linux-ext4 - Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries during lookup

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Date: Tue, 30 Jan 2024 16:47:24 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Gabriel Krisman Bertazi <krisman@...e.de>
Cc: viro@...iv.linux.org.uk, jaegeuk@...nel.org, tytso@....edu,
	amir73il@...il.com, linux-ext4@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 04/12] fscrypt: Drop d_revalidate for valid dentries
 during lookup

On Mon, Jan 29, 2024 at 05:43:22PM -0300, Gabriel Krisman Bertazi wrote:
> Unencrypted and encrypted-dentries where the key is available don't need
> to be revalidated with regards to fscrypt, since they don't go stale
> from under VFS and the key cannot be removed for the encrypted case
> without evicting the dentry.  Mark them with d_set_always_valid, to

"d_set_always_valid" doesn't appear in the diff itself.

> diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
> index 4aaf847955c0..a22997b9f35c 100644
> --- a/include/linux/fscrypt.h
> +++ b/include/linux/fscrypt.h
> @@ -942,11 +942,22 @@ static inline int fscrypt_prepare_rename(struct inode *old_dir,
>  static inline void fscrypt_prepare_lookup_dentry(struct dentry *dentry,
>  						 bool is_nokey_name)
>  {
> -	if (is_nokey_name) {
> -		spin_lock(&dentry->d_lock);
> +	spin_lock(&dentry->d_lock);
> +
> +	if (is_nokey_name)
>  		dentry->d_flags |= DCACHE_NOKEY_NAME;
> -		spin_unlock(&dentry->d_lock);
> +	else if (dentry->d_flags & DCACHE_OP_REVALIDATE &&
> +		 dentry->d_op->d_revalidate == fscrypt_d_revalidate) {
> +		/*
> +		 * Unencrypted dentries and encrypted dentries where the
> +		 * key is available are always valid from fscrypt
> +		 * perspective. Avoid the cost of calling
> +		 * fscrypt_d_revalidate unnecessarily.
> +		 */
> +		dentry->d_flags &= ~DCACHE_OP_REVALIDATE;
>  	}
> +
> +	spin_unlock(&dentry->d_lock);

This makes lookups in unencrypted directories start doing the
spin_lock/spin_unlock pair.  Is that really necessary?

These changes also make the inline function fscrypt_prepare_lookup() very long
(when including the fscrypt_prepare_lookup_dentry() that's inlined into it).
The rule that I'm trying to follow is that to the extent that the fscrypt helper
functions are inlined, the inline part should be a fast path for unencrypted
directories.  Encrypted directories should be handled out-of-line.

So looking at the original fscrypt_prepare_lookup():

	static inline int fscrypt_prepare_lookup(struct inode *dir,
						 struct dentry *dentry,
						 struct fscrypt_name *fname)
	{
		if (IS_ENCRYPTED(dir))
			return __fscrypt_prepare_lookup(dir, dentry, fname);

		memset(fname, 0, sizeof(*fname));
		fname->usr_fname = &dentry->d_name;
		fname->disk_name.name = (unsigned char *)dentry->d_name.name;
		fname->disk_name.len = dentry->d_name.len;
		return 0;
	}

If you could just add the DCACHE_OP_REVALIDATE clearing for dentries in
unencrypted directories just before the "return 0;", hopefully without the
spinlock, that would be good.  Yes, that does mean that
__fscrypt_prepare_lookup() will have to handle it too, for the case of dentries
in encrypted directories, but that seems okay.

- Eric