Commit 7f016eda authored by Mateusz Guzik's avatar Mateusz Guzik Committed by Eric Biggers
Browse files

fscrypt: try to avoid refing parent dentry in fscrypt_file_open 



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%)

v2:
- add a comment justifying rcu usage, submitted by Eric Biggers
- whack spurious IS_ENCRYPTED check from the refed case

Signed-off-by: default avatarMateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240508081400.422212-1-mjguzik@gmail.com


Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
parent dd5a440a

fs/crypto/hooks.c

+26 −6
Original line number Diff line number Diff line
@@ -30,21 +30,41 @@ 
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);



	/*

	 * Getting a reference to the parent dentry is needed for the actual

	 * encryption policy comparison, but it's expensive on multi-core

	 * systems.  Since this function runs on unencrypted files too, start

	 * with a lightweight RCU-mode check for the parent directory being

	 * unencrypted (in which case it's fine for the child to be either

	 * unencrypted, or encrypted with any policy).  Only continue on to the

	 * full policy check if the parent directory is actually encrypted.

	 */

	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 (!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);