_link(nd);
 	return step_into(nd, flags, dentry);
 }
@@ -2544,7 +2545,7 @@ static int link_path_walk(const char *name, struct nameidata *nd)
 		if (unlikely(!*name)) {
 OK:
 			/* pathname or trailing symlink, done */
-			if (!depth) {
+			if (likely(!depth)) {
 				nd->dir_vfsuid = i_uid_into_vfsuid(idmap, nd->inode);
 				nd->dir_mode = nd->inode->i_mode;
 				nd->flags &= ~LOOKUP_PARENT;