2024-05-08 08:15:13

by Mateusz Guzik

Subject: [PATCH v2] 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: Mateusz Guzik <[email protected]>
---
fs/crypto/hooks.c | 32 ++++++++++++++++++++++++++------
1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 104771c3d3f6..d8d5049b8fe1 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -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);
--
2.39.2

2024-05-08 17:43:01

by Eric Biggers

Subject: Re: [PATCH v2] fscrypt: try to avoid refing parent dentry in fscrypt_file_open

On Wed, May 08, 2024 at 10:14:00AM +0200, Mateusz Guzik wrote:
> 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 <[email protected]>
> ---
> fs/crypto/hooks.c | 32 ++++++++++++++++++++++++++------
> 1 file changed, 26 insertions(+), 6 deletions(-)

Applied to https://git.kernel.org/pub/scm/fs/fscrypt/linux.git/log/?h=for-next

Thanks!

- Eric