linux-kernel - [PATCH] fs: avoid calls to legitimize

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Message-ID: <20251109185409.1330720-1-mjguzik@gmail.com>
Date: Sun,  9 Nov 2025 19:54:09 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] fs: avoid calls to legitimize_links() if possible

The routine is always called towards the end of lookup.

According to bpftrace on my boxen and boxen of people I asked, the depth
count is almost always 0, thus the call can be avoided in the common case.

one-liner:
bpftrace -e 'kprobe:legitimize_links { @[((struct nameidata *)arg0)->depth] = count(); }'

sample results from few minutes of tracing:
@[1]: 59
@[0]: 147236

@[2]: 1
@[1]: 12087
@[0]: 5926235

And of course the venerable kernel build:
@[1]: 3563
@[0]: 6625425

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
 fs/namei.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 2a112b2c0951..d89937c2c0b2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -826,7 +826,7 @@ static inline bool legitimize_path(struct nameidata *nd,
 	return __legitimize_path(path, seq, nd->m_seq);
 }
 
-static bool legitimize_links(struct nameidata *nd)
+static noinline bool legitimize_links(struct nameidata *nd)
 {
 	int i;
 	if (unlikely(nd->flags & LOOKUP_CACHED)) {
@@ -845,6 +845,11 @@ static bool legitimize_links(struct nameidata *nd)
 	return true;
 }
 
+static __always_inline bool need_legitimize_links(struct nameidata *nd)
+{
+	return nd->depth > 0;
+}
+
 static bool legitimize_root(struct nameidata *nd)
 {
 	/* Nothing to do if nd->root is zero or is managed by the VFS user. */
@@ -882,8 +887,10 @@ static bool try_to_unlazy(struct nameidata *nd)
 
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 
-	if (unlikely(!legitimize_links(nd)))
-		goto out1;
+	if (unlikely(need_legitimize_links(nd))) {
+		if (unlikely(!legitimize_links(nd)))
+			goto out1;
+	}
 	if (unlikely(!legitimize_path(nd, &nd->path, nd->seq)))
 		goto out;
 	if (unlikely(!legitimize_root(nd)))
@@ -917,8 +924,10 @@ 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)))
-		goto out2;
+	if (unlikely(need_legitimize_links(nd))) {
+		if (unlikely(!legitimize_links(nd)))
+			goto out2;
+	}
 	res = __legitimize_mnt(nd->path.mnt, nd->m_seq);
 	if (unlikely(res)) {
 		if (res > 0)
-- 
2.48.1