Re: [PATCH v3 04/10] fscrypt: Drop d_revalidate once the key is added

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Jan 25, 2024 at 05:20:56PM -0300, Gabriel Krisman Bertazi wrote:
> Eric Biggers <ebiggers@kernel.org> writes:
> 
> > On Fri, Jan 19, 2024 at 03:47:36PM -0300, Gabriel Krisman Bertazi wrote:
> >> /*
> >>  * When d_splice_alias() moves a directory's no-key alias to its plaintext alias
> >>  * as a result of the encryption key being added, DCACHE_NOKEY_NAME must be
> >>  * cleared.  Note that we don't have to support arbitrary moves of this flag
> >>  * because fscrypt doesn't allow no-key names to be the source or target of a
> >>  * rename().
> >>  */
> >>  static inline void fscrypt_handle_d_move(struct dentry *dentry)
> >>  {
> >>  	dentry->d_flags &= ~DCACHE_NOKEY_NAME;
> >> +
> >> +	/*
> >> +	 * Save the d_revalidate call cost during VFS operations.  We
> >> +	 * can do it because, when the key is available, the dentry
> >> +	 * can't go stale and the key won't go away without eviction.
> >> +	 */
> >> +	if (dentry->d_op && dentry->d_op->d_revalidate == fscrypt_d_revalidate)
> >> +		dentry->d_flags &= ~DCACHE_OP_REVALIDATE;
> >>  }
> >
> > Is there any way to optimize this further for the case where fscrypt is not
> > being used?  This is called unconditionally from d_move().  I've generally been
> > trying to avoid things like this where the fscrypt support slows things down for
> > everyone even when they're not using the feature.
> 
> The problem would be figuring out whether the filesystem has fscrypt
> enabled.  I think we can rely on sb->s_cop always being set:
> 
> if (sb->s_cop)
>    fscrypt_handle_d_move(dentry);
> 
> What do you think?

That's better, I just wonder if there's an even better way.  Do you need to do
this for dentries that don't have DCACHE_NOKEY_NAME set?  If not, it would be
more efficient to test for DCACHE_NOKEY_NAME before clearing the flags.

- Eric