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

[Eric Biggers]

[+linux-fsdevel]

Thanks!  The general concept looks good.  A few nits below:

On Tue, May 07, 2024 at 11:36:53AM +0200, Mateusz Guzik wrote:
> fscrypto: try to avoid refing parent dentry in fscrypt_file_open

fscrypt, not fscrypto

> 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

Overly long line above

> 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();

It would be helpful for there to be a comment here that explains the
optimization.  How about something like the following?

	/*
	 * 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.
	 */

	dentry = file_dentry(filp);
	rcu_read_lock();
        ...

> +
> +	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;

The IS_ENCRYPTED() check above can be removed, because it becomes redundant due
to this patch.  I think it was intended to optimize the case of unencrypted
files, like your patch does.  But clearly it wasn't too effective, as it was
after the dget_parent().

- Eric