[RFC PATCH] vfs: dodge strlen() in vfs_readlink() when ->i_link is populated

[Mateusz Guzik <mjguzik@xxxxxxxxx>]

This gives me about 1.5% speed up when issuing readlink on /initrd.img
on ext4.

Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---

I had this running with the following debug:

if (strlen(link) != inode->i_size)
       printk(KERN_CRIT "mismatch [%s] %l %l\n", link,
           strlen(link), inode->i_size);

nothing popped up

I would leave something of that sort in if it was not defeating the
point of the change.

However, I'm a little worried some crap fs *does not* fill this in
despite populating i_link.

Perhaps it would make sense to keep the above with the patch hanging out
in next and remove later?

Anyhow, worst case, should it turn out i_size does not work there are at
least two 4-byte holes which can be used to store the length (and
chances are some existing field can be converted into a union instead).

Bench:
$ cat tests/readlink1.c
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <assert.h>
#include <string.h>

char *testcase_description = "readlink /initrd.img";

void testcase(unsigned long long *iterations, unsigned long nr)
{
        char *tmplink = "/initrd.img";
        char buf[1024];

        while (1) {
                int error = readlink(tmplink, buf, sizeof(buf));
                assert(error > 0);

                (*iterations)++;
        }
}

 fs/namei.c                     | 43 ++++++++++++++++++----------------
 fs/proc/namespaces.c           |  2 +-
 include/linux/fs.h             |  2 +-
 security/apparmor/apparmorfs.c |  2 +-
 4 files changed, 26 insertions(+), 23 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 9d30c7aa9aa6..7aced8aca0f6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -5272,19 +5272,16 @@ SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newna
 				getname(newname), 0);
 }
 
-int readlink_copy(char __user *buffer, int buflen, const char *link)
+int readlink_copy(char __user *buffer, int buflen, const char *link, int linklen)
 {
-	int len = PTR_ERR(link);
-	if (IS_ERR(link))
-		goto out;
+	int copylen;
 
-	len = strlen(link);
-	if (len > (unsigned) buflen)
-		len = buflen;
-	if (copy_to_user(buffer, link, len))
-		len = -EFAULT;
-out:
-	return len;
+	copylen = linklen;
+	if (unlikely(copylen > (unsigned) buflen))
+		copylen = buflen;
+	if (copy_to_user(buffer, link, copylen))
+		copylen = -EFAULT;
+	return copylen;
 }
 
 /**
@@ -5317,13 +5314,15 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
 	}
 
 	link = READ_ONCE(inode->i_link);
-	if (!link) {
-		link = inode->i_op->get_link(dentry, inode, &done);
-		if (IS_ERR(link))
-			return PTR_ERR(link);
+	if (link)
+		return readlink_copy(buffer, buflen, link, inode->i_size);
+
+	link = inode->i_op->get_link(dentry, inode, &done);
+	res = PTR_ERR(link);
+	if (!IS_ERR(link)) {
+		res = readlink_copy(buffer, buflen, link, strlen(link));
+		do_delayed_call(&done);
 	}
-	res = readlink_copy(buffer, buflen, link);
-	do_delayed_call(&done);
 	return res;
 }
 EXPORT_SYMBOL(vfs_readlink);
@@ -5391,10 +5390,14 @@ EXPORT_SYMBOL(page_put_link);
 
 int page_readlink(struct dentry *dentry, char __user *buffer, int buflen)
 {
+	const char *link;
+	int res;
+
 	DEFINE_DELAYED_CALL(done);
-	int res = readlink_copy(buffer, buflen,
-				page_get_link(dentry, d_inode(dentry),
-					      &done));
+	link = page_get_link(dentry, d_inode(dentry), &done);
+	res = PTR_ERR(link);
+	if (!IS_ERR(link))
+		res = readlink_copy(buffer, buflen, link, strlen(link));
 	do_delayed_call(&done);
 	return res;
 }
diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
index 8e159fc78c0a..c610224faf10 100644
--- a/fs/proc/namespaces.c
+++ b/fs/proc/namespaces.c
@@ -83,7 +83,7 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
 	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
 		res = ns_get_name(name, sizeof(name), task, ns_ops);
 		if (res >= 0)
-			res = readlink_copy(buffer, buflen, name);
+			res = readlink_copy(buffer, buflen, name, strlen(name));
 	}
 	put_task_struct(task);
 	return res;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 972147da71f9..7d456db6a381 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3351,7 +3351,7 @@ extern const struct file_operations generic_ro_fops;
 
 #define special_file(m) (S_ISCHR(m)||S_ISBLK(m)||S_ISFIFO(m)||S_ISSOCK(m))
 
-extern int readlink_copy(char __user *, int, const char *);
+extern int readlink_copy(char __user *, int, const char *, int);
 extern int page_readlink(struct dentry *, char __user *, int);
 extern const char *page_get_link(struct dentry *, struct inode *,
 				 struct delayed_call *);
diff --git a/security/apparmor/apparmorfs.c b/security/apparmor/apparmorfs.c
index 01b923d97a44..60959cfba672 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -2611,7 +2611,7 @@ static int policy_readlink(struct dentry *dentry, char __user *buffer,
 	res = snprintf(name, sizeof(name), "%s:[%lu]", AAFS_NAME,
 		       d_inode(dentry)->i_ino);
 	if (res > 0 && res < sizeof(name))
-		res = readlink_copy(buffer, buflen, name);
+		res = readlink_copy(buffer, buflen, name, strlen(name));
 	else
 		res = -ENOENT;
 
-- 
2.43.0