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fscrypt: try to avoid refing parent dentry in fscrypt_file_open

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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: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240508081400.422212-1-mjguzik@gmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
This commit is contained in:
Mateusz Guzik 2024-05-08 10:14:00 +02:00 committed by Eric Biggers
parent dd5a440a31
commit 7f016edaa0
1 changed files with 26 additions and 6 deletions
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32
fs/crypto/hooks.c
View File

@ -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);