linux-ext4 - Re: [PATCH v2 8/8] fscrypt: Move d_revalidate configuration back into fscrypt

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Date: Thu, 21 Dec 2023 23:58:30 -0600
From: Eric Biggers <ebiggers@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Gabriel Krisman Bertazi <krisman@...e.de>, jaegeuk@...nel.org,
	tytso@....edu, linux-f2fs-devel@...ts.sourceforge.net,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 8/8] fscrypt: Move d_revalidate configuration back
 into fscrypt

On Thu, Dec 21, 2023 at 07:39:40AM +0000, Al Viro wrote:
> 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.

fscrypt_prepare_lookup() handles unencrypted directories inline, without calling
__fscrypt_prepare_lookup() which is only for encrypted directories.  So the
logic to clear DCACHE_OP_REVALIDATE would need to be there too.

- Eric