Re: [PATCH v2 8/8] fscrypt: Move d_revalidate configuration back into fscrypt

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Dec 15, 2023 at 04:16:08PM -0500, Gabriel Krisman Bertazi wrote:

> +static const struct dentry_operations fscrypt_dentry_ops = {
> +	.d_revalidate = fscrypt_d_revalidate,
> +};
> +
>  int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
>  			     struct fscrypt_name *fname)
>  {
> @@ -106,6 +110,10 @@ int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
>  		spin_lock(&dentry->d_lock);
>  		dentry->d_flags |= DCACHE_NOKEY_NAME;
>  		spin_unlock(&dentry->d_lock);
> +
> +		/* Give preference to the filesystem hooks, if any. */
> +		if (!dentry->d_op)
> +			d_set_d_op(dentry, &fscrypt_dentry_ops);
>  	}
>  	return err;

Hmm...  Could we simply set ->s_d_op to &fscrypt_dentry_ops in non-ci case
*AND* have __fscrypt_prepare_lookup() clear DCACHE_OP_REVALIDATE in case
when it's not setting DCACHE_NOKEY_NAME and ->d_op->d_revalidate is
equal to fscrypt_d_revalidate?  I mean,

	spin_lock(&dentry->d_lock);
        if (fname->is_nokey_name)
                dentry->d_flags |= DCACHE_NOKEY_NAME;
        else if (dentry->d_flags & DCACHE_OP_REVALIDATE &&
		 dentry->d_op->d_revalidate == fscrypt_d_revalidate)
		dentry->d_flags &= ~DCACHE_OP_REVALIDATE;
	spin_unlock(&dentry->d_lock);

here + always set ->s_d_op for ext4 and friends (conditional upon
the CONFIG_UNICODE).

No encryption - fine, you get ->is_nokey_name false from the very
beginning, DCACHE_OP_REVALIDATE is cleared and VFS won't ever call
->d_revalidate(); not even the first time.  

Yes, you pay minimal price in dentry_unlink_inode() when we hit
        if (dentry->d_op && dentry->d_op->d_iput)
and bugger off after the second fetch instead of the first one.
I would be quite surprised if it turns out to be measurable,
but if it is, we can always add DCACHE_OP_IPUT to flags.
Similar for ->d_op->d_release (called in the end of
__dentry_kill()).  Again, that only makes sense if we get
a measurable overhead from that.