[PATCH] fscrypto: try to avoid refing parent dentry in fscrypt_file_open

[Mateusz Guzik]

Merely checking if the directory is encrypted happens for every open
when using ext4, at the moment refing and unrefing the parent, costing 2
atomics and serializing opens of different files.

The most common case of encryption not being used can be checked for
with RCU instead.

Sample result from open1_processes -t 20 ("Separate file open/close") from
will-it-scale on Sapphire Rapids (ops/s):
before:	12539898
after:	25575494 (+103%)

Arguably a vfs helper would be nice here.

Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
---
 fs/crypto/hooks.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 104771c3d3f6..16328ec14266 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -30,21 +30,32 @@
 int fscrypt_file_open(struct inode *inode, struct file *filp)
 {
 	int err;
-	struct dentry *dir;
+	struct dentry *dentry, *dentry_parent;
+	struct inode *inode_parent;
 
 	err = fscrypt_require_key(inode);
 	if (err)
 		return err;
 
-	dir = dget_parent(file_dentry(filp));
-	if (IS_ENCRYPTED(d_inode(dir)) &&
-	    !fscrypt_has_permitted_context(d_inode(dir), inode)) {
+	dentry = file_dentry(filp);
+	rcu_read_lock();
+	dentry_parent = READ_ONCE(dentry->d_parent);
+	inode_parent = d_inode_rcu(dentry_parent);
+	if (inode_parent != NULL && !IS_ENCRYPTED(inode_parent)) {
+		rcu_read_unlock();
+		return 0;
+	}
+	rcu_read_unlock();
+
+	dentry_parent = dget_parent(dentry);
+	if (IS_ENCRYPTED(d_inode(dentry_parent)) &&
+	    !fscrypt_has_permitted_context(d_inode(dentry_parent), inode)) {
 		fscrypt_warn(inode,
 			     "Inconsistent encryption context (parent directory: %lu)",
-			     d_inode(dir)->i_ino);
+			     d_inode(dentry_parent)->i_ino);
 		err = -EPERM;
 	}
-	dput(dir);
+	dput(dentry_parent);
 	return err;
 }
 EXPORT_SYMBOL_GPL(fscrypt_file_open);
-- 
2.39.2